aeration manual draft p.pdf

Copyright: Nopon Oy 1998. Printed in Finland. AER 04.98 ENG 395042

NOPOL®

AERATION MANUAL

1 Introduction and Table of Contents

2 Activated Sludge Process

3 Factors Affecting Dimensioning of Aeration Process

4 AOR and SOTR

5 Production of Air

6 Aeration Control

7 NOPOL® DDS Aeration System Design

8 NOPOL® O.K.I. Aeration System Design

9 Aeration in Pulp and Paper Industry

10 Glossary

NOPON OY Turvekuja 6 FIN-00700 Helsinki Finland Tel +358 9 351 730 Fax +358 9 351 5620E-mail: [email protected] internet: http://www.nopon.fi/

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Nopon Oy Aeration manual Document level 4 Date:03.07.1998

1 Introduction and Table of Contents Page: 1.1 (6)

Revision: 2 Written by: MR Inspected by: Accepted by:

1 INTRODUCTION

The purpose of this manual is to provide a useful reference of informationappertaining to NOPOL® aeration systems. The manual is divided into tenchapters each of which has a different purpose and may be used separately.

Chapter 1 contains a detailed table of contents of this manual.

The beginning of chapter 2 focuses on the working principle of the activatedsludge process. It then proceeds to detail variations in the activated sludgeprocess. The most important dimensioning parameters and their typicalvalues are given as a guide.

Chapter 3 deals with factors to be noted when dimensioning the aeration unitof the activated sludge process.

Chapter 4 explains the calculation of the Actual Oxygen Requirement AORand Standard Oxygen Transfer Rate SOTR values.

Chapter 5 contains information on air production. Different blowers and theiraccessories are discussed together with the general design principles ofblower plants and pipework.

Chapter 6 is useful to understand the most important features of operationalcontrol involved in the activated sludge process. Regulating and measuringthe oxygen content is examined, as is the selection of automation equipment.

The detailed design of an NOPOL® DDS aeration system is described inchapter 7. At the beginning, the oxygen demand and calculations of oxygenrequirements are covered. The manual then goes on to discuss featuresrelated to aeration system design where NOPOL® DDS aeration systems areused. The chapter concludes by giving some dimensioning examples.

The detailed design of a NOPOL® O.K.I. aeration system is described inchapter 8.

Chapter 9 contains two studies on aeration in pulp and paper industry.

Chapter 10 is a glossary chapter including symbols, a small dictionary ofwords used in waste water treatment and a table for transferring SI units tounits used in the U.S.A.




TABLE OF CONTENTS

1 INTRODUCTION 1.1

2 ACTIVATED SLUDGE PROCESS 2.12.1 Working Principle 2.12.2 Process Parameters 2.32.2.1 Aeration Method 2.32.2.2 Oxygen Requirement 2.42.2.3 Type of Flow 2.62.2.4 Process Efficiency 2.62.2.5 Sludge Age 2.72.2.6 Mohlmann Index or Sludge Volume Index SVI 2.82.2.7 Volumetric Load 2.92.2.8 Sludge Load F/M 2.92.2.9 Mixed Liquor Suspended Solids MLSS 2.102.2.10 Hydraulic Retention Time 2.102.2.11 Sludge Return Ratio 2.112.3 Process Modifications 2.122.3.1 Conventional Activated Sludge Process 2.122.3.2 Tapered Aeration 2.132.3.3 Step Feed 2.142.3.4 Complete Mixing Process 2.152.3.5 Contact Stabilisation 2.162.3.6 Kraus and Hatfield Processes 2.172.3.7 High-Rate Process 2.182.3.8 Extended Aeration 2.192.3.9 Oxidation Ditch 2.202.3.10 Carrousel Process 2.212.3.11 Aerated Lagoons 2.222.3.12 Two Stage Activated Sludge Process 2.232.3.13 Aerobic Anoxic Process 2.242.3.14 Anoxic Aerobic Process 2.252.3.15 BardenPho Process 2.262.3.16 Aerobic Contact Zone 2.272.3.17 Sequencing Batch Reactors 2.282.4 Summary of Process Modifications 2.30




3 FACTORS AFFECTING DIMENSIONING OF AERATION PROCESS3.1

3.1 Sewage Quantity and Composition 3.13.1.1 Design Flow 3.13.1.2 Biological Oxygen Demand BOD 3.33.1.3 Nitrogen Content, Ntot and N 3.43.1.4 BOD Load 3.53.2 Waste Water Properties 3.63.2.1 Coefficient of Total Oxygen Transfer KLa 3.63.2.2 Temperature 3.73.2.3 The α Coefficient 3.93.2.4 The β Coefficient 3.113.3 Aeration System 3.133.4 Operation Parameters 3.143.4.1 Dissolved Oxygen Level 3.143.4.2 Sludge Concentration 3.153.5 Plant Location 3.163.5.1 Atmospheric Pressure 3.163.6 Summary of Dimensioning Factors 3.17

4 AOR AND SOTR 4.14.1 Introduction 4.14.2 Actual Oxygen Requirement AOR 4.24.2.1 Eckenfelder O'Connor 4.34.2.2 Stall & Sherrad 4.44.2.3 "Abwassertechnik" 4.54.2.4 Eckenfelder - Boon 4.64.3 Standard Oxygen Transfer Rate SOTR 4.74.4 Clean Water Tests 4.84.4.1 General 4.84.4.2 Summary of Method 4.84.4.3 Definitions and Nomenclature 4.94.4.4 Apparatus and Methods 4.94.4.5 Chemicals 4.104.4.6 Samples 4.104.4.7 Air Flow Measurement 4.104.4.8 Timing Criteria 4.114.4.9 Calculations 4.114.5 Selection of Aeration Equipment 4.11

5 PRODUCTION OF AIR 5.1




5.1 Properties of Air 5.15.2 Calculation of Blower Air Flow 5.35.2.1 Cooling of Compressed Air in Pipework 5.35.2.1.1 Equation and Coefficients 5.45.2.1.2 Temperature Loss in a Pipe Surrounded by Air 5.75.2.1.3 Temperature Loss in a Pipe Surrounded by Earth 5.85.2.1.4 Temperature Losses in a Pipe Surrounded by Water 5.95.3 Air Intake 5.155.4 Silencers 5.175.5 Anti-vibration Control 5.175.6 Air Filtration 5.175.7 Different Types of Blowers 5.185.7.1 Positive Displacement Blowers 5.185.7.2 Dynamic Type Blowers 5.205.8 Delivery Control of Blowers 5.225.8.1 Rotary Blowers 5.225.8.2 Centrifugal Blowers 5.235.9 Blower Selection 5.235.9.1 Capacity Requirements 5.245.9.2 Delivery Control Requirements 5.245.10 Blower Plants 5.255.10.1 General Design Principles 5.255.10.2 Blower Accessories 5.265.11 Air Piping 5.275.11.1 Selection of Pipe Materials 5.275.11.2 Properties of Different Materials 5.275.11.3 Design Principles 5.285.12 Examples of Air Supply Systems 5.295.12.1 Waste Water Treatment Plant, P. E. 40,000 5.295.12.2 Waste Water Treatment Plant, P. E. 200,000 5.29

6 AERATION CONTROL 6.16.1 Benefits of Aeration Control 6.16.1.1 Process Benefits 6.16.1.2 Economic Benefits 6.26.2 Control System 6.26.2.1 Blower Air Delivery Control 6.26.2.2 Air Distribution Control 6.36.2.3 Example of Aeration Control System 6.56.3 Instrumentation 6.76.3.1 Dissolved Oxygen Probe 6.76.3.2 Air Flow Measurement 6.96.3.3 Pressure and Temperature 6.96.4 Mechanical Devices 6.9




7 NOPOL® DDS AERATION SYSTEM DESIGN 7.17.1 Air Flow 7.17.2 Mixing 7.37.3 Number of Diffusers 7.47.4 Layout 7.57.4.1 Layout Planning 7.57.4.2 Basin Geometry 7.107.4.3 Submersion Depth 7.117.4.4 Diffuser Layouts in Various Basins 7.117.5 Tapering Diffusers and SOTR 7.177.6 Calculation of Corrected SOTE Values 7.187.6.1 Expressing the Effect in Offers 7.187.7 Air Production 7.197.7.1 Dimensioning of the Blower 7.197.7.2 Dimensioning of Air Piping for NOPOL® DDS 7.267.7.3 Air Filtering System for NOPOL® DDS 7.277.8 Calculation Examples 7.297.8.1 Example 1 7.297.8.2 Example 2 7.39

8 NOPOL® O.K.I. AERATION SYSTEM DESIGN 8.18.1 NOPOL® O.K.I. Aerator Mixer 8.18.1.1 Scope of Delivery 8.28.1.2 Type of Aerator 8.48.1.3 Standard Oxygen Transfer Efficiency of Aerators 8.58.1.4 Basin Shape 8.68.1.5 Submersion Depth 8.68.1.6 Air Flow 8.68.1.7 Mixing 8.78.2 Designing the Aeration System 8.88.2.1 Number of Aerators 8.88.2.2 Cable and Hose Length Determination 8.88.2.2.1 Process Air Hose Length 8.88.2.2.2 Electric Cable Length 8.98.2.2.3 Lifting Cable Length 8.98.2.2.4 Protection Air Hose Length 8.98.3 Upgrading of O.K.I. 1000 Series Aerators 8.98.3.1 Example 1 8.98.3.2 Example 2 8.108.4 Layout Design 8.108.4.1 Aerator Location 8.118.4.2 Lifting Cable 8.128.4.3 Electric Cables and Protection Air Hose Attachments 8.12




8.4.4 Process Air Hose 8.128.4.5 Hose Flanges 8.158.4.6 Frequency Converter Control 8.158.4.7 Blower Air Delivery Control 8.168.4.8 Air Distribution Control 8.168.5 Electric System Design 8.178.5.1 Over Current Relay 8.178.5.2 Motor Protection 8.178.5.2.1 Thermistors 8.178.5.2.2 Thermal Units 8.188.5.3 Starting Current 8.188.5.4 Electro-magnetic Disturbances 8.188.6 Air Distribution Design 8.188.6.1 Process Air 8.188.6.1.1 Flow Rate 8.188.6.1.2 Valves 8.198.6.2 Hood Protection Air 8.208.6.2.1 Valves 8.208.6.2.2 Flow Rate and Flow Meters 8.208.7 Lifting System 8.218.8 Work Safety 8.228.9 Air Filtration 8.228.10 Water Separators 8.238.11 Installation, Operation and Maintenance 8.248.11.1 General 8.248.11.2 Manuals 8.248.11.3 Installation Supervision 8.248.12 Guarantees 8.25

9 AERATION IN PULP AND PAPER INDUSTRYDesign of the Activated Sludge Plant for the Pulp andPaper IndustryWaste Water Treatment in Pulp and Paper Industry

10 GLOSSARY 10.110.1 Symbols 10.110.2 Terms 10.410.3 Conversion Factors 10.18


2 Activated Sludge Process Page: 1.1 (30)


2 Activated sludge process2.1 Working Principle ...............................................................................2.12.2 Process Parameters ...........................................................................2.3

2.2.1 Aeration Method ..........................................................................2.32.2.2 Oxygen Requirement...................................................................2.42.2.3 Type of Flow................................................................................2.62.2.4 Process Efficiency .......................................................................2.62.2.5 Sludge Age..................................................................................2.72.2.6 Mohlmann Index or Sludge Volume Index SVI ............................2.82.2.7 Volumetric Load ..........................................................................2.92.2.8 Sludge Load F/M.........................................................................2.92.2.9 Mixed Liquor Suspended Solids MLSS .....................................2.102.2.10 Hydraulic Retention Time..........................................................2.102.2.11 Sludge Return Ratio..................................................................2.11

2.3 Process Modifications ......................................................................2.122.3.1 Conventional Activated Sludge Process ...................................2.122.3.2 Tapered Aeration ......................................................................2.132.3.3 Step Feed..................................................................................2.142.3.4 Complete Mixing Process..........................................................2.152.3.5 Contact Stabilisation .................................................................2.162.3.6 Kraus and Hatfield Processes ...................................................2.172.3.7 High-Rate Process ....................................................................2.182.3.8 Extended Aeration.....................................................................2.192.3.9 Oxidation Ditch..........................................................................2.202.3.10 Carrousel Process.....................................................................2.212.3.11 Aerated Lagoons .......................................................................2.222.3.12 Two Stage Activated Sludge Process .......................................2.232.3.13 Aerobic Anoxic Process ............................................................2.242.3.14 Anoxic Aerobic Process ............................................................2.252.3.15 BardenPho Process ..................................................................2.262.3.16 Aerobic Contact Zone................................................................2.272.3.17 Sequencing Batch Reactors......................................................2.28

2.4 Summary of Process Modifications ..................................................2.30

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2 Activated sludge process




2 ACTIVATED SLUDGE PROCESS

2.1 Working Principle

The activated sludge process is an aerobic biological process used for wastewater treatment. The process is based on the ability of micro-organisms toproduce and sustain biomass using the organic matter as carbon and nutrientsource.

The main parts of the aerobic waste water treatment are pretreatment,aeration and sedimentation. In practice, the aeration basin forms the reactorvessel for the activated sludge process, which comprises many simultaneousreactions. Aeration is necessary for maintaining aerobic conditions in thebasin and for creating sufficient mixing to keep the micro-organisms insuspension.

Following the aeration stage, the mixture of micro-organisms and water, socalled activated sludge, is conducted to a sedimentation basin where sludgeis separated from treated effluent. The bulk of the microbial sediment isreturned to the aeration basin to maintain the necessary concentration ofmicro-organisms there. The surplus microbial mass is removed from theprocess as excess sludge (see Figure 1).

AERATION BASIN

SEDIMENTATION BASIN

INFLUENT EFFLUENT

WASTE ACTIVATED SLUDGERETURNED ACTIVATED SLUDGE

MIXED LIQUOR

Figure 1: Diagram of activated sludge process

The oxygen demand of the micro-organisms is satisfied by aeration. Themicro-organisms need oxygen for the oxidation, synthesis, endogenousrespiration and nitrification.




OxidationDuring oxidation, organic material is biodegraded into carbon dioxide, waterand ammonia. Energy is released.

Equation 1

S O CO H O NH EnergyCOHNSbacteria enzyme+ → + + +2 2 2 3

,

SynthesisThe production of new micro-organisms is possible when organic materialand nutrients are present.

Equation 2

S P NH B CO H OCOHNSbacteria energy

C H NO+ + → + +3 2 25 7 2

,

Endogenous respirationEndogenous respiration means breaking down the biomass of dead micro-organisms. Energy is released:

Equation 3

B O CO H O NH EnergyC H NObacteria enzyme

5 7 25 5 22 2 2 3+ → + + +,

NitrificationOrganic material is not the sole substance which is biodegradable;ammonium nitrogen can be as well. This oxidation reaction is callednitrification:

Equation 4

NH O H H O NObacteria energy4 2 2 32 2+ + −+ → + +,

DenitrificationIn the presence of organic material and in the absence of oxygen, a reductionof nitrate may take place. Nitrogen will be released in gaseous form:




Equation 5

2223 212 COOHNHCNO bacteria

org ++ →++ +−

The quality of the effluent varies considerably depending on the abovereactions. That means that the purification result also varies. The results canbe determined for each particular case by using one or several of thefollowing processes:

• carbonaceous oxygenation equations Equation 1 - Equation 3• nitrification equations Equation 1 - Equation 4• nitrogen removal equations Equation 1 - Equation 5

The oxygenation of carboneous substances alone constitutes the "simplest"form of treatment. Complete nitrogen and phosphorus removal on the otherhand is the most complex and efficient one.

2.2 Process Parameters

The various process modifications discussed later on in Chapter 2.3 are eachdefined by the following technical criteria: aeration method, oxygenrequirement, type of flow, process efficiency, sludge age, sludge load,volumetric load, solids content, hydraulic retention time, and sludge returnratio. The criteria are defined as follows.

2.2.1 Aeration Method

The aeration system is normally based on bottom-installed aerationequipment. The air distribution profile may be either uniform, tapered orzonal, according to the diffuser layout (Figure 2). In Figure 3 various layoutsfor NOPOL® O.K.I. aerator mixers are shown.

Figure 2: Uniform, tapered and zonal diffuser arrangements




TAPERED SYSTEM

UNIFORM DISTRIBUTION

SELECTOR IN THE MIDDLE,AERATION BASIN AROUND IT

AERATORS DEVIDED TO SEPARATE BASINS

Figure 3: Various layouts with NOPOL® O.K.I. aerator mixers

2.2.2 Oxygen Requirement

Oxygen requirements determine the quantity of oxygen needed in the processmodification in relation to BOD5 or BOD7 (kg O2/kg BOD). The oxygenrequirement mainly depends on the value of the sludge load (F/M) andnitrification (see Figure 4).




Figure 4: Oxygen requirement as a function of sludge loading rate

Oxygen requirement can be expressed as AOR or SOTR which are discussedis Chapter 4.




2.2.3 Type of Flow

The type of flow and the shape of the aeration basin determine the flowconditions in the aeration basin, that is, whether there will be a so called plugflow or complete mixing. In plug flow, aeration conditions in different parts ofthe basin will vary. When the entire water volume is mixed, aerationconditions will be uniform throughout the basin (see Figure 5).

Figure 5: Schematic representation of plug flow and complete mixing

2.2.4 Process Efficiency

Process efficiency is defined as the reduction in pollution load achieved bytreating waste water, i.e. the difference between influent and effluent loads,often expressed as removal percentage or as residual concentration ofpollutants.

Equation 6

ES S

Si e

i

=−

•100%




whereE Process efficiency, %Si Influent substrate (BOD) concentration, kg/m3

Se Effluent substrate (BOD) concentration, kg/m3

This criterion is somewhat misleading as a measure of purification efficiencysince it does not say much about the final quality of the sewage afterpurification. For instance, if the efficiency is 90 % and the incoming BOD is300 mg/l, the resulting BOD will be 30 mg/l. If the incoming BOD is 100 mg/l,the resulting BOD will accordingly be 10 mg/l.

2.2.5 Sludge Age

Sludge age is defined as the average time during which the micro-organismsparticipate in the aerobic biological process. Sludge age θc is evaluated asthe ratio of total sludge mass in the system to the mass of sludge removedper day. The value of sludge age affects the final purification result and theMohlmann index (SVI) which reflects the sedimentation properties of thesludge. Sludge age may vary widely - from three to forty days, depending onwhat type of process configuration is used. In plants under normal loadlevels, sludge age varies from 5 to 10 days. Within this range a good BODreduction is achieved and the sludge will have good sedimentation qualities,as demonstrated in Figure 6. In nitrogen removal processes, the sludge agevaries from 20 to 30 days.

Equation 7

θ cw w e e

XV

X Q X Q=

+

whereθc Mean cell residence time (sludge age) dX Concentration of suspended solids in the

aeration basinkg/m3

V Aeration volume m3

Xw Concentration of suspended solids in excesssludge

kgMLSS/m3

Qw Excess sludge flow m3/dXe Concentration of suspended solids in the effluent kgSS/m3

Qe Effluent flow rate m3/d




Figure 6: BOD removal efficiency and sludge sedimentation quality versussludge age

2.2.6 Mohlmann Index or Sludge Volume Index SVI

The sludge volume index (SVI) is used to describe the settling properties ofsludge. It is determined by allowing a suspension of activated sludge to settlefor half an hour in a 1-litre graduated cylinder. SVI is defined as the ratiobetween the volume of settled sludge (g/1000 ml). SVI is therefore expressedas ml/g. SVI value in most cases varies between 50 and 200 ml/g. Valuesover 150 ml/g indicate poor settleability of sludge is caused by filamentousorganisms. Poor settleability of sludge is normally related to:

• low dissolved oxygen level in aeration• nutrient limitation• septic waste water• low F/M

Nutrient balance is rarely problem in municipal waste water treatment plantsbut in industrial waste water treatment plants it is more common. Nutrientdeficiency, particularly for nitrogen or phosphorus, produces poor settlingbulking sludge. The BOD: N: P ratio in the influent to aeration tanks shouldbe checked and adjusted if needed. A ratio of 100:5:1 should be targeted.However, deficiencies of other nutrients (such as iron) are also possible.




Low F/M bulking is often related to complete mixed aeration systems weresludge load F/M is relatively low. Especially difficult problems are arisenwhen waste water contains lots of soluble easily degradable carbon likewaste water from food, pulp and paper industry. Low F/M bulking can beavoided by using contact zones before main reactor.

2.2.7 Volumetric Load

The volumetric load (F/V) of the activated sludge process or basin is definedas the ratio between the daily organic load (BOD) fed into the aeration stageand the volume of the aeration basin.

The volumetric load is expressed as kg BOD/m3 d and it is defined as follows:

Equation 8

F VS Q

Vi i/ =

whereSi BOD concentration in influentQi influent flow rate, m3/dV volume of the aeration basin, m3

2.2.8 Sludge Load F/M

Sludge load F/M is the loading ratio of food-to-biomass of the activatedsludge process. Its value is the ratio between the daily incoming organic load(BOD) of the aeration stage and the suspended solids (MLSS) in the aerationbasin. It is expressed as kg BOD/kg MLSS d.

Equation 9

F MS Q

XVi i/ =

whereX concentration of activated sludge (MLSS), kg/m3

Sludge load is often calculated based on organic part of the activated sludge(MLVSS).




The various process modifications have optimum sludge load ranges whichgive the best process efficiency. Figure 7 illustrates SVI versus sludge load ina conventional process which only reduces the BOD.

Figure 7: SVI versus sludge load

2.2.9 Mixed Liquor Suspended Solids MLSS

Mixed liquor suspended solids (MLSS) is the concentration of solids in theactivated sludge process. The sludge content (X = MLSS) in the aerationbasin is usually 2 to 6 kg/m3. A higher solids content is maintained in thewinter than in the summer to compensate for the lower biological activity dueto lower temperature.

Approximately 60 to 80 % of the total solids content (MLSS) in the aerationbasin is organic matter (Mixed Liquor Volatile Suspended Solids MLVSS).The organic matter present is lower if primary sedimentation is not used or ifphosphorus is removed chemically. In the simultaneous precipitation process,the proportion is decreased to 50 - 70 %.

2.2.10 Hydraulic Retention Time

Hydraulic retention time describes the time taken for the micro-organisms toremove the oxygen-consuming soluble organic matter (BOD).




Equation 10

i

h Q

V=θ

whereθh hydraulic retention time, hQi influent flow, m3/hV aeration volume, m3

Hydraulic retention time can vary from 2 to 24 hours depending on wastewater composition and process modification. Industrial sewage often requiresconsiderably longer retention times than municipal waste waters. If retentiontime is below the specific value characteristic of the process modificationused, the sewage may be only partially purified.

2.2.11 Sludge Return Ratio

Sludge return ratio is the ratio between the volume of sludge returned to theaeration basin and the volume of influent water. The sludge return rationormally varies from 30 to 100 % in large plants and from 50 to 150 % insmaller plants, depending on the process modification used and on sludgesettling properties.

The ratio can be approximated by the following formula:

Equation 11

Q

Q

X

X Xr

i r

=−

whereQi influent flow, m3/hQr return sludge flow, m3/dXr return sludge suspended solids, kg MLSS/m3

X aeration basin sludge suspended solids, kg MLSS/m3




2.3 Process Modifications

2.3.1 Conventional Activated Sludge Process

In the conventional process, the sewage and return sludge are fed together tothe aeration basin inlet. Purification takes place progressively as the mixedsludge liquor advances in plug flow across the uniformly aerated aerationbasin.

Table 1: Process characteristics and parameters

Aeration method bottom aeration, constant air feedOxygen requirement 1,2 - 1,4 kg O2/kg BOD5

Type of flow plug flowPurification efficiency 85 - 95 % BOD5

Sludge age 5 - 10 dSludge load 0,2 - 0,4 kg BOD5/kg MLVSS • dVolumetric load 0,3 - 0,6 kg BOD5/m

3 • dSolids content 1,5 - 3,0 kg/m3

Hydraulic retention time 4 - 8 hSludge return ratio 30 - 100 %

Special features of the process• permits a relatively low sludge load• requires a rather long retention time• oxygen demand is highest in the feeding zone of the aeration basin• sensitive to variation of hydraulic and organic load• a constant sludge content level is maintained

AERATION BASIN

SEDIMENTATION BASIN

INFLUENT EFFLUENT


MIXED LIQUOR

Figure 8: Diagram of conventional activated sludge process




2.3.2 Tapered Aeration

In tapered aeration, process arrangements other than air feeding are exactlythe same as in the conventional process. The amount of air fed into theprocess is reduced in proportion to the decrease in the oxygen demand alongthe aeration basin.


Aeration method bottom aeration, progressively decreasing air feedOxygen requirement 1,0 - 1,2 kg O2/kg BOD5





Special features of the process• amount of air fed into the process is proportioned in accordance with

oxygen demand• energy savings are achieved• over and under aeration is eliminated better

AERATION BASIN

SEDIMENTATION BASIN

INFLUENTEFFLUENT


MIXED LIQUOR

DIMINISHING AERATION INTENSITY

Figure 9: Diagram of tapered aeration




2.3.3 Step Feed

In step feed process, the incoming sewage is fed to different points along theaeration basin. Return sludge is normally fed only to the aeration basin inlet.By distributing the incoming sewage to various feeding points, the absorptivecapacity of aeration sludge can be maintained at such a high level that arelatively short contact time is sufficient to ensure satisfactory purification.



Type of flow plug flow/complete mixingPurification efficiency 80 - 90 % BOD5




Special features of the process• offers flexibility of application• permits higher sludge concentration levels• not sensitive to sudden peak loads• aeration basin dimensions are smaller than in other process modifications• longer sludge age with same basin volume• effluent quality not as good as in tapered aeration

AERATION BASIN

SEDIMENTATION BASIN

INFLUENT EFFLUENT


MIXED LIQUOR

AERATION

Figure 10: Step feed




2.3.4 Complete Mixing Process

In the complete mixing process the mixture of incoming sewage and returnsludge is fed uniformly into the whole volume of the aeration basin. Oxygendemand and sludge concentration are thus constant throughout the basin.



Type of flow complete mixingPurification efficiency 80 - 90 % BOD5


3 • dSolids content 3 - 6 kg/m3


Special features of the process• evens out incoming load peaks efficiently• constant conditions are maintained for micro-organism activity• sometimes problems with sludge settling properties

Figure 11: Complete mixing process




2.3.5 Contact Stabilisation

In the contact stabilisation process, return sludge is fed to the aeration basininlet and aerated separately from the incoming sewage until the organicmatter contained in the sludge flocs has been stabilised, that is, used up forenergy release and the production of new micro-organisms. This stabilisationstage is followed by a contact stage in which the sewage is mixed with theaerated activated sludge.


Aeration method bottom aeration, constant air feed in contact andstabilisation basins

Oxygen requirement 0,8 - 1,2 kg O2/kg BOD5

contact 0,5 - 0,7 kg O2/kg BOD5

stabilisation 0,3 - 0,5 kg O2/1 kg BOD5


Sludge age 5 - 10 dSludge load 0,2 - 0,6 kg BOD5/kg MLVSS • d (contact stage)Volumetric load 1,0 - 1,2 kg BOD5/m

3 •d (contact stage)Solids content 2,0 - 4,0 kg/m3 (contact stage)

6,0 - 10,0 kg/m3 (stabilisation stage)Hydraulic retention time 1,0 - 3,0 h (contact) 3,0 - 6,0 h (stabilisation)Sludge return ratio 30 - 100 %

Special features of the process• well suited to sewage containing organic matter in colloidal form (low BOD

of soluble matter)• aeration volume required can be as low as one half of that required by the

conventional process• owing to the sludge buffer effect in the stabilisation section, the process is

able to accommodate load peaks and toxic matter without difficulty

SLUDGE REAERATION BASIN

CONTACT BASININFLUENT

WASTE ACTIVATED SLUDGE

RETURNED ACTIVATED SLUDGE

SEDIMENTATION BASIN

EFFLUENT

SLUDGE

SEDIMENTATION BASINPRIMARY FINAL

Figure 12: Contact stabilisation




2.3.6 Kraus and Hatfield Processes

In the Kraus process, some of the return sludge (10 -15 %) is fed into aseparate aeration basin into which sludge and supernatant from the digesterplant are also pumped. This mixture is aerated from 8 to 24 hours andsubsequently mixed with return sludge and fed to the aeration basin.

The Hatfield process differs from the Kraus process only in the fact that theentire return sludge is treated as in the Kraus process.


Aeration method bottom aeration, constant or tapered air feedOxygen requirement 1,0 - 1,4 kg O2/kg BOD5




Hydraulic retention time 4,0 - 8,0 hSludge return ratio 50 - 100 %

Special features of the process• well suited to sewage with low nitrogen content (for example, industrial

waste water containing carbohydrates)• settling qualities of activated sludge are improved by admixture of aerated

sludge which is denser and has better sedimentation characteristics• processes have an improved ability to accommodate peaks in organic load

since the aerated sludge mixture contains nitrogen in the form of nitratecompounds. This helps to maintain aerobic conditions in the process

REAERATION AERATION



SEDIMENTATION BASIN

EFFLUENT

INFLUENT

DIGESTER SUPERNATANT

DIGESTED SLUDGE

Figure 13: Schematic representation of Kraus and Hatfield processes




2.3.7 High-Rate Process

In the high-rate process the mixture of sewage and return sludge isdistributed uniformly over the whole volume of the aeration basin. Theaeration time is 0,5 - 2 h. Owing to the short aeration time the process issustained at the logarithmic growth phase in which micro-organismsreproduce at a very high rate, using large amounts of nutrients and oxygen.

The sludge return ratio has to be maintained at a high level to ensuresufficient feed of the activated sludge which makes flocculation moreeffective.

The principle of the high-rate process is the same as that shown in thediagram of the complete mixing process (Figure 11).






Hydraulic retention time 0,5 - 2 hSludge return ratio 100 - 500 %

Special features of the process• best suited to applications where a high-grade purification is not required• suitable as pretreatment for other processes




2.3.8 Extended Aeration

In the extended aeration process the mixture of sewage and return sludge isdistributed uniformly over the whole volume of the aeration basin, where it isaerated for 18 - 36 hours. Owing to the extended aeration time the process ismaintained at the phase of endogenous growth in which there is a fiercestruggle for nutrition between micro-organisms. The insufficient availability ofnutrients leads to a situation where micro-organisms use each others’ cellularmaterial as a source of nutrition. The principle is the same as inFigure 11.







Special features of the process• best suited to sewage containing organic matter with slow decomposition

characteristics• evens out sudden load variations efficiently• produces less sludge• high oxygen requirement




2.3.9 Oxidation Ditch

The purification process takes place in a closed annular channel or oxidationditch, into which the mixture of sewage and return sludge is fed. Thecirculatory movement of waste water is maintained in the channel normallywith brush aerators, but also large-diameter, slowly rotating propellers can beused to prevent settling of sludge. In the latter case aeration is achieved bymeans of diffusers arranged in groups and laid out in different sections on thebottom of the aeration ditch.


Aeration method bottom aeration with submersible mixers and airfeed by zones, surface aeration with brush aerators


Type of flow plug flow / complete mixingPurification efficiency 90 - 95 % BOD5




Special features of the process• complete nitrification is achieved in this process• nitrogen is removed from the process through denitrification• mechanical submersible mixers are used to create a sufficient current

velocity in waste water in order to prevent sludge settling

Figure 14: Schematic representation of oxidation ditch




2.3.10 Carrousel Process

In the carrousel process the incoming sewage and the return sludge aremixed together in the first aeration zone. Activated sludge moves in theendless channel through successive aeration zones. This process combinesthe features of complete mixing and plug flow. The complete mixing featureresults from the fact that the total liquid volume included in the circulatoryprocess is about 30 to 50 times greater than the influent flow rate. Thus theprocess can provide a marked buffer effect. The plug flow feature is due tothe great distance covered by one circuit. Improved denitrification results in areduced oxygen concentration in some parts of the aeration basin.


Aeration method bottom aeration with submersible mixers andair feed by zones


Type of flow plug flow / complete mixingPurification efficiency 95 - 98 % BOD5




Special features of the process• process can accommodate large variations in both quantity and quality of

incoming sewage• mechanical submersible mixers are used to create sufficient current

velocity in waste water in order to prevent the sludge settling

Figure 15: Principle of carrousel process




2.3.11 Aerated Lagoons

In this process, sewage is treated either by aeration alone or together withreturn sludge. In the latter case the process is equivalent to a normalextended aeration process. The aerated lagoon method is based on anaeration basin excavated in the ground and into which sewage and air are fedin order to oxidate organic matter.


Aeration method bottom aeration, air feed by zonesOxygen requirement 0,7 - 1,4 kg O2/kg BOD5


Sludge age not applicableLoad 0,002 - 0,05 kg BOD5/kg MLVSS • dSolids content 0,3 kg/m3

Hydraulic retention time 5 - 10 dSludge return ratio 50 - 200 % (if return sludge is used)

Special features of the process• low BOD reduction• low investment costs




2.3.12 Two Stage Activated Sludge Process

This process consists of two activated sludge processes operated in series.The first stage is operated under high load and the second one under normalload. These two separate activated sludge systems make it possible to createtwo separate biological sludge types which differ from each other as regardstheir microbial populations. Hence in the first stage the activated sludge has anormal microbial composition while in the second stage there is anabundance of nitrifying bacteria.


Aeration method bottom aeration, constant air feedOxygen requirement 0,5 - 0,6 kg O2/BOD5 (1st stage)

1,5 - 2,0 kg O2/BOD5 (2nd stage)Type of flow plug flowPurification efficiency > 95 % BOD5

Sludge age 1 - 3 d (1st stage) and 5 - 10 d (2nd stage)Sludge load 0,6 - 2,0 kg BOD5/kg MLVSS • d (I)

0,15 - 0,3 kg BOD5/kg MLVSS • d (II)Volumetric load 2,0 - 3,0 kg BOD5/m

3 • d (I)0,3 - 0,7 kg BOD5/m

3 • d (II)Solids content 2 - 3 kg/m3 (I); 1 - 1,5 kg/m3 (II)Hydraulic retention time 0,5 - 0,8 h (I); 1,6 - 3,2 h (II)Sludge return ratio 20 - 50 % (I); 25 - 50 % (II)

Special features of the process• efficient nitrification• high BOD reduction• suitable for concentrated sewage

AERATION BASIN AERATION BASIN

SEDIMENTATION BASIN

INFLUENT



MIXED LIQUOR

SEDIMENTATION BASIN

EFFLUENT



MIXED LIQUOR

Figure 16: Two stage activated sludge process in schematic form




2.3.13 Aerobic Anoxic Process

The process has two stages. The first stage is aerobic and results in theoxidation of organic carbon and ammonia. The second stage is anoxic and isaimed at denitrification, after which the treated sludge first passes through asecondary aeration stage before the sedimentation unit.



Type of flow plug flowPurification efficiency 90 - 95 % BOD5, 60 - 90 % NSludge age 7 - 15 dSludge load 0,05 - 0,15 kg BOD5/kg MLVSS • dVolumetric load 0,2 - 0,4 kg BOD5/m

3 • dSolids content 3 - 5 kg/m3

Hydraulic retention time 16 - 18 h (12 h, 4 h and 1 h)Sludge return ratio 75 - 150 %

Special features of the process• sludge produced by the process has poor settling qualities• mixing in the anoxic section is carried out by a mechanical mixer• denitrification rate is low due to limited availability of degredable organic

material in denitrification zone

AERATION MIXING AERATIONINFLUENT EFFLUENT

WASTE SLUDGERETURN SLUDGE

SEDIMENTATION BASIN

Figure 17: Schematic representation of aerobic anoxic process




2.3.14 Anoxic Aerobic Process

The process has two stages. In the first stage, the incoming sewage is mixedwith return sludge. Under anoxic conditions the mixture interreacts, resultingin the reduction of nitrate, which is released as nitrogen gas. The denitrifiedsludge mixture is then nitrified again during the second process stage, whichis aerobic. Following this, part of the sludge is recycled directly back to theanoxic stage.



Type of flow plug flowPurification efficiency 90 - 95 % BOD5 , 60 - 90 % NSludge age 10 - 20 dSludge load 0,05 - 0,15 kg BOD5 /kg MLVSS • dSolids content 3 - 5 kg/m3

Hydraulic retention time 18 h (6 h and 12 h)Sludge return ratio 100 - 150 % and 100 - 300 % for denitrification

recycle

Special features of the process• sludge produced has good settling qualities• process can be implemented by means of small modifications from the

nitrifying plug flow process• reduces the build-up of slime on diffusers

MIXING AERATIONINFLUENT

SEDIMENTATION BASIN

EFFLUENT

WASTE SLUDGE

RETURN SLUDGE

Figure 18: Principle of the anoxic aerobic process




2.3.15 BardenPho Process

The BardenPho process consists of five separate stages. The first stage isbased on an anaerobic reactor into which the incoming sewage is fedtogether with return sludge. Owing to the anaerobic conditions, phosphorusbound up in the return sludge is released. The second stage, which operatesunder anoxic conditions, is fed with a nitrate-rich sludge admixture from thethird stage. The nitrates are reduced and nitrogen gas is released. The thirdstage permits nitrification and the rebinding of phosphorus. The fourth stageconsists of another anoxic reactor where denitrification takes place again.The fifth and last stage comprises an aeration unit where the free phosphorusis bound to the sludge. This phosphorus-bearing sludge is then settled andsubsequently recycled back to the initial stage of the process.


Aeration method bottom aeration, constant air feedOxygen requirement 1,3 - 0,5 kg O2/kg BOD5 (III)

0,3 - 0,5 kg O2/kg BOD5 (V)Type of flow complete mixing / plug flowPurification efficiency 90 % BOD5; 80 - 90 % P; 90 % NSludge age 14 - 20 dSludge load 0,05 - 0,075 kg BOD5/kg MLVSS • dSolids content 3 - 5 kg/m3

Hydraulic retention time 1 h (I); 2 - 3 h (II); 4 - 5 h (III);2 - 3 h (IV); 1 h (V)

Sludge return ratio 100 % and 400 % (III ¤ II)

Special features of the process• provides biological removal of nitrogen and phosphorus• process produces sludge which is biologically stable and easy to treat in

further conditioning• total nitrogen content (nitrates, ammonia, organic nitrogen) of the effluent

leaving the process is very low (2 - 5 mg/l)

ANAEROBIC ANOXIC AEROBIC ANOXIC AEROBICINFLUENT EFFLUENT

WASTE SLUDGE

RETURN SLUDGE

RECYCLESEDIMENTATION BASIN

Figure 19: Principle of the BardenPho process




2.3.16 Aerobic Contact Zone

Poor sludge quality is a common problem in an activated sludge plant,especially in waste water treatment plants treating industrial, easilydegradable, waste water from food, pulp and paper industry.

Sludge settling properties can be improved with a selector basin. In theselector process the waste water and the return sludge are mixed in a smallbasin before it is introduced to the main aeration tank. In the selector-aerationsystem the well settling floc-forming bacteria have better conditions to grow.Therefore, these bacteria are selected into process after microbialcompetition. The selector establishes a substrate gradient that allows flocforming, which posses rapid uptake and storage capabilities, to compete thefilamentous bulking organisms.

The selector basin is dimensioned so that the soluble biologically degradableorganic compounds are either degraded or stored by bacteria in the selector.Contact times of 10 - 20 min have been used in a selector basin. BOD load ina selector is usually 3 - 6 kg BOD/ m3 d. Selector design is stronglydepending on the composition of the waste water.

Oxygen uptake rate in the selector basin is very high. It is affected by processconfiguration and type of the waste water. Actual Oxygen Demand can beestimated on the basis of soluble organic removal in the selector. The oxygendemand of the selector can be up to 50 % of the total oxygen demand of theprocess.




2.3.17 Sequencing Batch Reactors

Sequencing Batch Reactor SBR technology is a method of waste watertreatment in which all phases (aeration, clarification) of the treatment cycleoccur sequentially in one reactor basin. This basic cycle may be modified bythe designer to achieve the conditions necessary for carbonaceous oxidation,nitrification, denitrification and biological phosphorus removal.

The various phases in a typical cycle usually comprise the following:

Fill waste water enters the reactor basin and mixes with activatedsludge mixed liquor held in the tank.

React aeration of the tank contents. Biological reactions occur until thedesired degree of treatment has been achieved.

Settle aeration is stopped and the activated sludge suspended solidssettle to form a blanket on the bottom of the reactor vessel.

Decant clarified effluent is removed from the reactor without disturbingthe sludge blanket.

Idle unexpired time between cycles. Surplus sludge wasting mayoccur.

Completion of these five phases constitutes a cycle of typically six hoursduration which is then repeated. Influent fill operation is usually interruptedduring decanting to prevent effluent deterioration by short circuiting.

Typical SBR's may use the following modified 6 hour cycle sequence fornutrient removal:Time (h) 0 - 1.5 1.5 - 2.0 2.0 - 4.0 4.0 - 5.0 5.0 - 6.0

Fill Fill - Aerate Aerate Settle Decant

For a single basin operating as above, influent flow balancing would berequired to store the waste water during the non-filling aerate, settle anddecant phases.

SBR technology has the advantage of being much more flexible thanconventional activated sludge processes in terms of matching reaction timesto the concentration and degree of treatment required for a particular wastewater.




The volume between design bottom water level and top water levelrepresents the volume treated per batch or cycle fill volume. Cycle fillvolumes are typically up to thirty percent of the designated top water levelvolume and the overall basin depth is generally sized around 5 to 6 m.

FILLMixed liquor

Inflow

Air on or off

(2 hours)

REACTMixed liquor

Inflow optional

Air on

(2 hours)

SETTLESludge blanket

Air off

(1 hour)

DECANT

Sludge blanket

Air off

(1 hour)

Decant

IDLE

Sludge blanket

Air off or on

(Remainder ofcycle)

Waste sludge

Figure 20: Sequencing Batch Reactor




2.4 Summary of Process Modifications

A summary of the above mentioned process modifications is given in Table16. Purification efficiency, sludge age and volumetric load are given on thebasis of the process parameters.

Table 16: Summary of the process modifications

Modification Purification efficiency % Sludge age Volumetric loadBOD NH4/N / N-tot d kg BOD7/m

3•dConventional 85 - 95 - 5 - 10 0,3 - 0,6Tapered aeration 85 - 95 - 5 - 10 0,3 - 0,6Step feed 80 - 90 - 5 - 10 0,6 - 1,0Complete mixing 80 - 90 - 3 - 10 0,8 - 2,0Contactstabilisation

80 - 90 - 5 - 10 1,0 - 1,2

Kraus & Hatfield 85 - 95 - 3 - 10 0,6 - 1,6High-rate 60 - 80 - 1 - 3 2,0 - 6,0Extended aeration 85 - 95 > 90 / - 15 - 30 0,1 - 0,4Oxidation ditch 90 - 95 > 90 / 50 15 - 30 0,1 - 0,4Carrousel 95 - 98 > 90 / 50 15 - 30 0,1 - 0,4Aerated lagoon 50 - 75 - - 0,05 - 0,2Two stage > 95 > 90 / - 1-3 / 5-10 2,0-3,0 / 0,3-0,7Aerobic anoxic 90 - 95 > 90 / 50 7 - 20 0,2 - 0,4Anoxic aerobic 90 - 95 > 90 / 50 - 80 7 - 20 0,2 - 0,4BardenPho 90 - 95 > 90 / 60 - 90 14 - 20 0,1 - 03

P 80 - 90


3 Factors Affecting Dimensioning of Aeration Process Page: 1.1 (17)


3 Factors Affecting Dimensioning of Aeration Process3.1 Sewage Quantity and Composition ....................................................3.1

3.1.1 Design Flow.................................................................................3.13.1.2 Biological Oxygen Demand BOD ................................................3.33.1.3 Nitrogen Content, Ntot and N........................................................3.43.1.4 BOD Load....................................................................................3.5

3.2 Waste Water Properties.....................................................................3.63.2.1 Coefficient of Total Oxygen Transfer KLa....................................3.63.2.2 Temperature................................................................................3.73.2.3 The α Coefficient.........................................................................3.93.2.4 The β Coefficient .......................................................................3.11

3.3 Aeration System ...............................................................................3.133.4 Operation Parameters ......................................................................3.14

3.4.1 Dissolved Oxygen Level............................................................3.143.4.2 Sludge Concentration................................................................3.15

3.5 Plant Location...................................................................................3.163.5.1 Atmospheric Pressure ...............................................................3.16

3.6 Summary of Dimensioning Factors...................................................3.17

arto

3 Factor Affecting Dimensioning of Aeration Process

arto

Process




3 FACTORS AFFECTING DIMENSIONING OF AERATIONPROCESS

3.1 Sewage Quantity and Composition

3.1.1 Design Flow

Waste water flows may vary markedly hourly, daily and seasonally, and thevariations are typical for each treatment plant.

For the dimensioning of either the whole treatment plant or the aerationsystem alone, the following parameters are needed:

• hourly design flow, qdim (m3/h)• daily design flow, Qdim (m3/d)• maximum daily flow, Qmax (m

3/d)

As a general rule it can be said that the smaller the sewage network, thegreater the hourly variations. The daily variations depend on the weekday,season, weather conditions, etc. The daily variations of the influents(max/min) may vary in small plants from 5 to 10:1, whereas in large plantsthey are much less. A typical curve showing the variation of influents overone year is presented in Figure 1.

Figure 1: Typical daily variation of the influent.

The daily variations of the influents during the past year or over a longerperiod can be depicted as a duration curve (Figure 2). For dimensioningpurposes a certain projected value (m3/d) depending on the forecast of the




sewerage area (10 - 30 years forward) should be added to the duration curvevalues.

Figure 2: The variations of the influent presented (as AOR), duration curve.

The daily design flow (m3/d) and the maximum daily flow can be derived fromthe duration curve values. Normally the design flow can be chosen from theduration curve as the mean value, but where forecasts are not included in thiscurve, the design flow can be greater. The maximum daily flow (m3/d) can bedetermined by the permissible length of the period during which overflows areallowed.

To determine the hourly design flow (m3/h) the hourly variations should betaken into account. In Table 1 the hourly variation factor kdim is given as afunction of the relation between the maximum daily and average daily flows.

Table 1: kdim as a function of Qmax / Qaverage

Qmax / Qaverage kdim

1...2 1,0...1,22...4 1,2...1,44...8 1,4...1,6




If no existing data is available on daily variations, the hourly design flow ratecan be determined using Equation 1, in which factors from Table 1 can beused.

Equation 1

q kQ

t

Q

t

Qd

d

i

i

Ldim dim= • + +

24

whereqdim hourly design flow m3/hkdim hourly variation factorQd domestic sewage flow m3/dtd hours of domestic sewage flows per day hQi industrial sewage flow m3/dti hours of industrial sewage flows per day hQL leakages m3/d

For estimating domestic sewage flow the specific water consumption (l/P.E. •d) can be used. Normally the specific water consumption varies from 150 to300 l/P.E. • d.

Treatment capacity has to be designed to suit each individual case.

3.1.2 Biological Oxygen Demand BOD

The organic load expressed as biological oxygen demand (BOD) indicatesthe amount of oxygen required by micro-organisms for the biochemicaloxidation of organic matter. This amount is normally determined as theoxygen consumption occurring in a period of 5 or 7 days at a temperature of20 °C. In the analytical method nitrification is usually eliminated by theaddition of allyltiourea (ATU). The following equation permits a conversion tobe made from BOD5 to BODu:

Equation 2

( )BODBOD

eu kt

=− −

5

1

k 0,20 - 0,25t time, d




Figures for BOD5 can be converted to BOD7 by means of the followingequation:

Equation 3

BOD BOD7 5115= •,

If enough existing data is available on the influent BOD concentrations,duration curves can be used to determine the design BOD (So) concentration.Normally these values vary from 200 to 300 mg/l, but in the case of industrialwaste waters the fluctuations may be greater.

If there is no data available other than the population size, the populationequivalent value can be used. Values of 70 to 90 g BOD7/d • P.E. arenormally used.

The BOD concentration after the treatment process can be evaluated eitherby the process efficiency (%) or by the purification requirements (mg/l). In thiscase the process modification must be taken account. For normal domesticsewage the required level can be 10 - 20 mg/l, but for concentrated sewage(So > 1 000 mg/l) the level is usually 30 - 50 mg/l.

Organic load is expressed as kg BOD/d. The oxygen demand in the aerationstage is determined by the BOD value of the influent. Chemical and ormechanical treatment may significantly influence the capacity needed at thisstage.

Organic load can also be expressed as Chemical Oxygen Demand COD.COD indicates the total amount of organic material which can be oxidisedchemically in high temperature. COD includes both biologically degradableand undegradable organic compounds. For influent, the COD/BOD5 ratiovaries normally between 1,7 and 3,0 depending on the composition of wastewater.

3.1.3 Nitrogen Content, Ntot and N

The total nitrogen content of the influent (Ntot) is determined by the totalamount of nitrogen compounds (organic nitrogen, ammonium, nitrate, etc.).Normally it varies from 30 to 50 mg/l in municipal waste water. Sometimes thenitrogen content is expressed as Total Kjeldahl Nitrogen, which reflects onlythe total amount of organic and ammonium nitrogen. During the biological




treatment process, the organic compounds are transformed to ammonia(NH4

+).

If there is no nitrification (oxygenation of ammonium ions) in the treatmentprocess, almost all the nitrogen (N) passes through the process as ammonia(NH4

+). Only a certain percentage, normally 20 - 30 %, will be bound in thesludge and removed with it.

Nitrogen content of the excess sludge depends on the sludge age and sludgeproduction. Nitrogen content of biological sludge is normally 5 - 10 %.

Where there is nitrification, ammonium is oxidised to nitrate (NO3-) in a

biochemical reaction. Since ammonium is a soluble effluent, the ammoniumconcentration may be less than 1 mg/l after efficient nitrification.

Equation 4

NH O H H O NObacteria energy4 2 2 32 2+ + −+ → + +,

Denitrification converts nitrate nitrogen to molecular nitrogen (N2), whichbeing a gas, escapes into the atmosphere.

Equation 5

2223 212 COOHNHCNO bacteria

org ++ →++ +−

If denitrification phase is before the nitrification phase in the process, part ofthe organic load (BOD) is oxygenated in denitrification. This reduces theneed of oxygen in aeration phase and is expressed as a negative term in thecalculation of actual oxygen demand.

3.1.4 BOD Load

The BOD load is expressed as kg BOD/d. In determining the oxygen demandof aeration the influent BOD values of the aeration unit are used. Where extratreatment units exist before the aeration stage the reduced values can beused. The efficiency (%) of different kinds of pre-treatment units and methodsare quoted in Table 2.




Table 2: BOD reduction in different pre-treatment methods

Pre-treatment method Efficiency %Primary sedimentation 25 - 50Preprecipitation 50 - 70Biofilter 60 - 80

Nitrogen concentration is reduced in all cases by only 0 - 20 %.

Organic load can be expressed also as a Chemical Oxygen Demand (COD).COD indicates the total amount of the organic material, which can be oxidisedchemically in high temperature. COD-value includes both biologicallydegradable and undegradable organic compounds. For influent waste water,the COD/BOD5 -ratio varies normally between 1,7 - 3,0 depending thecomposition of the waste water.

3.2 Waste Water Properties

3.2.1 Coefficient of Total Oxygen Transfer KLa

The oxygen transfer rate into water (dm/dt) is expressed by the followingequation:

Equation 6

( )dm

dtD A

C C

LK A C CL

LL L= • •

−= • • −∞

∞

**

where

DL molecular diffusion of oxygen throughboundary fluid film

m/s

A area of air / water boundary surface m2

C*∞ saturation concentration of dissolved oxygenin water at process temperature

g O2/m3

CL existing oxygen concentration in water atprocess temperature

g O2/m3

L imaginary thickness of boundary fluid film mKL mass transfer coefficient of boundary fluid film g/m2

For evaluations using volumetric units the above equation may be expressedas follows:




Equation 7

( ) ( )dm

dt VK

A

VC C K a C CL L L L•

= • • − = • −∞ ∞* *

where

KLa apparent volumetric mass transfer coefficient in cleanwater at temperature T

A/V total mass transfer area per volumetric unit, m2/m3

The coefficient KLa is specifically characteristic to the whole aeration system.Among many factors affecting the value of this coefficient the following maybe mentioned:

• Temperature• Soluble organic and inorganic material• Aeration method• Renovation rate of the boundary surface (gas/fluid)• Depth and shape of the aeration basin

3.2.2 Temperature

Both the coefficient KLa and the saturation concentration of oxygen in waterCs are dependent on the water temperature. The value of the oxygen transfercoefficient KLa grows with an increase of water temperature. The temperatureeffect is generally expressed by means of the following equation:

Equation 8

( ) ( )K a T K a CL Lo T= • −20 20θ

A temperature of either 10 °C or 20 °C can be used as the standardtemperature for the application of this formula. The value used for thecorrection coefficient θ is usually 1,024. In related literature the values givenvary from 1,01 to 1,03.

The saturation concentration of oxygen decreases as the temperature of thewater increases. Table 3 shows the saturation concentration of oxygen (mg/l)in water as a function of temperature (atmospheric pressure).




Table 3: Saturation concentration of oxygen as a function of temperature(atmospheric pressure)

T (°C) Cs (mg/l) T (°C) Cs (mg/l) T (°C) Cs (mg/l)0 14,60 17 9,65 34 7,051 14,19 18 9,45 35 6,932 13,81 19 9,26 36 6,823 13,44 20 9,07 37 6,714 13,09 21 8,90 38 6,615 12,75 22 8,72 39 6,516 12,43 23 8,56 40 6,417 12,12 24 8,40 41 6,318 11,83 25 8,24 42 6,229 11,55 26 8,09 43 6,13

10 11,27 27 7,95 44 6,0411 11,01 28 7,81 45 5,9512 10,76 29 7,67 46 5,8613 10,52 30 7,54 47 5,7814 10,29 31 7,41 48 5,7015 10,07 32 7,28 49 5,6216 9,85 33 7,16 50 5,54

The value of C*∞ at different pressures can be calculated by means of thefollowing equation:

Equation 9

C H X ph

O∞ = • • +•

* δ2

where

H Henry’s constant (mg/l) / (kN/m2)Xo Molal fraction of oxygen in aeration air 0,209p Atmospheric pressure 101,325 kN/m2

δ Specific weight of water kg/dm3

h Submersion depth of diffusers m

The values of H and δ can be calculated from the following table:




Table 4: Temperature dependence of values H and δ

Temperature H δ0 0,697 9,8055 0,607 9,807

10 0,540 9,80415 0,484 9,79820 0,438 9,78925 0,393 9,77730 0,365 9,764

In the aeration process, the temperature of the water affects the oxygenationcapacity of aeration equipment in an analogous way. Thus a watertemperature increase from 10 °C to 20 °C increases the coefficient KLa by 25%, while the saturation concentration of oxygen simultaneously decreases by20 %. As a result, the above mentioned change in temperature decreases theoxygenation capacity of aeration equipment by 3 %, in case the oxygenconcentration maintained in the aeration basin is 2 mg/l. Under the sameconditions, the temperature change from 5 °C to 25 °C will decrease theoxygenation capacity by approximately 7 %.

For the calculation of the oxygen requirement the most important temperatureis the highest temperature existing during aeration. It is particularly importantto take this into account in a hot climate and when warm waste water fromindustry is being treated.

Waste water temperature in municipal plants is usually between 5 to 25 °C. Inindustrial plants the waste water temperature can be considerably higher, upto 40 °C.

3.2.3 The αα Coefficient

The α coefficient is defined as the ratio of the mass transfer coefficientsmeasured in sewage and in clean water:

Equation 10

α =K a sewage

K a cleanwaterL

L

( )

( )

The value of the coefficient depends on• concentration of surface active agents MBA




• intensity of mixing• geometry of the aeration basin• aeration method• process configuration• process parameters, sludge age

For fine bubble aeration systems it has been suggested that the value of αgrows from about 0,4 in the initial section to about 0,9 in the end section ofthe plug flow processes. In the high rate processes a figure of 0,5 and in thelow rate complete mixing processes a value of 0,8 can be used.

With submersible mechanical aerators, such as NOPOL® O.K.I. aeratormixer, the value of α varies from 0,70 to 0,95. Value of α for O.K.I. aerator ishigher compared to diffusers due to higher turbulence and different shape ofthe bubbles.

The high return sludge ratio and the high internal circulation in thedenitrification and phosphorous removal processes dilute the influent andthus increase the α value. In the following tables there are guidelines for α inthe most common processes.

Table 5: α values for diffusers in complete mix aeration processes

Conventionalnon nitrifying

Nitrifying Extendedaeration

SBRdenitrification

High rate 0,50 - - -Medium rate 0,60 0,75 - 0,75Low rate 0,75 0,80 0,85 0,80

Table 6: α values for diffusers in plug flow aeration reactors

Section Step feed Conventional Extended D/NNitrifying Aeration processes

1 0,50 0,40 0,60 0,652 0,60 0,50 0,65 0,703 0,70 0,60 0,70 0,754 0,75 0,70 0,75 0,805 0.80 0,80 0,80 0,856 0,90 0,90 0,85 0,90




3.2.4 The ββ Coefficient

The β coefficient is defined as the ratio of the saturation concentrations ofoxygen measured in sewage and in clean water:

Equation 11

( )( )

β = ∞

∞

C sewage

C cleanwater

*

*

The value of β depends on the amounts of suspended matter, soluble organiccompounds and dissolved mineral salts. A value of 0,98 is normally used formunicipal waste water. However, the influent waste water may in many casescontain large quantities of dissolved matter and this considerably decreasesthe saturation concentration. For industrial waste waters β may be smallerthan 0,95.

The β coefficient can be determined by measurements of oxygen saturationconcentration or by evaluation of the amount of the total dissolved solids(TDS) in the sewage. In the latter case the β can be calculated as a ratio:(dissolved oxygen in salty water)/(dissolved oxygen in clean water). Valuesfor dissolved oxygen in clean water and in sewage at various TDSconcentrations are given in the following table.

Since a low β value generates a higher oxygen requirement, a correctestimate of the β value is important.




Table 7: Dissolved oxygen concentration in water as a function oftemperature and salinity, barometric pressure 101,3 kPa

Dissolved oxygen concentration, mg/lTemp Salinity, parts per thousand°C 0 10 20 30 401 14,20 13,27 12,40 11,58 10,832 13,81 12,91 12,07 11,29 10,553 13,45 12,58 11,76 11,00 10,294 13,09 12,25 11,47 10,73 10,045 12,76 11,94 11,18 10,47 9,806 12,44 11,65 10,91 10,22 9,577 12,13 11,37 10,65 9,98 9,358 11,83 11,09 10,40 9,75 9,149 11,55 10,83 10,16 9,53 8,9410 11,28 10,58 9,93 9,32 8,7511 11,02 10,34 9,71 9,12 8,5612 10,77 10,11 9,50 8,92 8,3813 10,53 9,89 9,30 8,74 8,2114 10,29 9,68 9,10 8,55 8,0415 10,07 9,47 8,91 8,38 7,8816 9,86 9,28 8,73 8,21 7,7317 9,65 9,09 8,55 8,05 7,5818 9,45 8,90 8,39 7,90 7,4419 9,26 8,73 8,22 7,75 7,3020 9,08 8,56 8,07 7,60 7,1721 8,90 8,39 7,91 7,46 7,0422 8,73 8,23 7,77 7,33 6,9123 8,56 8,08 7,63 7,20 6,7924 8,40 7,93 7,49 7,07 6,6825 8,24 7,79 7,36 6,95 6,5626 8,09 7,65 7,23 6,83 6,4627 7,95 7,51 7,10 6,72 6,3528 7,81 7,38 6,98 6,61 6,2529 7,65 7,26 6,87 6,50 6,1530 7,54 7,14 6,75 6,39 6,0531 7,41 7,02 6,65 6,29 5,9632 7,29 6,90 6,54 6,19 5,8733 7,17 6,79 6,44 6,10 5,7834 7,05 6,68 6,33 6,01 5,6935 6,93 6,58 6,24 5,92 5,6136 6,82 6,47 6,14 5,83 5,5337 6,72 6,37 6,05 5,74 5,4538 6,61 6,28 5,95 5,66 5,3739 6,51 6,18 5,87 5,58 5,3040 6,41 6,09 5,79 5,50 5,22




3.3 Aeration System

Aeration equipment are divided into three classes according to the size of theair bubbles:

• fine bubble aeration equipment (bubbles Ø 1 - 3 mm)• medium bubble aeration equipment (bubbles Ø 3 - 10 mm)• coarse bubble aeration equipment (bubbles Ø > 10 mm)

The coefficient of total oxygen transfer KLa increases as the size of thebubbles decreases. Scientific research shows that this coefficient attains itsmaximum value when the diameter of the bubbles is between 1,0 and 2,5 mm.The increase in the value of KLa is due to the increased total mass transfersurface obtained through the smaller bubble diameter. If the air feed is kept ata constant level, the decrease of the bubble size from 5 mm to 2 mm willresult in an approximately 6 times larger air-to-water transfer surface area.

Aeration efficiency does not increase in direct proportion to bubble size. As ageneral guidance, efficiencies are in relation to each other as follows:

fine bubble 1medium bubble 0,7coarse bubble 0,4 - 0,5

The bubble size increases in proportion to an increase in the air flow throughone diffuser.

The decrease in bubble size is caused to a considerable degree by thesurface-active substances contained in the sewage. The increase of the masstransfer area caused by the decrease in bubble size partially counteracts thedecreased rate of oxygen transfer due to the surface-active substances.

The spherical area of the bubble, which in theory should increase by 5 % foreach metre the bubble ascends, decreases by about 10 % for each metre ofupward movement. The natural reason for this phenomenon is to be found inthe decrease of the partial pressure of oxygen in the bubble owing to gastransfer from air to water.

The ascent velocity of the bubbles produced by fine bubble diffusers is 25 to30 cm/s.




3.4 Operation Parameters

3.4.1 Dissolved Oxygen Level

The dissolved oxygen level in the aeration basin should normally be between1 - 2 mg/l. If the concentration falls below 1 mg/l, it may limit the growth ofaerobic micro-organisms. Oxygen concentrations exceeding 2 mg/l meanunnecessary expenditure of energy, as the cellular synthesis of micro-organisms does not require higher concentrations.

For nitrifying process, the oxygen concentration is usually kept around 2 mg/l.Concentrations under 2 mg/l limit the growth of nitrifying bacteria. Increasingof the oxygen concentration from the level necessary for the bacteria is wasteof energy. Aeration efficiency decreases linearly when the oxygenconcentration increases.

The higher the maintained oxygen concentration the smaller the efficiency ofthe aeration system. At an oxygen concentration of 1 mg/l the aerationprocess operates at 90 % of its maximum performance. At an oxygenconcentration of 2 mg/l the efficiency reaches only 78 % of the maximumpossible (see Figure 3).

The distribution of dissolved oxygen along the length of the basin can bedepicted graphically by an oxygen concentration profile.

Figure 3: Relationship between oxygen concentration and efficiency of theaeration system

The oxygen profile of the aeration basin should typically be near to the formof curve B (Figure 4) to achieve considerable savings in aeration energy.




Figure 4: Two oxygen profiles, A and B, in the aeration basin

The oxygen profile can be adjusted by the proper distribution of aerationequipment along the length of the basin. The following distribution values forSOTR (or for number of diffusers) can be used as a guideline. Values foreach zone are percentage (%) of total SOTR (No. of diffusers).

steppedsewage 34 % 26 % 22 % 18 %feed

0.25 0.5 0.75 1Basin length

plugflow 14 % 28.5% 27 % 17.5% 13 %

0.07 0.25 0.5 0.75 1basin length

3.4.2 Sludge Concentration

The amount of oxygen required for endogenous respiration depends on theactivated sludge concentration (kg MLSS/m3). The higher the concentration,the higher the oxygen demand due to higher amount of breathing micro-organisms. Normally the values range from 2 to 6 kg MLSS/m3, but in someapplications values up to 10 kg/m3 are used.




More precisely, the oxygen demand depends on the organic matter present inthe activated sludge (kg MLVSS/m3). In a biological treatment plant thisamounts to 60 - 80 % of the MLSS, and it decreases if primary sedimentationis not used or phosphorous is reduced chemically. In the simultaneousprecipitation process the proportion is 50 - 60 %.

The concentration of sludge in the aeration basin affects also the oxygentransfer rate. The higher the sludge concentration, the lower the resultingoxygen transfer coefficient (KLa). An increase in sludge concentration resultsin increased viscosity and a reduced oxygen transfer rate from the gaseous tothe fluid state (dissolution of oxygen into water). Since the area of the masstransfer rate is reduced by the increase in viscosity, the total oxygen transferrate is decreased.

3.5 Plant Location

3.5.1 Atmospheric Pressure

The elevation of the treatment plant with respect to sea level is an importantaspect of the design of the aeration system. This is due to the fact that athigher elevations the ambient atmospheric pressure and the oxygen contentof the air are lower.

The oxygen content of the air can be calculated from the equation:

Equation 12

XP

TO280 0= •,

whereXO2 oxygen content of air kg O2/m

3

p atmospheric pressure barT air temperature K

The effect of plant elevation on atmospheric pressure is shown in Table 8.

A low atmospheric pressure must be taken into account in the dimensioningof both the blower units and the oxygenation capacity of the diffuser system.




Table 8: Atmospheric pressure at different elevations

Elevation from sea level Pressurem bar0 1,013

100 1,001200 0,989400 0,966600 0,943800 0,921

1000 0,899

3.6 Summary of Dimensioning Factors

All the factors mentioned in this chapter affecting dimensioning of theaeration process are summarised in Table 9.

Table 9: Summary of dimensioning factors for aeration process

Factor Normal valueQdim -qdim -So 200 - 300 mg BOD/l

70 - 90 g BOD/d•P.E.S 10 - 20 mg BOD/lNo 30 - 50 mg N/l

12 - 15 g N/d•P.E.BOD load efficiency of pre-treatment must be taken account, kg BOD/dN load efficiency of pre-treatment must be taken account, kg N/dTemperature Tmax in aeration basinθ 1,024α varies 0,4 - 0,9β 0,98C*∞ depends on Tmax and submersion depthMLSS 2 - 5 kg MLSS/m3

e2 depends on the type of diffuser chosen


4 AOR and SOTR Page: 1.1 (12)


4 AOR and SOTR4.1 Introduction.........................................................................................4.14.2 Actual Oxygen Requirement AOR......................................................4.2

4.2.1 Eckenfelder O'Connor .................................................................4.34.2.2 Stall & Sherrad ............................................................................4.44.2.3 "Abwassertechnik".......................................................................4.54.2.4 Eckenfelder - Boon......................................................................4.6

4.3 Standard Oxygen Transfer Rate SOTR..............................................4.74.4 Clean Water Tests .............................................................................4.8

4.4.1 General .......................................................................................4.84.4.2 Summary of Method ....................................................................4.84.4.3 Definitions and Nomenclature .....................................................4.94.4.4 Apparatus and Methods ..............................................................4.94.4.5 Chemicals..................................................................................4.104.4.6 Samples ....................................................................................4.104.4.7 Air Flow Measurement...............................................................4.104.4.8 Timing Criteria...........................................................................4.114.4.9 Calculations...............................................................................4.11

4.5 Selection of Aeration Equipment ......................................................4.11

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4 AOR and SOTR




4 AOR AND SOTR

4.1 Introduction

In Chapter 2 the principles of estimating the process parameters and inChapter 3 the dimensioning values were introduced. In this chapter thecalculations will be continued from AOR to SOTR. The air flow needed,number of aerators / diffusers and the layout depend on the aerationequipment selected. There are separate chapters for designing an aerationsystem using either NOPOL® DDS diffusers or NOPOL® O.K.I. aerator mixers.A summary of the set of calculations is given in Figure 1.

LAYOUT PLANNING

NUMBER OFDIFFUSERS OR AERATORS

CALCULATIONOF AIR FLOW

SOTRCALCULATION

AORCALCULATION

PROCESSPARAMETERS

DESIGNFACTORS

SEE CHAPTER 3

SEE CHAPTER 2NO

YES

NO

YES

Figure 1: Design of an aeration system




4.2 Actual Oxygen Requirement AOR

Several methods are available for evaluating the Actual Oxygen Requirement(AOR). For rough estimates, the oxygen demand of organic matter (BOD) canbe considered to be 0.7 x BOD in high-load plants, 1.0 x BOD in normallyloaded plants, and 1.5 x BOD in low-load plants. However such estimates arebecoming more and more obsolete as increasing demands are placed on thetreatment process.

The actual oxygen requirement can be calculated from the following equation:

Equation 1

AOR COR EOR NOR DOR= + + −

where

AOR actual oxygen requirement kg O2/dCOR oxygen requirement of organic compounds kg O2/dEOR oxygen required for endogenous respiration

of activated sludgekg O2/d

NOR oxygen required for nitrification kg O2/dDOR oxygen released in denitrification kg O2/d

Actual oxygen demand can be calculated also based on COD balance of thebiological part:

AOR = CODin - CODeff - CODs

The surplus sludge production is depending on type of the waste water andthe operating parameters of the process. The surplus sludge production isusually between 0,3 - 0,6 kg CODS / CODin.

The methods used vary from one country to another. In the United States,Germany and the United Kingdom there are specific equations fordetermining oxygen demand in aeration.




4.2.1 Eckenfelder O'Connor

In the United States, the Eckenfelder O'Connor empirical equation has beenused:

Equation 2

( ) ( ) ( )( )

AOR a S S Q b X V k NH NH Q NH N

AOR a S Q b X V k NH Q NH N

o O O D

O D

= • − • + • • + • − • − • −

= • • + • • + • • − −

*, ,

*,

' ,

' ,

4 4 4

4 4

2 8

2 8∆ ∆

where

a* substrate respiration rate; 0,5 (0,4 - 0,63)SO BOD5 of influent (before aeration) kg/m3

S BOD5 of effluent (after aeration) kg/m3

∆S substrate reduction kg BOD5/m3

b endogenous respiration rate0,1 (0,10 - 0,15) biological treatment0,066 biological - chemical treatment

ND total nitrogen in effluent kg/m3

X sludge concentration kg MLVSS/m3

Q influent flow m3/dV aeration basin volume m3

k' ammonium oxygenation coefficient; 4,6NH4,O ammonia nitrogen (NH4-N) concentration of influent kg/m3

NH4 ammonia nitrogen (NH4-N) concentration of effluent kg/m3

∆ NH4 (NH4-N) reduction kg/m3

Varying values have been measured for the coefficients a and b. Coefficient ais usually 0,5 and b 0,1. However, a value of 0.066 is also used for b inSweden.




4.2.2 Stall & Sherrad

Modern dimensioning practices in particular in the United States are basedon the following equation:

Equation 3

( ) ( )AORQ S S

fP NH NH Q N

AORQ S

fP NH Q N

O eX O R

X R

=• −

− • + • − • − •

=•

− • + • • − •

1 42 4 57 1 7

1 42 4 57 1 7

4 4

4

, , ,

, , ,

,

∆

whereQ water flow m3/dSO BOD5 of influent kg/m3

Se BOD5 of effluent kg/m3

NH4,O ammonia nitrogen concentration in influent kg/m3

NH4 ammonia nitrogen concentration in effluent kg/m3

∆ NH4 ammonia nitrogen reduction kg/df coefficient of the conversion BOD5 (0,86)PX net production of biomass kg MLSS/dNR total nitrogen reduction kg N/dS BOD5 reduction kg/m3




4.2.3 "Abwassertechnik"

In Germany, current dimensioning practice is generally based on the methodpresented in the Lehr - und Handbuch der Abwassertechnik manual:

Equation 4

( )[ ]OV a B b x TS f q N NO NR R R r R A D= • • + • • • + • • + •* , ,η 4 6 1 73

where

OVR actual oxygen requirementAOR • V 1/24 kg O2/m

3 • ha* substrate respiration rate (0,4 - 0,65)BR volumetric load kg BOD / m3 • dη purification efficiency (0,7 - 1,0)b endogenous respiration rate, 0,24 kg O2/kg TSR • dx proportion of active biomass (organic

matter)TSR concentration of suspended solids in

the aeration basinkg/m3

fr temperature correction coefficient,1,072T-15

qR hydraulic load m3 of sewage /m3 of basin • dN(NO3)A nitrate concentration of effluent kg/m3

ND total nitrogen concentration of effluent kg/m3

Equation 5

N N N N N ND ges Z NH A org A us NO A= − − − −( ) ( ) ( ) ( )4 3

where

N(ges)Z total nitrogen of influent (0,040) kg/m3

N(NH4)A ammonium nitrogen of effluent (0,028 - 0,0) kg/m3

N(org)A organic nitrogen of effluent (0,002) kg/m3

Nüs nitrogen bound to excess sludge (0,010) kg/m3

N(NO3)A nitrate concentration of effluent (0 - 0,027 - 0,017) kg/m3




4.2.4 Eckenfelder - Boon

In the United Kingdom the following equation is generally used:

Equation 6

R a B N N b X VH T= • + • − • + • •* , ,4 34 2 85

where

R actual oxygen requirement kg O2/da* substrate respiration (0,75 - 1,0)B amount of BOD removed kg/dNH amount of ammonium nitrogen removed kg N/dNT total amount of nitrogen removed kg N/db endogenous respiration (0,048) kg O2/kg MLSS • dq temperature coefficient (1,024)




4.3 Standard Oxygen Transfer Rate SOTR

The next step is to convert the actual oxygen requirement (AOR, kg O2/d) tothe Standard oxygen transfer rate (SOTR, kg O2/h) which is the clean waterrequirement in specified standard conditions. The following equation isnormally used:

Equation 7

SOTRC

C CAOR k

L

T= •• −

• • • •∞

∞

−1 1

2420 20

1α βθ,

*

*

whereAOR actual oxygen requirement kg O2 /hSOTR standard oxygen transfer rate (101.3 kPa, 20 °C) kg O2 /hα alpha coefficient, generally 0,4 - 0,9β beta coefficient, generally 0,9 - 1,0θ temperature correction coefficient, 1,024k1 flow rate correction coefficientC*∞ steady state dissolved oxygen (DO) saturation

concentration attained at infinite time at watertemperature T and field atmospheric pressure. Thevalue can be estimated as follows:C*∞ = CST • (1 + 0,035 (h - 0,25))

mg O2/l

CST table value for dissolved oxygen (DO) at temperature Tat surface level

mg O2/l

C*∞, 20 steady state dissolved oxygen (DO) saturationconcentration attained at infinite time at watertemperature 20 °C and standard atmospheric pressure(101.3 kPa). The value can be estimated as follows:C*∞, 20 = CST20 • (1 + 0,035h)

mg O2/l

CST20 table value for dissolved oxygen (DO) at temperature 20°C at surface level, 9,07

mg O2/l

CL actual oxygen concentration in aeration basin mg O2/l

The value of the flow rate correction factor k1 depends on the retention timeof waste water in aeration basin. The shorter the retention time, the bigger isthe value k1.

The retention time t is calculated as follows:




Equation 8

tV

q=

dim

where

V aeration basin volume m3

qdim dimensioning water flow m3/h

Guideline values for k1 are shown in Table 1. Note that these values are forsmaller waste water treatment plants. Larger plants have smaller variationsand normally value for k1 is known in the design stage.

Table 1: Flow rate correction factor

t 24 12 8 4 2k1 1.10 1.25 1.35 1.40 1.50

4.4 Clean Water Tests

4.4.1 General

The SOTR guarantee tests for NOPOL® aeration systems should be carriedout according to “Measurement of Oxygen Transfer in Clean Water”, ASCE(American Society of Civil Engineers, 1984). Other test methods like ÖNORMM5888 are not accepted unless all technical details are agreed before thetest.

The ASCE test method is applied in the General Guarantee Terms forNOPOL® Aeration Systems.

In the following text some of the important factors are collected from ASCE-standard. Following text do not include all the necessary information that isneeded for a correct SOTR test. Therefore ASCE standard shall be studiedcarefully before testing the aeration equipment.

4.4.2 Summary of Method

The test method is based upon removal of dissolved oxygen (DO) from thewater volume by sodium sulfite followed by reoxygenation to near saturationlevel. The DO inventory of the water volume is monitored during the




reaeration period by measuring DO concentrations at several determinationpoints selected to best represent tank contents. These DO concentrationsmay be either sensed in situ using membrane probes or measured by theWinkler or probe method applied to pumped samples.

The data obtained at each determination point are then analysed by asimplified mass transfer model to estimate the apparent volumetric masstransfer coefficient, Kla, and the equilibrium concentration, C∞*. The basicmodel is the following:

Equation 9

C = C∞* - (C∞*-C0) exp (-KLa * t)

Non-linear regression is employed to fit equation to DO-profile measured ateach determination point during reoxygenation. In this way, estimates of KLaand C∞*are obtained in each determination point. These estimates areadjusted to standard conditions and standard oxygen transfer rate is obtainedas the average of the products of the adjusted point KLa values, thecorresponding adjusted point C∞* values and the tank volume.

4.4.3 Definitions and Nomenclature

Standard Oxygen Transfer Rate (SOTR) is mass of oxygen per unit timedissolved in a volume of water by an oxygen transfer system operating undergiven standard conditions when the DO concentration is zero.

Standard Oxygen Transfer Efficiency (SOTE) is the fraction of oxygen in aninjected gas stream dissolved under given conditions of temperature,barometric pressure, gas rate and zero DO concentration.

4.4.4 Apparatus and Methods

For determination of a Standard Oxygen Transfer Rate, the water to whichoxygen is transferred should be equivalent in quality to public water supply.Repetitive testing may be conducted in the same water provided that TDSdoes not exceed 1 500 mg/l.

Membrane Electrode Measurement of DO either on pumped samples or insitu shall be in accordance with section 421F of Standard Methods (1).Minimum four determination points shall be used. One should be in shallowdepth, one should be at deep location and one should be at mid depth. The




points should be at leas 0,6 meter from walls, floor and surface and no closerto the surface than 10 percent of the minimum tank dimension.

Water temperature measurement shall be in accordance with section 212 ofStandard Methods (1).

4.4.5 Chemicals

Either reagent or technical grade sodium sulfite (Na2SO3) shall be used fordeoxygenation. Sodium sulfite shall be added in solution. This may beaccomplished by dissolving the sulfite in a separate mixing tank prior to itsaddition to the test tank.

The theoretical sodium sulfite requirement for deoxygenation is 7,88 mg/l per1 mg/l DO concentration. Sulfite additions are made in excess ofstochiometric amounts. The amount of excess varies from 20 to 250 %.Sufficient sulfite solution shall be added to depress the DO level below 0,5mg/l at all points in the test water. Dissolved sulfite shall be distributed rapidlyinto test tank. Extreme care should be exercised to assure adequatedispersion in the test tank. Final mixing prior to test can be achieve bystarting the aeration for very short period 5 – 10 seconds. Air bubbles aremixing the chemical effectively. Oxygen level in the tank should remain zerodue to excess dose of the sodiumsulfite.

The cobalt catalyst should normally be added once for each test. A solution ofcobalt salt shall be added to test to achieve a soluble cobalt concentrationbetween 0,10 mg/l and 0,5 mg/l in the test water. The solution shall be addedprior to beginning of oxygen transfer testing with the aeration systemoperating 30 minutes after addition.

4.4.6 Samples

Water samples according to ASCE standard shall be taken and analysed.Analyses include determination of Total Dissolved Solids and Soluble Cobaltin the beginning and the end of every test. Also temperature of test water isdetermined in beginning and end of every test.

4.4.7 Air Flow Measurement

Gas flow apparatus shall be capable of measuring the gas flow with anaccuracy of 5 %. Full-scale plant gas flows should be used with caution sincethe precision and accuracy of the measurement device may not be adequate




for test flow rates. It is desirable to provide a back-up or supplemental as acheck. The instataneous gas flow rate should not vary by more than 5 %during the test.

4.4.8 Timing Criteria

The purpose of the criteria is to ensure that the data points are representativeof the reaeration curve and that adequate number of points are obtained insensitive regions of the curve. The lowest DO value shall be not greater 20 %of C∞*. The highest DO value shall be not less than 98 % of C∞*.

4.4.9 Calculations

Non-linear method for calculation the result shall be used. This method isbased on non-linear regression of the model (Equation 9) through the DOversus time. The best estimates for parameters are selected as values whichdrive the model equation through the DO concernation versus time datapoints with a minimum residual sum of squares.

Best fit log deficit method is acceptable to evalute the SOTR result. The ciefadvantage of the method is that in can be applied with relatively simplecalculation procedure with normal spredsheet program. The method is basedon linear regression of the logaritmic form of the model equation usinglogaritmic function of DO data. The logaritmic equation shall be fit to the DOdata for each determination point by performing a linear regression ofln(C∞* - C) versus time.

Other calculation methods are not acceptable for evaluating result accordingASCE standard.

4.5 Selection of Aeration Equipment

In normal waste water applications both disc diffusers and O.K.I. aerators areoften alternatives. O.K.I. aerator is more adequate to heavy industrialapplications were a disc diffuser system cannot grant reliable operation.

Examples on applications especially suitable for O.K.I. aerators are thefollowing:

- mixing without air is needed (two speed motors, AM-models)




- liftable system is needed (plants having only one aeration line)- water contains compounds which can clog the disc diffusers- water contains compounds, such as oil, that can damage the EPDM

membrane- installation in waterfilled basin is required- deep basin (from 8 meters)- sludge handling solutions

Applications especially suitable for DDS disc diffuser systems:

- standard aeration solutions- very high efficiency is needed- lower investment cost is demanded- normal municipal waste water- maintenance of aeration equipment by emptying the basin is possible

Guidelines given above are not strict. For instance liftable systems can bemade mounting disc diffusers on stainless steel grids. Chemical resistance ofthe diffuser is characteristic that varies from one diffuser type to another.


5 Production of Air Page: 1.1 (30)


5 Production of Air5.1 Properties of Air..................................................................................5.15.2 Calculation of Blower Air Flow............................................................5.3

5.2.1 Cooling of Compressed Air in Pipework......................................5.35.2.1.1 Equation and Coefficients....................................................5.45.2.1.2 Temperature Loss in a Pipe Surrounded by Air...................5.75.2.1.3 Temperature Loss in a Pipe Surrounded by Earth...............5.85.2.1.4 Temperature Losses in a Pipe Surrounded by Water..........5.9

5.3 Air Intake ..........................................................................................5.155.4 Silencers...........................................................................................5.175.5 Anti-vibration Control........................................................................5.175.6 Air Filtration ......................................................................................5.185.7 Different Types of Blowers ...............................................................5.18

5.7.1 Positive Displacement Blowers .................................................5.185.7.2 Dynamic Type Blowers..............................................................5.21

5.8 Delivery Control of Blowers ..............................................................5.235.8.1 Rotary Blowers ..........................................................................5.235.8.2 Centrifugal Blowers ...................................................................5.24

5.9 Blower Selection...............................................................................5.245.9.1 Capacity Requirements .............................................................5.255.9.2 Delivery Control Requirements .................................................5.25

5.10 Blower Plants ................................................................................5.265.10.1 General Design Principles.........................................................5.265.10.2 Blower Accessories ...................................................................5.27

5.11 Air Piping ......................................................................................5.285.11.1 Selection of Pipe Materials........................................................5.285.11.2 Properties of Different Materials................................................5.285.11.3 Design Principles ......................................................................5.29

5.12 Examples of Air Supply Systems ..................................................5.305.12.1 Waste Water Treatment Plant, Population Equivalent 40,000..5.305.12.2 Waste Water Treatment Plant, Population Equivalent 200,0005.30

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5 Production of Air




5 PRODUCTION OF AIR

5.1 Properties of Air

Air is a mixture of several permanent gases, in which nitrogen, oxygen, argonand carbon dioxide predominate. Together they account for about 99,9 % byweight of the air.

The composition of the atmosphere is fairly constant throughout the world,but changes markedly with altitude. At higher altitudes, the concentration ofthe heavier gases, notably oxygen, decreases.

Table 1: Composition of air, main components

Gas Symbol Content % by weight Content % by volumeNitrogen N2 75,51 78,08Oxygen O2 23,15 20,95Argon Ar 1,28 0,95Carbon dioxide CO2 0,046 0,03

Air normally also contains varying amounts of water vapour and solids. At lowtemperatures the water vapour content of the air ranges from a minimum ofnearly 0 % to maximum of about 3 % by weight or about 4 % by volume. Inlarger cities the solids content of air may be up to 500 000 particles per m3.

Air possesses some physical constants:molecular weight 28,96 kg/moldensity (+ 20 °C, 100 kPa) 1,188 kg/m3

gas constant 287,1 J/K kg

The density of air is dependent on its temperature. The following values applyto dry air at a pressure of 101,3 kPa.

temperature, °C density, kg/m3

-50 1,534-30 1,4530 1,293

20 1,204540 1,126760 1,0595




The standard atmosphere (U.S., 1962) is based on a constant referencedistribution of atmospheric pressure in normal conditions and at differentaltitudes, as follows (dry air).

Table 2: Altitude dependence of atmospheric pressure, temperature anddensity of air

altitude, m pressure, bar temperature, °C Density, kg/m3

0 1,013 15,00 1,225100 1,001 14,95 1,213200 0,989 13,70 1,202400 0,966 12,40 1,179600 0,943 11,10 1,156800 0,921 9,80 1,1341000 0,899 8,50 1,1122000 0,795 2,00 1,0073000 0,701 -4,50 0,9094000 0,616 -11,00 0,819

Compression affects air temperature. A rapid compression of air with amechanical blower causes a significant temperature rise of the air. The rise isabout 10 °C per each 10 kPa (0.1 bar or approximately 1 m water depth)pressure increase: For instance, if the air temperature at the intake is 15 °Cand the pressure on the delivery side of the compressor is 55 kPa (quitenormal for an aeration basin of about 4 m depth) the temperature of the airwill be approximately 70 °C.

The relationship of the different units used in regard to air pressure ispresented in Table 3.

Table 3: Air pressure units

mmHg bar mbar N/m2 kp/cm2 kPa760 1,0132 1013,2 101325 1,033 101,325750,1 1 1000 100000 1,020 100,000735,6 0,9807 980,7 98070 1 98,070




5.2 Calculation of Blower Air Flow

The actual air flow required from the blower is calculated as follows:

Equation 1

q qp

p

T

Ta as

i

i

s

, = • •

whereq´a Blower air flow in real conditions at maximum

summertime temperature at the altitude of thetreatment plant

m3/h

qa Air flow calculated under standard conditions m3/hpi Atmospheric pressure at the plant altitude kPaps Standard atmospheric pressure, 1.013 kPaTi Maximum intake air temperature during summer,

expressed as absolute temperatureK (= ti °C + 273)

Ts Air temperature under standard conditions K (20 °C = 293 K)

5.2.1 Cooling of Compressed Air in Pipework

When compressing air adiabatically in the blowers, temperature increases byabout 10 °C per each 10 kPa ( equivalent to 1 m of water column ). E.g. in a10 m deep basin, when ambient air is 30 °C, the temperature of the airleaving the blower is about 130 °C.

High temperature may be harmful for the materials of the systems havingpolymer components.

Air is cooled down in a different way for each stage of the pipework: blowerroom, main header and distribution header in open air (in sunshine and inrain) and especially in the dropleg pipes. Sometimes the pipes are buried inthe ground, sometimes isolated in order to reduce noise. Heat transfer differsconsiderably according to circumstances and it has to be calculatedseparately for each section of pipe, when accuracy is required.

In the following pages there are the basic equations for calculating cooling invarious conditions. Values for the essential parameters are given andexamples of calculations in selected common applications are calculated.




5.2.1.1 Equation and Coefficients

5.2.1.1.1 Main Equation

T0 = Temperature of surrounding medium ( Co ) (air, earth, water)T1 = Temperature of air after compressor ( Co )T2 = Lower air temperature ( Co )L = length of pipe (m)m = Air flow, mass (kg / h)c = Specific heat of air = 0.25 kcal / kg Co

k = Heat flow rate through a pipe wall per one meter of pipelength

(kcal / m . h . Co )

di = Inside diameter of pipe (m)do = Outside diameter of pipe (m)ë = Heat transfer coefficient between air and inside surface of

pipe (kcal / m2. h . Co )

áá = Heat transfer coefficient between outside surface of

pipe and surrounding medium (kcal / m2. h . Co )ë = Thermal conductivity of pipe material (kcal / m . h . Co )

Carbon steel = 45 kcal / m . h . Co

Stainless steel = 22 kcal / m . h . Co

PVC = 0.14 kcal / m . h . Co

Air flow, mass

V

A•W=m

w = Velocity of air (m/s)A = Area of pipe cross section (m2)V = Specific volume of air (m3/kg)

c•mk•L

-

01

02 e=T-TT-T

i

0

s0ai dd

ln•21

+•d•

1+

•d•1

1=k

λπαπαπ




Temperature + 80 Co V = 0.760 m3 / kgTemperature + 130 Co V = 0.580 m3 / kg

5.2.1.1.2 Equation of Heat Transfer Coefficients

5.2.1.1.2.1 Between Air and Inside Surface of Pipe

25.0

75.0

8.3id

w=α

(kcal / m2 . h . Co )

w = Velocity of air in normal conditions (m/s)dI = Inside diameter of pipe (m)

5.2.1.1.2.2 Between Outside Surface of Pipe and Air (Horizontal Pipe, ImmobileAir)

1sá = ∆ t 0.233 / do0.3 (kcal / m2 . h . Co )

∆ t = Tempreture difference T1 – T0

d0 = Outside diameter of pipe (m)

5.2.1.1.2.3 Between Outside Surface of Pipe and Water

αs1 = 2900 . w1 0.85 ( 1+ 0.14 tm ) (kcal / m2 . h . Co )

w1 = Velocity of water ( m / s ) = 0.3 m/s

tm = Average temperature of water ( Co ) = 20 Co

αs2 = 2900 . 0.3 0.85 ( 1+ 0.14 . 20 ) = 1336 kcal / m2 . h. Co

to be chosen 1100 kcal / m2 . h . Co (mixture of air and water)




5.2.1.1.3 Cooling Off of Air, Pipe Surrounded by Earth

Earth calculated as insulation :λe = 2 kcal / m . h . Co

b = 2 m

Heat loss :

00

0

01

1ln2

1)(

Dd

D

TTQ

e αλ

π

+⋅

−=

T1 = Temperature of air ( Co )T0 = Temperature of earth ( Co )d0 = Outside diameter of pipe (m)Do = Outside diameter of insulation = d0 + 4 (m)λe = Thermal conductivity of earth = 2 (kcal / m . h. Co )α = 1.6 . ( T1 – T0 )

0.24 ( kcal / m . h . Co )

Temperature loss

cm

LQt

⋅⋅

=∆

∆ t = T1 – T2

Q = Heat loss (kcal / m . h)L = Length of pipe (m)




m = Air flow, mass (kg/h)c = Specific heat of air = 0.25 (kcal/kg Co )

5.2.1.2 Temperature Loss in a Pipe Surrounded by Air

Example

Pipe d0 = 406.4 mm, di = 400.4 mmPipe material : stainless steel, λ= 22 (kcal / m . h . Co )

T0 = + 30 Co

T1 = + 80 Co

L = 100 mm = 9920 kg/hw = 15 m/swn = 16.6 m/s (in normal conditions)c = 0.25 kcal / kg Co

35400.0

158.3

25.0

75.0

==aα kcal / m2 . h . Co

3.3400.0

)3080(3.0

233.0

1 =−

=sα kcal / m2 . h . Co (immobile air)

8.3ln

1

400.0406.0

2221

3.34.01

354.01

=++

=⋅⋅⋅⋅ πππ

k

mCeT

100/1.73080

3025.099208.3100

2 o==−−

⋅⋅−

Temperature loss = 0.07 Co / m of pipe lengthNormally temperature losses are 0.15 – 0.05 Co /m.




5.2.1.3 Temperature Loss in a Pipe Surrounded by Earth

Example

Pipe d0 = 406.4 mm, di = 400.4 mmPipe material : stainless steel, λ= 22 ( kcal / m . h . Co )

T0 = + 5 Co

T1 = + 80 Co

L = 100 mm = 9920 kg/hc = 0.25 kcal / kg Co

D0 = 4.4 m

α =1.6 ( T1 – T0 )0.24 = 1.6 ( 80 – 5 )0.24 = 4.5

mCt

hmkcalQ

100/6.1425.09920

100363

/363ln

)580(

4.45.41

4.04.4

221

o=⋅⋅

=∆

⋅=+−

=⋅⋅

π

Temperature loss = 0.15 Co / m of pipe length.




5.2.1.4 Temperature Losses in a Pipe Surrounded by Water

5.2.1.4.1 Pipe Material : Carbon Steel

Length L = 5 m T1 = 30 + 50 = + 80 Co (air)pressure = 1.5 bar abs. T0 = + 20 Co (water)

T2 = lower air temperature Co

Pipe diameters ( mm )

42,4 60,3 88,9 114,3

38,4 55,7 83,1 107,9

A 0,116 0,244 0,542 0,914

5 m / s m kg / h 28 58 128 216

10 55 116 256 433

15 83 174 384 650

20 110 232 512 866

25 138 290 640 1082

5,54 m / s 31 28 26 24

11,08 52 47 43 40

16,62 71 64 58 55

22,16 88 80 72 68

27,7 104 94 86 80free air

5 m / s k 3.64 4,78 6,53 7,96

10 6,00 7,97 10,80 13,08

15 8,02 10,60 14,38 17,75

20 9,81 13,06 17,64 21,70

25 11,51 15,17 20,82 25,27

5 m / s 25 32 42 49

10 27 35 46 53

15 29 38 48 55

20 30 40 50 56

25 31 41 51 58

0d

id

2dm

aα

2T

2/ mkcal

2/ mkcal

co




5.2.1.4.2 Pipe Material: Carbon Steel

Length L = 10 m T1 = 30 + 100 = + 130 Co ( air )pressure = 2.0 bar abs. T0 = + 20 Co ( water )



42,4 60,3 88,9 114,3

38,4 55,7 83,1 107,9

A 0,116 0,244 0,542 0,914

5 m / s m kg / h 36 75 168 284

10 72 150 336 568

15 108 225 504 852

20 144 300 672 1134

25 180 375 840 1420

7,25 m / s 38 35 31 29

14,5 64 58 52 49

21,75 86 79 71 67

29,00 107 98 88 83

36,25 126 115 104 126free air

5 m / s k 4,44 5,94 7,87 9,58

10 7,31 9,64 12,96 15,90

15 9,65 12,81 17,41 21,40

20 11,81 15,76 21,27 26,15

25 13,69 18,24 24,80 30,49

5 m / s 21 25 37 49

10 22 28 44 56

15 23 31 48 60

20 24 33 51 6625 25 36 54 67

0d

id

2dm

aα

2T

2/ mkcal

2/ mkcal

oC




5.2.1.4.3 Pipe Material: Stainless Steel

Length L = 5 m T1 = 30 + 50 = + 80 Co (air)pressure = 1,5 bar abs. T0 = + 20 Co (water)



42,4 88,9 114,3 168,3 219,1 273

38,4 84,9 110,3 164,3 215,1 268

A 0,116 0,566 0,955 2,12 3,63 5,64

5 m / s m kg / h 28 134 226 502 860 1336

10 55 402 452 1004 1720 2672

15 83 536 678 1506 2580 4008

20 110 670 904 2008 3440 5344

25 138 375 1130 2510 4300 6680

5,54 m / s 31 26 24 22 20 19

11,08 52 44 40 36 34 32

16,62 71 59 55 49 46 43

22,16 88 73 68 61 57 54

27,7 104 87 80 72 67 64free air

5 m / s k 3,64 6,76 8,16 11,10 13,25 15,66

10 5,99 11,25 13,32 17,92 22,21 26,10

15 8,00 14,89 18,07 24,10 29,70 34,63

20 9,80 18,19 22,08 29,67 36,41 43,04

25 11,47 21,41 25,69 34,50 42,39 50,52

5 m / s 25 42 49,53 59 64 67

10 27 46 55 62 66 69

15 29 49 57 64 68 70

20 30 50 58 65 69 7125 31 52 54 66 69 72

aα

2T

Cmkcal o2/

oC

Cmkcal o2/

do

di

dm2




5.2.1.4.4 Pipe Material: Stainless Steel




42,4 88,9 114,3 168,3 219,1 273

38,4 84,9 110,3 164,3 215,1 268

A 0,116 0,566 0,955 2,12 3,63 5,64

5 m / s m kg / h 36 176 296 658 1127 1750

10 72 351 592 1316 2253 3500

15 108 527 888 1974 3380 52500

20 144 702 1184 2632 4506 7000

25 180 878 1480 3290 5632 8750

7,25 m / s 38 31 29 26 25 23

14,5 64 52 49 44 41 39

21,75 86 71 67 60 56 53

29,00 107 88 83 75 70 66

36,25 126 104 98 88 82 78free air

5 m / s k 4,43 8,03 9,76 13,06 16,47 18,89

10 7,30 13,21 16,19 21,74 26,60 31,53

15 9,63 17,78 21,77 29,21 35,81 42,28

20 11,79 21,64 26,58 36,01 44,16 52,00

25 13,65 25,22 30,97 41,76 51,15 60,77

5 m / s 21 37 94 70 81 92

10 22 44 57 77 89 97

15 23 49 61 81 92 100

20 24 52 65 84 94 10225 25 55 68 86 97 103

aα

2T

Cmkcal o2/

oC

Cmkcal o2/

do

di

dm2




5.2.1.4.5 Pipe Material : PVC PN 10




63 90 110 140 160 225

57 81,4 99,6 126,6 144,6 203,4

A 0,255 0,52 0,779 1,26 1,64 3,25

5 m / s m kg / h 60 123 185 299 3,89 770

10 120 246 369 597 777 1540

15 180 369 554 896 1166 2310

20 240 492 738 1194 1554 3080

25 300 615 923 1493 1943 3850

5,54 m / s 28 26 24 23 22 20

11,08 48 43 41 39 37 35

16,62 64 58 56 52 50 47

22,16 80 72 69 65 63 58

27,7 94 86 82 77 74 68free air

5 m / s k 3,15 3,73 4,02 4,43 4,61 5,15

10 4,16 4,80 5,17 5,53 5,67 6,23

15 4,86 5,41 5,79 6,07 6,22 6,70

20 5,31 5,82 6,18 6,45 6,59 7,00

25 5,63 6,14 6,47 6,71 6,82 7,20

5 m / s 41 53 59 65 67 73

10 47 61 65 70 72 75

15 55 65 69 72 74 77

20 59 67 71 74 75 7725 61 69 72 75 76 78

aα

2T

Cmkcal o2/

oC

Cmkcal o2/

do

di

dm2




5.2.1.4.6 Pipe Material : PVC PN 10




63 90 110 140 160 225

57 81,4 99,6 126,6 144,6 203,4

A 0,255 0,52 0,779 1,26 1,64 3,25

5 m / s m kg / h 79 161 242 391 509 1009

10 158 322 484 782 1018 2018

15 237 483 726 1173 1527 3027

20 316 644 968 1564 2036 4036

25 395 805 1210 1955 2545 5045

7,25 m / s 35 31 30 28 27 25

14,5 58 52 50 47 46 42

21,75 79 71 68 64 62 57

29,00 98 88 85 80 77 71

36,25 115 104 100 94 91 83free air

5 m / s k 3,60 4,11 4,50 4,85 5,04 5,61

10 4,66 5,19 5,57 5,89 5,07 6,53

15 5,29 5,80 6,15 6,42 6,68 6,98

20 5,70 6,18 6,53 6,76 6,88 7,25

25 5,99 6,45 6,78 6,98 7,09 7,42

5 m / s 38 60 72 87 94 108

10 54 78 90 102 107 117

15 65 88 98 108 112 120

20 73 95 104 113 116 12225 80 100 108 115 118 124

aα

2T

Cmkcal o2/

oC

Cmkcal o2/

do

di

dm2




5.3 Air Intake

The following recommendations are given for air intake location:

• air intake distance from the ground should be at least 2.5 m• intake air flow velocity in icy conditions must not exceed 2 m/s• intake should be located leeward of the prevailing wind direction• intake should be oriented in the direction where air impurities, humidity and

also direct solar radiation are the smallest.

The following factors should be taken into account in the design of the airintake:

• the air intake should be located so that air enters it at right angles andwithout turbulence

• the air intake should be equipped with a screen against leaves and similarmatter

• a properly located shield helps to avoid the effects of snow and rain (seeFigure 1)

• in summertime, intake air may be conducted through cool structures orspaces in order to lessen the risk of overheating the blowers

• in wintertime, intake air may be conducted through heated premises• in case where a sudden clogging by ice (or other agent) is possible, the air

intake should be equipped with a bypass gate which opens automatically ifthe pressure drops too low

• steel and lightweight alloys are the most suitable materials for the airintake




Figure 1: Air intake, schematic representation

Figure 2: Air intake opening can be widened without increasing the duct size

Typical location errors for the air intake include

• air intake located too near the ground• air intake located in a gravel- or dirt-covered area with traffic dust (causes

wear to pipework)• air intake exposed to corrosive chemical vapours• air flow velocity too high• large water areas located near the intake (humidity)• intake located near a wall exposed to sunshine• intake exposed to flue gases• intake exposed to airborne pollen and seeds causing obstruction of filters




Figure 3: Two examples of air intake locations

5.4 Silencers

The inlet and discharge noise of the blowers has to be silenced forenvironment and work safety reasons. The inlet and discharge silencers haveto be selected according to the silencing needs and requirements and blowertype. Usually the package type blowers are equipped with silencers as astandard.

Rotary piston blowers are recommended to be equipped with pulsationdampers to reduce the noise of the downstream piping. Centrifugal and turboblowers do not need pulsation dampening.

Many blower manufacturers are providing the blowers with acoustic hoods toreduce the noise radiation from the machine casing. Noise radiation can bereduced also by noise insulation of the blower room.

5.5 Anti-vibration Control

The blowers are normally installed on flexible machine mountings. This isdone in order to reduce the solid borne noise and vibrations. The dampeningof the solid borne noise and vibration is especially important when usingrotary piston blowers.




If the blower is installed on flexible machine mountings the connection to theplant piping has to be flexible as well. The connection can be a rubber sleevecoupling in normal air temperatures. In higher temperatures a stainless steelbellow type joint is used.

5.6 Air Filtration

The purpose of air filtration is to avoid internal clogging of aeration equipmentand excessive wear to both blowers and piping.

A waste water treatment plant must be operated continuously 24 hours a day.Hence filtration should preferably consist of several units so that any one ofthem may be shut off for maintenance while the others are working. Anotherimportant point is for the unit under maintenance to be isolated from theothers so that there is no risk of short circuit air flows.

Large plants are normally equipped with a combined system, where the airintake and filters are connected to blowers by piping. In this kind of a systemthere is enough spacing for maintenance work. The pressure differencebetween different rooms must be taken into account when designing theirdoors and sealing. All surfaces must be made of dust free materials.

All air filters must be equipped with control accessories to avoid overloadingof the filters and possible damage to the installation. In large plants, thepressure difference of the various filters and their alarms are usuallyindicated in the control system of the plant.

Detailed information on air filtration requirements of NOPOL® DDS ANDNOPOL® O.K.I. aeration equipment are given in chapters 7 and 8.

5.7 Different Types of Blowers

The following two types of blowers are the most widely used in waste waterapplications:• positive displacement blowers• dynamic type blowers

5.7.1 Positive Displacement Blowers




In positive displacement blowers suction air enters a compression chamberwhere its volume is reduced until the pressure has reached a certain pre-setvalue. Air is conducted by forced flow.

There are different types of positive displacement blowers:• piston blowers• lamellar blowers• screw blowers• rotary blowers

Rotary blowers have so far been the most commonly used positivedisplacement blowers for waste water aeration purposes. However, there isno technical impediment to the use of screw blowers. The working principle ofthese two blower types is shown in Figure 4 and Figure 5.

Figure 4: Working principle of a rotary blower




Figure 5: Working principle of a screw blower

When using rotary blowers the air flow is continuous but pulses ratherstrongly, a fact that must be taken into account, for instance, as a possiblecause of vibrations in pipework.

Typical with rotary blowers is that the air flow is virtually independent of themagnitude of counterpressure at low pressure ratios. When counterpressureincreases, the back flow of air through clearances into the suction side alsoincreases. The power requirement grows linearly with the increasingcounterpressure (see Figure 6), but it does not grow with decreasing suctionair temperature. The maximum delivery pressure reached by this type ofblower is 60 - 100 kPa (0,6 - 1,0 bar), depending on the size and constructionof the machine. The capacities of up to 85.000 m3 can be attained. Thedelivery air of rotary blowers is oil free because the compression chamberhas no lubrication.




Figure 6: Specific curves for rotary blowers

Silencers are usually needed on both sides of the blower. In some cases aspecial silencing device must be installed in the delivery side. The blowersmust always be located such that no water can enter them.

5.7.2 Dynamic Type Blowers

Dynamic type blowers are usually classified into two main groups:• radial (centrifugal) flow blowers, which may have one or several impellers

and where air flows radially (Figure 7)• axial flow blowers having a multistage series of impellers including an axial

air flow (Figure 8)




Figure 7: A single-impeller centrifugal blower

Figure 8: Axial flow blower

The performance curves of centrifugal blowers are often labile at lowerstages, e.g. there are two different delivery values for a given value ofpressure. Operation of the blower must be designed so that it will not berunning within this labile region. Except for momentary starting runs,centrifugal blowers must not run at delivery rates smaller than the pumpinglimit as this may cause breakdown of the blower (see Figure 9). The flatnessof the characteristic curves requires the suitability of simultaneous running ofseveral blowers.




Figure 9: Typical characteristic curves of centrifugal blowers

The delivery rates of centrifugal blowers are dependent on the inlet airtemperature, owing to which it is always necessary to check that• the drive motor power is sufficient at the lowest inlet air temperature and at

the highest inlet air temperature• the delivery rate is sufficient at the highest inlet air temperature

The maximum attainable delivery pressure of dynamic type blowers is 1.000(3.000) kPa with centrifugal blowers and 600 kPa with axial blowers. Thecharacteristic performance ranges are correspondingly 2.000 - 35.000 m3/hand 35.000 - 100.000 m3/h. Rotation ranges from 5.000 to 15.000 rpm andthe circumferential velocity of the impeller ranges from 150 to 300 m/s. Thetransmission oil requires a cooling system complying with the specificationsgiven by the manufacturer of the blower.

5.8 Delivery Control of Blowers

5.8.1 Rotary Blowers

The most common delivery control methods for rotary blowers are thefollowing:• simultaneous use of various blowers• variable speed control

By the simultaneous use of various blowers step by step delivery control canbe attained. When using this method energy is lost, since there is no delivery




control between different start-ups. The best result in delivery control isobtained by the variable speed method which can be executed either byfrequency converters in the case of AC drives or by DC drives. When usingfrequency converters, up to 10 % of the energy is lost as heat and this shouldbe taken into account in economic calculations.

5.8.2 Centrifugal Blowers

The most common delivery control methods for centrifugal blowers are thefollowing:• inlet valve throttling• outlet valve throttling or variable diffusor control at delivery side

Inlet valve throttling is often used for the delivery control of centrifugalblowers. The valves used for this purpose are usually operated eithermanually or by a motorised device. Surge limits for turbo blowers can belowered by up to 45 % of the rated capacity when using a throttling valve.Where inlet vanes are employed surge limits can be lowered by up to 30 % ofthe rated capacity.

Outlet valve throttling can also be resorted to for delivery control, but thismethod uses more energy compared with inlet valve throttling. Somecentrifugal blowers are equipped with variable diffusors at the delivery sidethrough which a delivery control range of 45 - 100 % of the rated capacity canbe achieved. In such blowers the change of inlet air temperature andpressure can be eliminated by automatic inlet vane control.

5.9 Blower Selection

The blower selection must be based on the requirements set by good controlof the whole aeration process. Furthermore, the technical characteristics ofthe blowers must be sufficiently known to make proper comparisons betweendifferent alternatives.

The main technical requirements that should be taken into account in acomparison of different blowers are normally the capacity and the deliverycontrol range. The noise and maintenance aspects should be studied as well.




5.9.1 Capacity Requirements

Fluctuations in the air demand under varying load conditions must be known.The consideration of design conditions alone is generally not sufficient. Theoptimum oxygen profiles of the aeration basins under varying load conditionsshould also be known. The necessary delivery control range, as well as therequirements for the control system, are established by means of the oxygenprofile and the fluctuations in the air demand.

The air delivery rate of each blower should be given as a commensuratevalue so that the free air flow rate is expressed in terms of volume reducedaccording to both the pressure and the temperature of the intake air. Thiscomparative index value is called the actual air delivery rate. Whencomparing blowers, all the performance values which may be given ondifferent bases must always be reduced to the same intake air pressure andtemperature conditions.

As far as ambient conditions are concerned, the maximum and minimumtemperatures of the intake air are the most important factors that should beconsidered when selecting and dimensioning blowers as well as operationalequipment. When using turbo blowers it is necessary to make sure that theseare able to deliver a sufficiently high pressure even at maximum intake airtemperature.

5.9.2 Delivery Control Requirements

The delivery control range of the blowers should coincide with the respectiveaeration equipment’s value (1:5) in order to take full advantage of theproperties of aeration equipment. If the air flow rate adjustment is notstepless, the blowers should be selected which permit delivery control insteps of 20 to 25 %.

In a nitrifying plant the oxygen demand of the aeration process is at least attimes markedly smaller than the air demand necessary to ensure sufficientmixing in the aeration basin. If diffusers are used as aeration equipment, itmay be convenient to equip the basin with an additional mechanical mixingdevice to complete the mixing requirement of the activated sludge process.

In a nitrifying plant aeration is closed off periodically. In such cases it isconvenient to equip the plant with NOPOL® O.K.I. aerator mixers toguarantee sufficient mixing of the activated sludge process.




5.10 Blower Plants

5.10.1 General Design Principles

In connection with large treatment plants it is often most convenient toconstruct a separate blower station. One of the advantages of this is theisolation of noise and vibration effects from laboratory and control premises.The blower station should be located so that the distances to the aerationbasins are the shortest possible and lowest possible noise level in theimmediate environment is achieved.

At large treatment plants the blowers are usually located inside specialpremises equipped with good soundproofing. At smaller plants it is often moreconvenient to install each blower separately within a soundproofed casing.

The blower room must be properly ventilated using a sufficient amount of airin order to keep the maximum temperature rise at level of 10 - 15 °C. As abasis for the evaluation of ventilation air demand it may be assumed that withrotary blowers the quantity of heat generated is about 10 % of the energy fedto the blower drive shaft. This percentage includes both the blower and theelectric motor. In addition, all hot pipework surfaces (60 - 80 °C) with theblower station naturally must be taken into account when assessing theventilation need.

Blowers should be located above the water surface level of the aerationbasins. If this is not possible, then the pipework must be designed so that nowater can enter the blowers through the pipework.

Within capacities of up to about 10.000 m3/h, it is often appropriate to provideeach blower with a separate filter-equipped air intake directly from the outsideair. At larger plants a special air intake room should be constructed andequipped with soundproofing.

No additional air consuming equipment (for example airlift pumps for returnsludge) should be connected to the aeration system as this might causeharmful fluctuations in the air feed serving the aeration process.




5.10.2 Blower Accessories

Blowers should normally be equipped with accessories (see Figure 10) suchas:

1. air filter (blower air intake filter)2. suction noise muffler3. pressure noise muffler. The working principle of this depends essentially

on the blower system. Adsorption muffler is best suited for use with singleblowers working continuously at a constant running speed. If the systemincludes several blowers in parallel, or if the blower running speed isregulated, special mufflers, for example resonance or combined absorption/ resonance mufflers, may be necessary.

4. Flexible coupling of the pressure pipe. This is essential when the blower ismounted on a vibration-absorbing base (direct mounting on a vibrationabsorber is not suitable for blowers with controlled running speed).

5. Safety valve. Compulsory for all forced-drive blowers.6. Non-return valve. Indispensable in any system incorporating several

blowers. Its use is also advisable in a single-blower system as it preventsreverse running of the blower when stopping against a full working load.The construction of the non-return valve must be suitable for compressedair.

7. Shut-off valve. This must not have any structural parts exposed tovibrations.

Figure 10: Rotary blower equipment




There are also other accessories which can be used in conjunction withblowers, including:• a manometer, at least one on the delivery side• a thermometer for blower temperature control• an ammeter for the control of blower running conditions• a kWh meter for aeration energy measurement

5.11 Air Piping

5.11.1 Selection of Pipe Materials

The following factors should be taken into account when selecting the pipematerials.

• the requirements imposed by aeration equipment (for example, internalcorrosion of pipes must not cause clogging of diffusers)

• pipework must stand up to the external effects of weather and waste water• pipework located in free air must be capable of withstanding temperatures

of 80 - 120 °C• joining methods, the need for T-junctions, and the degree of precision

required in the installation work• transportation, stockpiling at site, various cost factors

Pipework may be the main source of noise at a waste water treatment plant.From this point of view lightweight piping materials may cause more problemsthan heavier materials. Noise can be efficiently reduced by dividing thepipework into convenient sections installing flexible joints and by mounting iton supports at appropriate intervals. All these factors should be consideredwhen comparing the total costs of different pipe materials.

5.11.2 Properties of Different Materials

Steel pipes must be surface treated both internally and externally. Thismaterial is best suited to the main headers. Stainless steel and acid-resistantstainless steel can be used without surface treatment and are the mostcommon piping material in waste water treatment plants in the Nordiccountries. Owing to the small wall thickness, special attention must be paid toreducing pipework noise and avoiding pipework vibrations. Excessivevibrations may result in breakage in stainless steel and acid-resistantstainless steel pipes.




Reinforced plastic pipes are used to some extent. Special attention should begiven to reducing pipework noise, i.e. by means of specially designedresonance absorbers. Thermal elongation must be taken into account whenplanning the location of junctions, branches, and pipe supports. Cast ironpipes must have internal coating of epoxy-based or similar material. On theexternal surface a coat of paint is usually sufficient.

When using thin-walled pipes (stainless steel) or lightweight pipe materials(reinforced plastic) the wall thickness cannot be selected on the basis ofmaximum working pressure alone (usually this does not exceed 1,0 bar).Other factors that must be given equal attention to are the reduction ofpipework noise, suppression of pipe vibrations, and supporting of thepipework. Wall thicknesses chosen according to pressure classes PN4 -PN10 have proved to be appropriate for matching the combined requirementsimposed by these factors.

5.11.3 Design Principles

The main header and the distribution headers should be located above thewater surface and outside the basin walls to avoid harmful effects frombacklash or freezing of the waste water. The main header must have aninclination of 1:200 to 1:100 and the shallowest point should be equipped witha drainage valve.

The slanted junctions between the blower delivery side and the main headerare necessary if the blower running speed is regulated, and advisable ifseveral simultaneously running blowers are connected to the main header. Ifthe blowers have a common suction main, all the branches should likewise bejoined at a slant to it, as illustrated in Figure 11.

Figure 11: Connection of blower pressure side to the suction main




No section of the pipework may be located inside or under adjacentstructures. All junctions and accessories must be easily accessible formaintenance purposes.

5.12 Examples of Air Supply Systems

5.12.1 Waste Water Treatment Plant, Population Equivalent 40,000

At the blower station, air filter units are mounted on a wall and connected tothe common intake main of the blowers. A noise-reducing casing has beeninstalled on the air intake grille to eliminate ambient noise effects in thenearby housing areas. The floor channel for the main header is wide enoughto permit good accessibility for maintenance.

Figure 12: Example of a blower station at a smaller treatment plant

5.12.2 Waste Water Treatment Plant, Population Equivalent 200,000

A separate air intake structure is located on the roof of the blower station.The building has been provided with heavy concrete walls. Additionally, eachblower has an individual noise reduction casing. The main header has beenprovided with a flexible pipe joint outside the building for the isolation ofpipework-born noise and to accommodate thermal expansion. Stainless steelis used as a pipe material and the shut-off is provided with non-return valves.




Access to the air filter is via stairs outside the building, and the blower stationis equipped with a bridge crane.

Figure 13: Example of a blower station at a larger treatment plant

In the air piping stainless steel (nonsubmerged sections) and acid resistantsteel (submerged sections) are used as pipe materials. Appropriately spacedcut-off junctions (at every 10 - 15 m) along the pipelines are an efficient wayof eliminating noise travel in the pipework. The flexible junctions have flangesto provide the pipes with more rigidity and to reduce their vibrations.

Figure 14: Example of aeration pipework


5 Aeration Control Page: 1.1 (10)


6 Aeration Control6.1 Benefits of Aeration Control ............................................................... 6.1

6.1.1 Process Benefits .........................................................................6.16.1.2 Economic Benefits.......................................................................6.2

6.2 Control System...................................................................................6.26.2.1 Blower Air Delivery Control .........................................................6.26.2.2 Air Distribution Control ................................................................6.36.2.3 Example of Aeration Control System...........................................6.5

6.3 Instrumentation...................................................................................6.76.3.1 Dissolved Oxygen Probe.............................................................6.76.3.2 Air Flow Measurement.................................................................6.96.3.3 Pressure and Temperature .........................................................6.9

6.4 Mechanical Devices ...........................................................................6.9

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6 Aeration Control




6 AERATION CONTROL

6.1 Benefits of Aeration Control

In this chapter the different controlling alternatives are outlined. The design ofa control system and its necessity have to be taken into account already invery early stage. By doing this it is the best way to get the appropriate andeconomical control system for the end user. The consideration of controlsystem in the very beginning of the design process may affect the selection ofaeration equipment and furthermore enables the flexibility in operation of thesystem later on.

The benefits achieved by aeration control are summarised as follows• process benefits• economic benefits

6.1.1 Process Benefits

Marked fluctuation in the influent load is a characteristic of waste watertreatment plants. How strongly these fluctuations affect the aeration systemdepends on the type of treatment process. In a normally loaded activatedsludge plant, fluctuations in influent load very rapidly affect the oxygencontent in aeration, while in extended aeration plants the effect of loadfluctuation is slower and smaller. The more rapidly the effects show up in theaeration process, the more advisable it is to use automatic aeration control.

Process benefits are achieved by maintaining a constant mixed liquordissolved oxygen (DO) concentration and an appropriate oxygen profile inaeration. Using automated DO control the following process benefits areachieved:• improved process reliability• better sludge settleability• better nitrogen removal efficiency• better effluent quality

Where aeration control is not automated and there is an inadequate DOconcentration in aeration, the following problems will appear:• sludge bulking in secondary sedimentation• inhibition of biological activity• filamentous bacterial growth




On the other hand, excessive aeration may result in settleability problems byshearing activated sludge flocs.

6.1.2 Economic Benefits

The energy consumption of aeration normally accounts for 50 - 60 % of thetotal energy demand and up to 30 % of the operating costs in activatedsludge plants. Due to these high figures great savings can be achieved bysmall improvements in the control system. In general, by automating the DOcontrol cuts of 25 to 40 % in energy costs can be made, but figures as high as50 % are possible.

The best economic benefits are reached by stepless control of the blowersand electrical motors, by frequency control and by automated DO controlachieved through adjustable valves according to on-line DO measurement.

6.2 Control System

The degree of aeration control to be implemented can vary from manualmanipulation based on manual measurements to a comprehensive fullyautomated control system based on on-line measurements and an intelligentautomation system to control electric motors, blower air delivery and airdistribution hardware.

In a new treatment plant the value of a sophisticated control system shouldbe quite clear. The savings achieved by the control system compared to thecapital investment costs are so large that it is obvious that the plant should beprovided with a DO control system.

The aeration control system is usually divided as follows:• blower air delivery control• air distribution control

6.2.1 Blower Air Delivery Control

Blower air delivery control is discussed earlier in Chapter 5. Stepless controlof the blowers should be the target, but step by step control is alsoappropriate.

Air delivery control can be arranged in various ways as follows• DO measurement control• main header pressure control




DO measurement control is employed where there is a separate blowersystem or no control valves are used. A separate blower system means thatevery process line has its own blower, the delivery of which is controlleddirectly through the DO measurement on the line. By this, aeration control isusually successful as no control valves are needed. This solution isexpensive and it is used in cases where only a few process lines exist.

A common blower station and one main header for the whole treatment plantis the most common solution to the aeration system. As well in this case, theblowers can be controlled directly by DO measurement. In this set-up, forexample the lowest DO measurement value always controls the blowers.However, excessive aeration inevitably takes place in some basins, and theresult achieved is not the best.

The most common means of blower control is based on the main headerpressure. The pressure is usually kept constant so that the air flow isincreased when the pressure is decreasing and decreased when the pressureis increasing.

When the use of positive displacement blowers and stepless control is theintention, at least two blowers should be equipped with frequency converters,or there should be two blower types with different delivery values. In thiscase, the blower with the greater delivery is controlled by the frequencyconverter and the blower with the smaller delivery starts up when the first onehas reached its maximum delivery. When using frequency converters it mustbe remembered that these consume up to 10 % of the energy applied asheat.

With centrifugal blowers, control is achieved by inlet valve control or by outletdiffusers. If inlet valve control is used, the system must be equipped withsurge protection, because the blower cannot develop enough pressure toovercome the downstream process pressure. Most blower manufacturersoffer an independent surge protection system but the automation systemshould also be provided control algorithms. Where guide vanes are used tocontrol blower output, the motor amperage draw corresponding to the surgepoint is also reduced. Thus, care must be taken to set the point of minimumamperage so that this is not too high.

6.2.2 Air Distribution Control

At treatment plants with various process lines the general design guideline isfor each line to be measured and controlled separately. In order to achieve




faultless air distribution to different process lines it is most important for thepipe diameters to have been selected and graded correctly.

Every process line should be fed by at least one individual distributionheader, but several distribution headers can also be used. Distributionheaders are equipped with air flow meters and pressure gauges. Controlvalves should be installed in droplegs or distribution headers. Droplegsshould also be equipped with pressure gauges to control pressure loss of thediffusers.

In order to achieve successful air distribution, the general behaviour of theprocess must be sufficiently well known, that is the optimum oxygen profilesof the aeration basins and their variations should be known for varying loadconditions (see Figure 1). On the basis of these factors it is possible todecide how many DO measuring points are needed and how they should belocated. In addition, the effects of various process parameters should beknown, including sludge age and the amounts of return and excess sludge.

Figure 1: Typical oxygen profile variation range at optimum aeration level(normally loaded activated sludge plant)

Air distribution is usually controlled by control valves according to the DOmeasurement. Usually there are several aeration zones in one aerationbasin. In the optimum situation there are the same number of DO probes aszones, so that every zone can be controlled to keep the oxygen profile asdesired. Figure 2 shows an example in which the aeration basin is dividedinto four aeration zones, each with their own distribution header. Each headeris equipped with an air flow meter, pressure gauge, and control valve. In




addition, each zone has a DO probe so that DO measurement directlycontrols the control valves according to the desired DO setting point.

Figure 2: Aeration equipment is divided according to load conditions. The airfeed control design is aimed to maintain a suitable oxygen profilecontinuously.

Although the control algorithms should be designed so that the control valveswill be kept as open as possible in order to minimise overall system pressureand aeration energy expenditure, it is also necessary to sacrifice some headloss in order to maintain control.

6.2.3 Example of Aeration Control System

Figure 3 shows the aeration system of a waste water treatment plant. In thiscase the aeration system consists of a common blower station with twocentrifugal blowers and two aeration basins, each with its own distributionheader and control valves. The control system is manipulated by a digitalprocess automation system.




QR

l

Q1

QT

Q1

O2

QT

Q2

Q2

Q2

I II

Q2

QC

Q1

QM

Q1

QC

QN

M

QN

C

Q1

HS

Q1

Z1

ZT

Q1

QN

C

QC

Q2

Q2

ZT

Q2

ZI

ZN

M

PT

LOGIIKKA

ZN

C

ZN

M

ZC

Control room operation

Field operations

O2

Figure 3: Flow chart of an automatic control of an aeration line

QT oxygen measurement transmitterQC oxygen concentration controlZT valve position transmitterZI valve position indicatorHS valve position control switchQM generation of corrective action upon faulty condition of basin IIZM generation of valve position adjustment sumZC control of delivery adjustmentZI valve position indicatorQRI indication and plotting of oxygen measurement data




The blower air delivery is controlled by regulating the inlet valves accordingto the pressure of the main header. Additionally, the positions of the controlvalves play a part in air delivery control. One control valve is always as openas possible.

The aeration basins are of the plug flow type and each of them is providedwith two DO probes. The probes are located so that one lies in the first half,and the other in the second half of the basin. The mean value of thesemeasurements is used for valve control. Although the air flow of eachdistribution header is measured, such measurement does not play a part inthe control of the system. The figure shows only one basin.

6.3 Instrumentation

6.3.1 Dissolved Oxygen Probe

Dissolved oxygen (DO) monitoring equipment is the most important of all theon-line instruments used in aeration control. It is also most often the mainreason for control system failure. System failure may be traceable in somecases to faulty equipment, but it is at least as likely to be due to improperapplication, poor installation, lack of attention and maintenance by plantpersonnel or a combination of thereof.

All the DO probes available today are electrochemical cells in contact withthe fluid through an oxygen-permeable membrane. The cells are equippedwith electrodes in which chemical reactions induce changes in voltage acrossthem. The subsequent current flow across the electrodes produces anelectrical signal in proportion to the oxygen content of the fluid.

In instrument selection environmental conditions, operating ranges, anddesign requirements should be identified. Existing comparative instrumenttest data and the experience of other users under both bench and fieldconditions should be taken into account.

Each manufacturer has its own recommended calibration procedures. Ingeneral, these are simple one or two point calibrations. The field verificationprocedure for the installed instruments should include accurate outputmeasurements over the entire expected operating range. Other performancechecks, such as response time, hysteresis, and repeatability should also bemade.




The main reasons for errors in measurements are the following• build-up of slimy biofilm on the probes• non-desired reactions taking place on the probes resulting in chemical

precipitates or extraneous currents affecting instrument readings• location of probes at a point where circulation is insufficient• adherence of small air bubbles to the probes

The slimy biofilm can usually be effectively removed from the membranesurface by carefully wiping with a wet tissue soaked in a 10 % HCI solution.

Probe cleaning frequency depends on the process loading characteristicsand operating configuration. To minimise maintenance requirements, it isimportant to clean the probes only when necessary.

The measuring devices should be mounted so that the probes can easily beremoved and their location can be altered when necessary in order to locatethe most appropriate measuring point for control purposes. An example of aninstallation method is shown in Figure 4.

Figure 4: Floating oxygen measuring probe attached to a lever rod




6.3.2 Air Flow Measurement

Air flow measurement equipment is an important component of aerationcontrol systems. An even air distribution to different parts of the aerationsystem requires accurate air flow measurement to ensure that the correctquantity of oxygen is being delivered.

The different air flow metering systems used today are based either ondifferential pressure across a control element or mass flow. The differentialflow meters use plate orifices, Pitot or Venturi tubes to produce a measurabledifferential head or pressure. This pressure differential is then converted tovelocity and mass flow for which temperature and pressure corrections haveto be made.

Mass flow meters generally operate on the principle of a hot wireanemometer. A wire is placed in the flow stream with an electric currentapplied to maintain the wire at a pre-set temperature. The rate of cooling thewire, based on the current required to maintain it at the pre-set temperature,is proportional to the mass flow rate of air.

Care must be taken when selecting air flow meters to ensure the correctdimensioning of a meter, the size of which should accommodate only theimmediate future expected ranges in air flows.

Because the probes are not in contact with waste water, much lessmaintenance is required compared to DO probes. Periodic conformancechecks are, however, recommended.

6.3.3 Pressure and Temperature

Pressure and temperature measurements are used in the aeration controlsystem to monitor blower suction and discharge conditions. Pressure gaugesare also used to control the pressure loss of diffusers.

Pressure and temperature measurements are also used to provide on-lineinformation for converting volumetric field flow rates to standard flow rates.

6.4 Mechanical Devices

Mechanical devices necessary for the aeration systems are the blowers andthe control valves. Blowers have already been discussed in Chapter 5 andwill not therefore be dealt with here.




The most commonly used control valves for air distribution control purposesare butterfly valves. Ball valves may also be used, but their higher pricegenerally makes them uneconomical in this context.

When selecting and dimensioning the control valves the followingrequirements should be met• the air flow rate achieved by throttling the control should be as linear as

possible to the travel of the valve• the right dimensioning of the valve is important; it does not need to satisfy

requirements far into the future

The automation hardware used for the aeration control system is usually onlya part of the automation hardware of the whole plant. The hardware used forthis purpose is either digital processing units or programmable logiccontrollers. The main requirement for these is for them to be freelyprogrammable.


7 NOPOL® DDS Aeration System Design Page: 1.1 (51)


7 NOPOL® DDS Aeration System Design7.1 Air Flow ..............................................................................................7.17.2 Mixing .................................................................................................7.37.3 Number of Diffusers............................................................................7.47.4 Layout.................................................................................................7.5

7.4.1 Layout Planning ..........................................................................7.57.4.2 Basin Geometry.........................................................................7.107.4.3 Submersion Depth.....................................................................7.117.4.4 Diffuser Layouts in Various Basins ...........................................7.11

7.5 Tapering Diffusers and SOTR ..........................................................7.177.6 Calculation of Corrected SOTE Values ............................................7.18

7.6.1 Expressing the Effect in Offers..................................................7.187.7 Air Production...................................................................................7.19

7.7.1 Dimensioning of the Blower.......................................................7.197.7.2 Dimensioning of Air Piping for NOPOL® DDS ...........................7.267.7.3 Air Filtering System for NOPOL® DDS ......................................7.27

7.8 Calculation Examples .......................................................................7.297.8.1 Example 1..................................................................................7.297.8.2 Example 2..................................................................................7.39

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7 NOPOL® DDS Aeration System Design

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7 NOPOL® DDS AERATION SYSTEM DESIGN

To design a disc diffuser aeration system, it is necessary to know the ActualOxygen Requirement AOR and Standard Oxygen Transfer Rate SOTR of theprocess. The calculation of these values is explained in Chapter 4.

7.1 Air Flow

Once the Standard Oxygen Transfer Rate has been determined, the air flowrequired can be calculated from Equation 1:

Equation 1

qSOTR

C eai

=•

where

qa air flow in NTP (+20 °C, 101,3 kPa) m3/hCi oxygen content of the air (+20 °C, 101,3 kPa)

0,280 kg O2 /m3

e standard oxygen transfer efficiency, SOTE %

The value of e depends on the choice of diffuser type, aeration depth, air flowrate and diffuser spacing. Efficiency curves of NOPOL® disc diffusers are inthe NOPOL® DDS product manual.

The greater the submersion depth of the diffusers, the greater the mass ofoxygen transferred. The higher oxygen transfer efficiency is due to the longercontact time between waste water and air bubbles, and to the higher oxygensaturation concentration at higher pressure. Oxygen absorption at differentdepths can be estimated by means of the following equation:

Equation 2

eh

h

en

=

•

2

2

100

where




e oxygen adsorption at the chosen aeration depth %h aeration depth mh2 depth of the oxygen absorption curves measured me2 degree of oxygen absorption given by the curves %n exponent

1,0 for aeration depths 2 - 5 m0,8 for aeration depths 5 - 8 m

The minimum air flow to meet the oxygen requirement and the respectiveminimum air flow per diffuser needed for mixing should be calculated to avoidunacceptably low air flow values. Adequate sludge circulation in the aerationbasin may determine the minimum air flow necessary for the process, and itsvalue may well be higher than that required by the minimum oxygen demand.

Calculation of the air flow required is usually an iterative process becausecertain variables must initially be based on assumptions which can only bechecked after all the calculations have been carried out. Therefore Nopon Oyhas created a simple computer program package for the calculation ofNOPOL® Disc Diffuser Systems.

For disc diffusers, the air flow is expressed in cubic metres per diffuser discarea (m3/h•m2). The shape and material of the aeration element of the diffuserand the pressure loss caused by the diffuser orifice element determine theappropriate range of air flow into the diffuser.

The minimum air flow is determined by the clogging tendency of the diffuser,while the maximum air flow is determined by the amount of pressure lossoccurring in the diffuser.

The diffuser’s oxygenation efficiency (kg O2/kWh) decreases as the air flowper diffuser increases. This is due to the following reasons:

• water circulation in the aeration basin is improved• the retention time of the bubbles is shorter• the bubbles produced by a diffuser are larger in diameter• the counteracting pressure caused by the porous material of the diffuser

increases.

The oxygenation efficiency of fine bubble diffusers is about 20 to 30 % lowerwhere the maximum air flow is used than it is when the minimum air flow isused.




7.2 Mixing

Sufficient mixing energy must be generated by the aeration equipment in allconditions to keep the activated sludge in suspension and prevent it fromsettling.

The minimum air flow required for this mixing effect can be estimated on thebasis of the fact that activated sludge remains in suspension with G values of20 to 75 1/s (G = root mean square velocity gradient).

In normal aeration processes the G values range from 90 to 220 1/s. In thecomplete mixing process the G values are higher than in the plug flowprocess. The greater the sludge concentration the higher the G valuesrequired, because a greater amount of aeration capacity is used per unitvolume. The smallest G values are achieved with fine bubble diffuser layoutscovering the whole bottom of the aeration basin.

Under these conditions the sludge flocs show the least tendency todisintegrate and thus a sludge with good settling qualities is obtained forsedimentation. By entering the chosen G value into the equation:

Equation 3

G V= •µ 2

where

P required power kWµ dynamic viscosity Ns/m2

G velocity gradient 1/s

The required power per unit volume P/V (kW/m3) can be calculated when thedynamic viscosity µ of the sludge suspension is known.

Equation 4 can also be used to calculate the required amount of isothermallyexpanding air needed to meet the power requirement.

Equation 4

( )P q ha= • • +0 0281 1 0 097, ln ,




where

qa air flow m3/hh submersion depth m

The above two formulas make it possible to evaluate the required minimumair feed rate for an aeration arrangement covering the entire bottom as afunction of the submersion depth of the diffusers.

The critical air flow rate for keeping the sludge in suspension can vary from1,2 to 3,0 m3/h • m2. The minimum air flow rate depends on the settlingcharacteristics of the sludge and on the installation density of the diffusers.

The width of the basin can be discounted when determining the requiredminimum air feed rates with diffusers covering the entire bottom.

When one-sided or two-sided lateral installations are being used, therequired minimum air feed rate can be from 0,6 to 1,2 m3/h • m3 of the basinvolume.

In some cases mixing can be achieved with mechanical mixers (propellers).Especially in the DN-process the denitrification stage should be provided onlywith propellers. Propellers can also be used when the minimum air flow isunable to keep the activated sludge in suspension.

7.3 Number of Diffusers

The number of diffusers is calculated from Equation 5:

Equation 5

Nq

qa

a d

',

=

where

N' number of diffusersqa total air flow m3/hqa,d air flow / diffuser m3/h

The maximum and minimum air flow per diffuser must be checked from thetechnical data given for the particular diffuser type (dimensioning value).Technical data for the diffusers is in the NOPOL® DDS product manual.




The area density of diffusers (diffuser spacing) is determined from Equation6:

Equation 6

DDN

l w=

•'

where

DD area density of diffusers 1/m2

l length of aeration basin mw width of aeration basin m

Diffuser density can also be calculated as a percentage:

Equation 7

DDN d

l wa=

••

•'

100

where the value for da is 0.025 m2 for the NOPOL® KKI 215, HKL 215, andMKL 215 disc diffusers, and 0.060 m2 for the PRK 300 and PIK 300 diffusers.

When calculating diffuser density, use the bottom area where diffusers are tobe installed to and ignore the effect of slopes to the basin dimensions. Slopesaffect the SOTE value and there is a separate instruction on how todetermine this effect.

7.4 Layout

7.4.1 Layout Planning

Dividing the total number of diffusers required by the number of aerationbasins yields the number of diffusers per basin.

In small aeration basins, a single group of diffusers is used with one dropleginstalled vertically, one zone header installed horizontally on the basinbottom, and the necessary number of diffuser elements branching out at rightangles from the zone header. In addition, there is the necessary number ofwater collection pipes. In small diffuser groups the zone header is normally




located at either one end or one side of the basin, while in bigger groups it isusually installed in the middle of the basin.

Figure 1: Diffuser layout, zone header in the middle

Figure 2: Diffuser layout, zone header on one side




For general purposes, the following limit values can be used for each diffusergroup:

PIK 300 PRK 300 KKI 215HKL 215MKL 215

maximum total air flow Qmax 3 m3/s 3 m3/s 3 m3/smaximum air velocity in pipes vmax 18 - 25 m/s 18 - 25 m/s 18 – 25 m/smaximum length of one row ofdiffusers

Lmax 40 m 40 m 40 m

maximum spacing of diffusers on thebranches

Emax 1250 mm 1250 mm 1000 mm

minimum spacing of diffusers on thebranches

Emin 450 mm 450 mm 350 mm

maximum width of the diffuser group Bmax 12 m 12 m 12 mmaximum distance between branches Dmax 1250 mm 1250 mm 1000 mmminimum distance between branches Dmin 500 mm 500 mm 350 mm

number of diffusers per branch NLmax 30 30 50Zone Header diameter (size ofconnector flange)

F DN80 - DN350 DN80 - DN350 DN80 - DN350

Values deviating from those given above should not be used withoutconsulting with Nopon Oy.

If the limit values cannot be fulfilled, it is necessary to install two or morediffuser groups in the basin. When determining the size of the diffuser group,all inclined surfaces possibly contained in the basin must also be taken intoaccount, as the group installation normally covers only the horizontal part ofthe basin bottom.

The final dimensioning of the group is carried out according to the followingprinciples. Initially, the necessary number of diffusers in the group and thebasin bottom surface area available for the installation must be known. Thenumber of diffuser rows is obtained by dividing the width (A) of the basinbottom by the distance between successive diffuser rows (D). The result isrounded down to the nearest integer = C. The integer gives the number ofdiffuser rows necessary.

One should check that the outer rows are not too near the basin walls or theedges of inclined sections (see Figure 3 and Figure 4).




Figure 3: Diffuser layout in a basin with straight edges

Figure 4: Diffuser layout in a basin with slants

The number of diffusers per row (2 x L in Figure 1 and L in Figure 2) isobtained by dividing the total number of diffusers by the number of rows. Ifthe group is arranged as in Figure 1, and an even distribution of diffusers isdesired, the number of diffusers obtained should be rounded to the nearesteven number.

Finally, the distance between successive diffusers along the rows is obtainedsimply by dividing the available basin bottom length by the number ofdiffusers to be mounted in each row.

Before carrying out the calculations, the space needed for the zone header,the water collection pipes and the junction must be subtracted from the totalbasin bottom length. This subtractive space is about 1,2 m (Figure 1) or about0,85 m (Figure 2).




If the distance between successive diffusers (E) obtained by calculation isgreater than or equal to Emin, the plan can be accepted and developedfurther.

If E is too small, the above calculations must be repeated, changing the initialdata so that the distance between the diffuser rows is smaller, and hence thenumber of diffuser rows is greater.

If E is too long, the calculations should be repeated for a greater distancebetween the diffuser rows.

To determine the final distance between diffusers (diffuser spacing) along theelement, the value obtained by calculation is rounded to the nearest multipleof five (mm).

As is obvious from the above, the calculations are iterative and may thusneed to be repeated several times before a final suitable diffuser and elementspacing is obtained.

To permit subsequent extension of the aeration system or changing the basinoxygen profile, the advisable diffuser spacing is longer than Emin, whichpermits the mounting of new diffusers between the original ones.

When determining the final equipment parameters of the aeration system, thediffuser rows are selected so that the total length of one continuous row ofdiffusers is not greater than 5000 mm (Figure 5). Once all the equipmentparameters have been established (including the zone header, diffuserelement, junctions and water collection pipes), the dimension of the groupsystem and its suitability to the available basin space should be verified.

Figure 5: Diffusers installed on an element

The distance from the diffuser element to the centre of the nearest diffuser isnormally one half of the distance between the diffusers (E/2).




7.4.2 Basin Geometry

The geometry of the aeration basin affects the oxygenation efficiency and themixing characteristics achieved in the basin. An appropriate aeration basinshape may help to improve the oxygenation efficiency by as much as onehalf.

Aeration basins in which diffusers are to be installed should be about 3 to 6 min depth to ensure efficient performance of the diffusers. In the plug flowprocess, the ratio between basin width and depth should be approximately1,0 to 2,0, which in practice means basin widths of from 3 to 12 m. Forreasons of process technology the ratio between basin length and widthshould not exceed 15:1. A typical rectangular basin will thus be 4 m in depth,6 m in width and 90 m in length, resulting in a volume of 2160 m3.

As the depth to width ratio increases, the contact time of air bubbles in thewater also increases, improving the oxygen transfer efficiency figures. If thediffuser layout covers the entire bottom of the basin, this ratio is of noimportance. In the latter case chambered bottom edges are alsounnecessary.

To facilitate maintenance operations it is advisable to build an inclined bottomand provide it with facilities for discharging by means of either gravity orpumping.

The layout of diffusers in the aeration basin affects the absorption rate andoxygenation efficiency in a fundamental way.

If the diffusers uniformly cover the entire bottom of the basin, spiral flows areeliminated and the bubbles rise in the water at their specific speed of ascentwith respect to stagnant water. With this installation arrangement an oxygenutilisation degree of 20 to 25 % and an oxygenation efficiency of 3,2 to 4,1 kgO2/kWh can be attained with clean water in a 4 m deep basin.

In diffuser layouts covering the entire bottom of the basin, the diffuserdensity, that is, the number of diffusers per unit area (m2) of the bottom, has asignificant effect on the oxygen utilisation degree and oxygenation efficiencyobtainable in the basin. The greater the diffuser density the greater theperformance values.




7.4.3 Submersion Depth

The oxygenation rate can be raised by increasing the submersion depth ofthe aeration equipment. The improved efficiency of oxygen transfer thusobtained is mainly due to the longer contact time between sewage andbubbles and to the higher oxygen saturation concentration achieved. Oxygenabsorption at different depths can be estimated by means of Equation 8:

Equation 8

eh

h

en

=

•

2

2

100

where

e oxygen adsorption at the chosen aeration depth %h aeration depth mh2 depth of the oxygen absorption curves measured me2 degree of oxygen absorption given by the curves %n exponent

1,0 for aeration depths 2 - 5 m0,8 for aeration depths 5 - 8 m

Oxygenation efficiency (kg O2/kWh) remains constant within the normalsubmersion depths (3 to 8 m). Even though the oxygen transfer rate is higherat greater submersion depths, the effect of improved absorption iscounteracted in terms of economy. This is caused by the increased energyconsumption of blowers owing to increased hydrostatic pressure.

The normal submersion depth of diffusers ranges from 3 to 6 m. If very limitedground space for the aeration plant is available, submersion depths of up to12 m can be used. Submersion depths of less than 3 m are not advisableowing to the process technology.

The attachment distance from the basin bottom is from 0,2 to 0,35 m with discdiffusers.

7.4.4 Diffuser Layouts in Various Basins

Bottom diffusers can be installed in various types of aeration basins. Arectangular form is the most common, round and other types of aerationbasins are also available.




Figure 6, Figure 7 and Figure 8 show different placing examples of diffusersin round and ring basins. Figure 9 shows a placing example in an oxidationditch equipped with a flow generator.

Figure 6: Placing of diffusers in a round basin

Figure 7: Placing of diffusers in a ring basin




Figure 8: Placing of diffusers in a round basin equipped with flow generator

min 1.5-2 m = water depth appr 2-3 m min 1.5 m

Aerationgroup

Aerationgroup

Aerationgroup

Aerationgroup

Influent

Figure 9: Placing of diffusers in an oxidation ditch (race track) equipped withflow generators




There are various “non standard installations”, that is all shapes of aerationbasins where diffusers partially cover the basin bottom and all shapes ofbasins equipped with flow generators or with mixers. In these cases thewater speed differs from the standard case where the whole bottom iscovered and thus affects the SOTE value. Corrections must be madeaccording to following instructions.

The present Quotation Manager (QM) design programs take intoaccount the effect of flow generators and slopes in the basin as explained inthese instructions. The purpose of this is to understand the magnitude ofthese effects on cases where the system has been designed using "standard"SOTE curves for complete bottom coverage.

7.4.4.1 Partial Coverage, No Flow Generators

Basin bottom areas not covered by diffusers exist in following cases:• basin has slopes at the sides, typically from 0,5 m to several meters• basin bottom is not level• basin has been designed for spiral flow (old tube diffuser

installations)• basin is foreseen with various constructions (old surface aerators,

concrete supports, pockets or channels to empty the basin, etc.)• anaerobic zones in the basins.

The uncovered area always creates a spiral flow in the basin decreasing thedetention time of the bubbles in the basin and thus reducing SOTE values.The influence of the spiral flows on the system efficiency must be correctedby multiplying the air flows calculated for “standard installation” by thecoefficient f1, shown in Table 1.

Recalculate the system considering that the maximum air flow per diffuser isnot exceeded. Note also the necessary correction on the SOTE value whenexpressed in offers.

Table 1: Correction coefficients for spiral flows

Width of the uncovered area, m(distance of diffusers from the wall)

multiply the air flow byf1

1 1,102 1,253 1,304 1,35

>5 1,35




These values are approximate figures for total aeration basin width from 5 to8 meters when slope is on one side of basin or for basins of total width of 10to 16 m when slopes are on both sides. For more accurate figures and forwider basins, see Table 2.

Table 2: Air flow correction coefficients for non standard installations, diffuserscover part of the basin bottom - no flow generators

Width ofthe notcoveredbottomarea

Correction coefficient f1

(m) width of the aeration basin, m3 4 5 6 7 8 9 10 12 14 16 18 20 >20

1 1,18 1,12 1,09 1,07 1,06 1,05 1,04 1,04 1,03 1,03 1,02 1,02 1,02 1,022 1,35 1,35 1,23 1,18 1,14 1,12 1,10 1,09 1,07 1,06 1,05 1,04 1,04 1,043 1,35 1,35 1,35 1,26 1,21 1,18 1,15 1,12 1,10 1,08 1,07 1,06 1,064 1,35 1,35 1,35 1,35 1,28 1,23 1,18 1,14 1,12 1,10 1,09 1,095 1,35 1,35 1,35 1,35 1,35 1,25 1,19 1,16 1,13 1,12 1,12

>5 to be defined case by case

Multiply the air flow calculated for the whole bottom coverage by coefficient f1.

Use total basin width when slope is on one side only. When slopes are onboth sides use the half width of the basin: from centre line to the wall.

7.4.4.2 Carrousel and Ring Basins, Flow Generators

Carrousel type nitrogen removal processes typically have aerated zones inthe straight part of the basin. Flow generators are normally installed in thebeginning of the straight part having free space before and after the diffusersto protect the bearings and the operation of the flow generators. Flowgenerators create a horizontal steady speed of over 0,3 m/s. Aerated zonesare located in this straight area and the curved ends of the basins aretypically anoxic.

Sometimes ring shape aeration basins are also equipped with flow generatorsin order to create a horizontal flow. In practice the actual flow pattern is tosome extent vertical but also has spiral flow components.

According to the experiences of Nopon Oy and to published data thehorizontal water velocity effects on the SOTE value by -30 ... +30 % depending




on the diffuser density.

Ask more detailled design rules from Nopon Oy.

The above mentioned flow generator correction can be used only if thehorizontal flow is min. 0,30 m/s, not for horizontal flow caused by sewage andsludge volumetric flows (water speed is much lower). The water velocity hasto be achieved when aeration is in use.

Aeration air is affecting the water flow pattern especially when air isintroduced at high volumes per basin area. These unpredictable changes inflow pattern might affect significantly on the oxygen transfer efficiency of theaeration system. Consult Nopon Oy before using disc densities over 10 % inflow generator basins.

In the rounded ends of the basins a combination of horizontal, vertical andspiral flows appear. Avoid placing diffusers in that area.

7.4.4.3 Mixers and Aeration

Mixers for mixing only, not creating a horizontal flow, cause strong verticalflows and turbulence in the basin. This type mixer typically has a smallerpropeller and a higher speed than flow generators. The vertical up and downflows compensate the influence of each other. Turbulence causes thecoalescence of bubbles and improves the SOTE value as well as increasesthe alpha value. The overall effect can be regarded as negligible andtherefore does not have to be taken into consideration.




7.5 Tapering Diffusers and SOTR

In some activated sludge processes the spacing of the diffusers is taperedacross the length of the aeration basin so that diffuser density is highest onthe entrance side and decreases towards the outlet, for instance in four stepsequal in length to one quarter of the system length. When necessary,tapering should be taken into account while designing the diffuser groupinstallation.

Table 3: Tapering of diffusers, municipal waste water

Basinlength

Cumulative number of diffusers from the basin beginning%

% 0 1 2 3 4 5 6 7 8 90 1,8 3,6 5,3 7,2 9,0 10,8 12,6 14,4 16,2

10 18 19,8 21,6 23,4 25,2 27,0 28,8 30,6 32,4 34,220 36 37,2 38,4 39,6 40,8 42,0 43,2 44,4 45,6 46,830 48 49,2 50,4 51,6 52,8 54,0 55,2 56,4 57,6 58,240 60 61,0 62,0 63,0 64,0 65,0 66,0 67,0 68,0 69,050 70 70,7 71,4 72,1 72,8 73,2 74,2 74,9 75,6 76,360 77 77,6 78,2 78,8 79,4 80,0 80,6 81,2 81,8 82,470 83 83,7 84,4 85,1 85,8 86,5 87,2 87,9 88,6 89,380 90 90,5 91,0 91,5 92,0 92,5 93,0 93,5 94,0 94,590 95 95,5 96,0 96,5 97,0 97,5 98,0 98,5 99,0 99,5

100

The cumulative number of diffusers expresses the proportion of diffusers inthe end of each zone.

How to use Table 3:

1. Calculate the total number of diffusers for the aeration basin2. Calculate the percentage length or surface area for each aerated zone3. Read the proportional number of diffusers from the table for the first zone4. Read the proportional number of diffusers from the table for the end of the

second zone5. Deduct the proportion of the first zone6. Repeat the calculation for the rest of the zones7. Calculate the actual number of diffusers for each zone

Note: The values of this table can also be applied in proportioning the SOTRfor aerated zones.




7.6 Calculation of Corrected SOTE Values

Corrected SOTE values are calculated as follows:

Equation 9

SOTESOTR

Q ci

=•

where: Q is the corrected air flowci is oxygen content of air (0,280 kg/m3 at 20 °C, 101,3 kPa)

7.6.1 Expressing the Effect in Offers

The corrections made have to be expressed in all offers as follows:

A) Partial coverageThe normal dimensioning of the NOPOL® DDS systems gives the SOTR.....kg O2/h at the air flow ......m3/h (T = 20°C; p = 101,3 kPa). According to ourexperiences the partial coverage of basin bottom decreases the SOTE valueof the system and the SOTR ... kg O2/h is achieved by the air flow..... m3/h (T= 20°C; p = 101,3 kPa).

B) Flow generatorsThe normal dimensioning of the NOPOL® DDS systems gives the SOTR ......kg O2/h at the air flow ........ m3/h (T = 20°C; p = 101,3 kPa). According to ourexperiences the flow generators increase the SOTE value of the system andthe SOTR ..... kg O2/h is achieved by the air flow........ m3/h (T = 20°C; p =101,3 kPa).

The offers are subject to normal guarantees (Nopon Oy guarantees the givenSOTR at the expressed air flow) for the corrected values. However, in case oflarge projects, Nopon Oy must be consulted.




7.7 Air Production

7.7.1 Dimensioning of the Blower

Blower discharge pressure for a NOPOL® DDS system can be calculatedusing Equation 10:

Equation 10

p p p p p pi h p a b2 = + + + +

where

p2 required blower discharge pressure kPapi atmospheric pressure at the plant altitude kPaph hydraulic pressure (10 kPa / 1m) kPapp head loss in pipework including control valves, flow

meters and non return valveskPa

pa head loss in the diffuser kPapb head loss in inlet valves and air filters kPa

The head losses of piping have to be calculated in the normal way usingrelevant tables, nomograms or formulae. A general figure for the “average”plant is a pressure loss in the pipework of 6 kPa (4 - 10 kPa), or 0.06 bar.Head losses for diffusers are shown in the graphs, see the NOPOL® DDSproduct manual. For example, a typical value is 5 kPa for a KKI 215 diffuserat 3 m3/h air flow. The head losses for inlet filters and valves are usuallyabout 2 kPa.

Thus, the total pressure for the blower design in a typical 4 m deep basinwould be the water pressure, plus the above, or about 6 + 5 + 2 + 40 = 53kPa (0.53 bar).

Having defined and calculated the above, the power requirement of theblowers can also be calculated. Normally with the above figures, the blower ischosen from manufacturers’ catalogues, and power thus specified accordingto the type of blower chosen.




The blower power requirement is determined as follows:

Equation 11

Pk

kp q

p

p

i a

i

k

k

= •−

• • •

−

−

0 0002781

12

1

, ,

η

whereP Power kWp2 Blower discharge pressure kPak Constant factor for blowers 1,395η total efficiency %/100

This is the combined value of the blower, electric motorand transmission efficiencies (=ηkj•ηm•ηv)

Equation 11 for calculating blower power can also be expressed in thefollowing form:

Equation 12

P p q

p

pi a

i= • • •

−

0 000982

12

0 283

, ,

,

η

Blower efficiency (ηk) varies between 0,65 and 0,75 in rotary displacementblowers and between 0,7 and 0,8 in turbo blowers. The value 0,9 can be usedfor the combined efficiency of the electric motor and transmission (ηm•ηv).

7.7.1.1 Pressure Losses in the Pipework

Pressure losses in the pipework of an aeration system can be evaluated onthe basis of a nomogram (Figure 10). This nomogram is proposed for straightpipes.




Figure 10: Pipe pressure loss nomogram




The pressure loss in T -connections, 90° bows, valves, flanges, etc. can beevaluated as an equivalent length of the pipe. Typically this equivalent isexpressed as a multiplier of the pipe diameter. Values are expressed invarious technical handbooks and manufacturers' technical information.

Below there are two examples of how to use the nomogram.

Example 1. Select zone header (and flange) diameter. Air flow is 1500 m3/h(20°C, 101,3 kPa). KKI 215 diffuser, 2 m3/h/diffuser. Water depth is 6 m.

1. Correct the volumetric air flow into actual conditions:• Pressure is 101,3 + 5,75 (= diffuser submergence) + 4 (= diffuser pressure

loss) = 162,8 kPa• Temperature is 273 + 20 (= ambient) + 60 (= warming up in blower) - 30 (=

estimated cooling in main and distribution headers and dropleg) = 323 K.• Corrected air flow is 1500 • (101,3/162,8) • (323/293) = 1029 m3/h 2. Start reading the nomogram at bottom line at 1029 (see Figure 11).

3. Move vertically until the line “Velocity of air” = 20 m/s

4. Read the value of the line “Inside diameter” = 0,135 m

5. Select pipe 150 mm (= next standard size).

6. Of the left vertical axis you can also read that the pressure loss in pipe(150 mm) is 0,022 Pa/m.




Figure 11: Use of the nomogram, example 1




Example 2. There are 40 pcs of KKI 215 diffusers installed on a 40 m longheader pipe. Air flow per diffuser is 2 m3/h. Water depth is 6 m and ambientair temperature is 20 °C. Air cooling in pipework is assumed to be 30 °C.What is maximum air velocity in header pipe? What is the pressure differenceat the beginning and at the end of the pipe? What is the pressure differencecompared to pressure loss in a diffuser? What is the difference of air flow tothe first and last diffusers in the header pipe?

1. Correct the air flow (as in example 1)Pressure is 101,3 + 57,5 + 4 = 162,8 kPaTemperature is 273 + 20 + 60 - 30 = 323 KAir flow = 40 • 2 • (101,3/162,8) • (323/293) = 54,8 m3/hAir velocity is 54,8 • (4/(π • 0,092 • 3600)) = 2,39 m/s

2. Air velocity can be read from the nomogram by starting from the base lineat 54,8 m3/h and moving vertically until “Inside diameter” = 0,09 m. We canread: Maximum air velocity is 2,4 m/s.

3. We can read the maximum pressure loss to be in the beginning of the pipe1,1 Pa/m. Because air flow is decreasing along the pipe we can not use thisvalue for the whole pipe. To simplify the calculations we can split the pipe infour sections having axial air flows of 54,8; 41,1; 27,4 and 13,7. Using thenomogram as in point 2, we get the pressure losses correspondingly: 1,1;0,39; 0,27 and < 0,1. Total pressure loss in the pipe is10 • 1,1 + 10 • 0,65 + 10 • 0,27 + 10 • 0,1 = 21,3 Pa.

4. Proportional pressure difference is 21,3/400 • 100 = 5,1 %

5. Because the pressure loss versus air flow of the diffuser is practicallylinear, the difference in the air flow from the first and last diffusers ismaximum 5,1 % = 0,051 • 2 = 0,10 m3/h.

6. Note that the values used are maximum values and if integrating thepressure losses along the pipe in a smaller section a somewhat lower valuefor the air flow difference would be obtained.




Figure 12: Use of the nomogram, example 2




7.7.2 Dimensioning of Air Piping for NOPOL® DDS

The recommended actual air flow velocity ranges for different air distributionpipes are as follows (see Figure 13):• main header (1) 6 - 9 m/s• distribution headers (2) 10 - 15 m/s• droplegs (3) and zone headers (4) 18 - 25 m/s

Figure 13: Recommended actual air flow velocities in distribution pipes

Even distribution of air flow to different aeration basins and diffuser groups isachieved by an appropriate grading of flow velocities. Smaller flow velocitiesand larger pipe diameter also reduce vibration and noise.

Air flow velocity can be calculated using Equation 13 which corrects thepressure and temperature change caused by the compression of air:

Equation 13

v qD

T

T

P

PaD

p

s

s= •• •

• •4

360022π




wherev air velocity m/sqa air intake rate m3/hDD diameter of pipe mTp average temperature in distribution pipe KTs air temperature under standard conditions KP2 blower discharge pressure kPaPs atmospheric pressure under standard conditions kPa

The head losses in the pipework can be taken from nomograms or calculatedusing the Darcy-Weisbach equation.

7.7.3 Air Filtering System for NOPOL® DDS

The purpose of air filtration is to avoid internal clogging of the diffusers andexcessive wear to both blowers and piping.

Fine bubble diffusers have a specific need for air filtration. The pores in theporous discs and rubber membranes are very small, ranging from a fewmicrons to a hundred microns. Therefore, in order to avoid even partialclogging, with a consequent loss of performance and increase in backpressure, filtration should always be included in the air production plant.

Feed air must be conducted through a fine filter. In order to reduce the loadon fine filters and to prolong their life, coarse filters should always be used aspre-treatment before fine filtering.

There are many types of filters available on the market, but here is acombination suggested for normal cases:

For MKL 215 and HKL 215:Coarse filter G3 CEN Class (European Committee for Standardisation) withan efficiency of 38 – 42 % at 2.0 - 3.0 µm particle, and fine filter F8 CENClass with an efficiency of 95 - 99 % at 0.75 - 1.0 µm particle.

For PIK 300, KKI 215 and PRK 300:Coarse filter as above. Fine filter F6 (35 – 40 % at 0.75 - 1.0 µm particle and80 - 85 % at 2.0 - 3.0 µm).

If the intake air contains excessive amounts of harmful gases like SO2, Cl2 orH2O, it may be necessary to complement the air filtering system, for instance




with an active carbon filter. In special cases the manufacturer of the diffusersshould be consulted.

Note that the normal blower intake filters are for the protection of the bloweralone, and generally not enough for the protection of the diffusers. Whenfiltration is carried out according to the above requirements, the blowersuction filters can be omitted.

Fibreglass type of filters or silencers may not be used!

The choice of filter arrangement depends on the size of the air productionplant:

• In small plants (air flow up to 2 - 3 m3/s) a simple single fixed filter unit canbe used, utilising a single housing containing both coarse and fine filters

• In large plants (> 3 - 4 m3/s) the use of separate filters should beconsidered, the fine filters being wallmounted after the coarse filters

Automatic roller filters (dry filters) have proved to be the most appropriate foruse as coarse filters. The exchange interval of the filter fabric is normally 1 - 2years. Fine filters cannot be regenerated. The exchange interval for them isnormally 0,5 - 1 year.

The following conditions should be taken into account when dimensioning thefiltration system:• icy conditions and / or a high dust concentration in the air; guide value for

initial head loss is approximately 15 mm water column (150 Pa) and forhead loss before filter change approximately 60 mm water column (60 Pa)

• recommended maximum values are the followinginitial head loss 50 mmfinal head loss 150 mm


8 NOPOL® O.K.I. Aeration System Design Page: 1.1 (25)


8 NOPOL® O.K.I. Aeration System Design8.1 NOPOL® O.K.I. Aerator Mixer.............................................................8.1

8.1.1 Scope of Delivery ........................................................................8.28.1.2 Type of Aerator............................................................................8.48.1.3 Standard Oxygen Transfer Efficiency of Aerators .......................8.58.1.4 Basin Shape................................................................................8.68.1.5 Submersion Depth.......................................................................8.68.1.6 Air Flow .......................................................................................8.68.1.7 Mixing ..........................................................................................8.7

8.2 Designing the Aeration System ..........................................................8.88.2.1 Number of Aerators .....................................................................8.88.2.2 Cable and Hose Length Determination .......................................8.8

8.2.2.1 .............................................................Process Air Hose Length 8.88.2.2.2 .................................................................. Electric Cable Length 8.98.2.2.3 .................................................................... Lifting Cable Length 8.98.2.2.4 ..........................................................Protection Air Hose Length 8.9

8.3 Upgrading of O.K.I. 1000 Series Aerators..........................................8.98.3.1 Example 1....................................................................................8.98.3.2 Example 2..................................................................................8.10

8.4 Layout Design ..................................................................................8.108.4.1 Aerator Location........................................................................8.118.4.2 Lifting Cable ..............................................................................8.128.4.3 Electric Cables and Protection Air Hose Attachments ..............8.128.4.4 Process Air Hose.......................................................................8.128.4.5 Hose Flanges ............................................................................8.158.4.6 Frequency Converter Control ....................................................8.158.4.7 Blower Air Delivery Control .......................................................8.168.4.8 Air Distribution Control ..............................................................8.16

8.5 Electric System Design.....................................................................8.178.5.1 Over Current Relay ...................................................................8.178.5.2 Motor Protection ........................................................................8.17

8.5.2.1 ..................................................................................Thermistors 8.178.5.2.2 .............................................................................. Thermal Units 8.18

8.5.3 Starting Current .........................................................................8.188.5.4 Electro-magnetic Disturbances .................................................8.18

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8 NOPOL® O.K.I. Aeration System Design




8.6 Air Distribution Design......................................................................8.188.6.1 Process Air ................................................................................8.18

8.6.1.1 .................................................................................... Flow Rate 8.188.6.1.2 ..........................................................................................Valves 8.19

8.6.2 Hood Protection Air (100 and 200 series O.K.I. aerators).........8.208.6.2.1 ..........................................................................................Valves 8.208.6.2.2 .........................................................Flow Rate and Flow Meters 8.20

8.7 Lifting System...................................................................................8.218.8 Work Safety......................................................................................8.228.9 Air Filtration ......................................................................................8.228.10 Water Separators..........................................................................8.238.11 Installation, Operation and Maintenance ......................................8.24

8.11.1 General .....................................................................................8.248.11.2 Manuals.....................................................................................8.248.11.3 Installation Supervision .............................................................8.24

8.12 Guarantees ...................................................................................8.25

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8 NOPOL® O.K.I. AERATION SYSTEM DESIGN

8.1 NOPOL® O.K.I. Aerator Mixer




8.1.1 Scope of Delivery

For the design of O.K.I. aeration system it is necessary to make differencebetween the components, which are part of the aerator, and the components,which are necessary for aeration. The components of the O.K.I. aerator canbe divided into two categories:

Preinstalled:

• Power and Signal Cables with airlocks• Protection Air Hose of EPDM rubber, only in 100- and 200-series• Lifting Cable with breaking strength of more than 100kN Separate:

• Process Air Hose of EPDM rubber• Hose flanges, hose clamps and rubber straps• Installation and Operation Manual

The difference between the traditional O.K.I. 100 or 200 series and the newO.K.I. 1000 series is in the motor and the gear reducer. The new 1000 serieshas a submersible motor and a gear reducer. O.K.I. 100 or 200 seriesaerators have a hood to protect the motor. In addition, there is a protection airhose made of EPDM rubber. The protection air system is designed topressurise the hood above the level of the surrounding hydrostatic pressure,no matter how deep O.K.I. Aerator is installed. Thanks to the fact that themotor and the gear reducer are submersible, no protection air is needed inO.K.I. 1000 series.

NOPOL® O.K.I. serves also as a mixer in normal aeration operation. Mixingwithout air supply is permitted only at the lower speed of the two-speedaerator and with O.K.I. 200 series aerators. It is not allowed to operate O.K.I.100 series aerator at high speed without air.




1 Frame

2 Rotor

3 Stator and stator ducts

4 Nameplate

5 Hose flange seal

6 Hose flange

7 Hose clamp

8 Rubber strap

9 Process air hose

10 Shaft seal

11 Drive unit (geared motor)

12 Lifting bail

13 Lifting cable

14 Power cable(s)

15 Signal cable

16 Cable support

Figure 1: Exploded view of O.K.I. 1000 series

1 Frame

2 Rotor


4 Nameplates

5 O-rings

6 Inlet and outlet valve systems

7 Cable inlet element

8 Shaft seal


10 Cooling jackets

11 Cooling fan

12 Hood

13 Lifting cable

14 Hose flange seal

15 Hose flange

16 Hose clamps

17 Expansion system

18 Rubber strap

19 Process air hose

20 Signal cable

21 Protection air hose

22 Power cable(s)

Figure 2: Exploded view of O.K.I. 100 series.





2 Rotor

3 Extension channels

4 Nameplates

5 O-rings

6 Inlet and outlet valve systems

7 Cable inlet element

8 Hose flange seal

9 Shaft seal


11 Hood

12 Lifting cable

13 Hose flange

14 Hose clamps

15 Oil expansion system

16 Rubber strap

17 Process air hose

18 Signal cable

19 Protection air hose

20 Power cable(s)

Figure 3: Exploded view of O.K.I. 200 series.

8.1.2 Type of Aerator

Before choosing the type of the O.K.I. aerator the criteria of aeration systemhave to be determined by the system designer. Together with oxygentransfer, mixing, process and process control are examples of these criteria.

NOPOL® O.K.I. 100 series is used in applications were stainless steelmaterial is required.

NOPOL® O.K.I. 1000 A models are only meant for continuous aeration. AMmodels are designed for continuous aeration and mixing. Both A and AMmodels are equipped with one speed motor. AM2 models are equipped withtwo-speed motors and can be used for continuous aeration at high speed andmixing at low speed without air.

It is advisable to follow the rule that the biggest aerator possible for the caseis chosen and the calculation is carried out accordingly. With this aerator thepossibility of designing the system is checked.




The minimum required number of the aerator is determined according tofollowing. Under any circumstances the aerators must have enough aerationand mixing capacity and all the other criteria are filled.

8.1.3 Standard Oxygen Transfer Efficiency of Aerators

SOTR curves are based on clean water test at standard conditions (+10 oC,101,3 kPa). In the curves the air flow is given in standard conditions (+20 oC,101,3 kPa). The values presented in the curves are valid for standardinstallations. The SOTR curves are in the NOPOL® O.K.I. product manual.

NOPOL® O.K.I. aerator has an active area where oxygen transfer efficiency isthe same as presented in the efficiency curves. In cases where O.K.I.aerators have to cover wider area than presented in the curves, a SOTRvalue correction factor should be used.

Unaerated area creates spiral flows in the basin and thus decreases thedetention time of the bubbles and consequently reduces SOTR values. Theinfluence of the spiral flows on the system efficiency shall be corrected bymultiplying the SOTR value with the correction factor shown in Table 1.Corrected SOTR is calculated with following formula:

SOTR = SOTRstd * k2

Table 1: Correction factors for SOTR efficiency

Unit area / Activearea

Correctioncoefficient, k2

m2 / m2

1,0 1,001,5 0,952,0 0,902,5 0,853,0 0,80

>3,5 0,75

Unit area of one O.K.I. unit is obtained by dividing the basin bottom area withnumber of aerators:

AOKI = Ab / N

In cases of tapered aeration, selectors etc. the calculations are made usingzone area and number of aerators in aeration zone.




8.1.4 Basin Shape

Aeration basin depth has major impact to Oxygen Transfer Efficiency. Deepaeration basin should be designed to reach high aeration efficiency. Theoptimum water depth for aeration with O.K.I. aerators is 6 - 12 m. In the plugflow process, the ratio between basin width and depth should beapproximately 1,0 to 2,0, which in practice means basin widths of 6 to 24 m.For process technology reasons the ratio between basin length and widthshould not exceed 15:1.

To reach optimum aeration configuration, the mixing areas of O.K.I. aeratorsshould be taken into account as well as lifting possibilities. More detailedinformation about mixing areas are presented in chapter 5.2 and informationabout lifting is in chapter 5.9.

8.1.5 Submersion Depth

The oxygenation rate can be raised by increasing the submersion depth ofthe aerators. The improved efficiency of oxygen transfer thus obtained ismainly due to the longer contact time between water and bubbles and to thehigher oxygen saturation concentration achieved. Submersion depth alsoaffects to the efficiency of the aerator, maximum and minimum air flow due tocompressibility of the air. In deeper basin actual flow in cubic meters is lowerthan shallow basins.

8.1.6 Air Flow

With NOPOL® O.K.I. aerators, air flow is expressed in cubic meters (20 oC,101,3 kPa) per minute per aerator unit m3/min.

The model of the aerator determines the appropriate range of the air flow intoaerator. Too high air flow /unit will dramatically decrease the efficiency. Alsothe risk of mechanical failure increases due to unbalanced air distribution inaerator.

The power consumption of the O.K.I. aerator increases with decreasing airflow. Running with too low air flow overloads the electrical motor and mightcause serious mechanical damage in the motor. The minimum air flow foraerators is shown in efficiency curves. AM models of the 1000-series arecapable to handle air flow down to zero without overloading the motor.




The optimum air flow / aerator depends on the water depth and compressorefficiency.

The design air flow of the aerators is a result of the number of aerators andthe required SOTR. When designing the aeration system it is not advisable touse the maximum allowable air flow rate per unit, since the limitations formaximum air flow rates are for individual aerators only. The air distributionsystem has to be designed in such a way that in any running circumstancesthe air flow rate for each O.K.I. aerator remains between the given minimumand maximum limits.

8.1.7 Mixing

In order to function properly the different processes require different mixingcapabilities in the basin. It has to be noted that the given curves for mixingcharacteristics are valid only in aeration use, where the rising air bubbleshave a remarkable mixing effect.

100-series aerators directly connected to electric net are not meant to beused without process air flow. This restricts the use of single speed O.K.I. inother circumstances than continuous aeration. When using these aeratorsthrough frequency converters it is possible to use them also for pure mixing. Itis required, however, that the speed of rotation has to be limited so that thepower consumption decreases. It has to be remembered that, due to thelower speed of rotation of the rotor, mixing capability decreases. The givenmixing characteristics are not valid any more in aeration.

Figure 4: Mixing requirement curve

100 series double speed aerators are to be used also for pure mixing, but atlower speed. At lower speed the mixing characteristics curves are not valid.

3

53

[m³]

12

Water Depth

6

Rated Power per cubic meterbasin volume15 [W/m³]




200-series single speed aerators are to be used for aeration and for mixing.Combined with frequency converter the energy consumption can beoptimised.

The maximum mixing capacity is not always needed in processes. From timeto time less mixing is sufficient to keep the process working. Double speedaerators and aerators used through frequency converters are suitable forthese purposes.

8.2 Designing the Aeration System

8.2.1 Number of Aerators

Calculation example

Design data:Water depth 8 mSOTR required 630 kg O2/hBasin dimensions 27 m x 70 m = 1890 m2

1. Shape and dimensions of the basin are known (70 x 27 x 8 m3)2. Required standard oxygen transfer is known (630 kg O2/h)3. O.K.I. 133-51 is chosen using SOTR curves4. Active area of one O.K.I. 133-51 aerator is 590 m3 (O.K.I. mixing

characteristics table)5. Minimum number of O.K.I. 133-51 aerators is calculated by dividing the

basin area by the active area of one unit. 1890 m3 / 590 m3 = 3,2 => 4units are enough for mixing

6. Required SOTR for one aerator is 630 kg O2/h / 4 = 157,5 kg O2/h7. Five (5) O.K.I. 133-51 aerators are needed to meet the SOTR required8. Required SOTR per one O.K.I. 133-51 is 630 kg O2/h / 5 = 126 kg O2/h9. 126 kg O2/h can be reached with O.K.I. 133-51 at 22 m3/min air feed10. Layout has to be finalised

8.2.2 Cable and Hose Length Determination

8.2.2.1 Process Air Hose Length

After the final layout for O.K.I. aerators has been done, the determination ofthe process air hose length has to be done carefully. Table 2 can be used asaid.




8.2.2.2 Electric Cable Length

The length of the electric cable has to be determined after the layout is ready,process air hose determination has been done and the location of the electricconnection box is known. The length of the electric cable is the distancebetween the process air hose connection and the electric connection box plusthe length of the process air hose.

8.2.2.3 Lifting Cable Length

The length of the lifting cable is recommended to be as short as possible.However, it has to be long enough to reach the attachment point at the basinedge.

8.2.2.4 Protection Air Hose Length

Protection air hose is only needed for 100 and 200 series O.K.I. aerators.The length of the protection air hose has to be determined after the layout isready, process air hose determination has been done and the location of theprotection air supply joint is known. The length of the process air hose is thedistance between the process air hose connection and the protection airsupply joint plus the length of the process air hose. The length of theprotection air hose has to be accurate in order to prevent condensation ofwater in the hose.

8.3 Upgrading of O.K.I. 1000 Series Aerators

8.3.1 Example 1

Original choice O.K.I. 1090A-18AMrated power 18,5 kWair flow 20,0 m3/minSOTR 133 kg O2/hwater depth 10 m

Upgraded aerator O.K.I. 1090B-18Arated power 18,5 kWair flow 20,0 m3/minSOTR 150 kg O2/hwater depth 10 m




What is required to achieve this?The first stage in the gear box is replaced by a new one 103 rpm => 112 rpm

8.3.2 Example 2

Original choice O.K.I. 1050A-03Arated power 3,0 kWair flow 7,5 m3/minSOTR 19 kg O2/hwater depth 7 m

Upgraded aerator O.K.I. 1050B-04Arated power 4,0 kWair flow 7,5 m3/minSOTR 26 kg O2/hwater depth 7 m

What is required to achieve this?Motor is replaced by a new one 3,0 kW => 4,0 kWThe first stage in the gear box is replaced by a new one 142 rpm => 157 rpm

8.4 Layout Design

Generally O.K.I. aerators are located in a basin so that there is adequatemixing in every spot of the basin. The aeration system has to be adapted tothe needs, because usually the needed oxygen amount in different parts ofthe basin is unequal.

The easiest way to put the aerators into a basin is to place them evenly onthe bottom. Diversified density can be advisable if the oxygen demand isbigger in some parts of the basin, like usually in the beginning of the aerationbasin. It is also, however, easy to alter the aerator density afterwards.

Uneven need for aeration is better to take into account beforehand.Otherwise, the required oxygen has to be reached for example by means ofvalves. Immoderate valve control could lead to excessive oxygen amount andthat could load the aerators excessively. Extreme amount of pumped air couldlead even to decline in aeration capacity.




Figure 5: Aerators in different layouts

If the aerator density decreases substantially in some parts of the basin, thereis a risk that the mixing capacity will not be adequate.

8.4.1 Aerator Location

When placing the aerators in the basin, sufficient space is to be left betweenthem as well as between them and the basin walls. If the aerators are tooclose one to the other or to the wall, a fall in the oxygenation capacity is to beexpected at the aerators in question. The height difference between theaerators must not exceed 50 mm in the basin.

There are, for each aerator type, minimum lead values for the distances fromthe wall and from the next aerator corresponding to the guarantee conditions.If these values cannot be respected, a consultation with the nearest NOPOL®

O.K.I. representative is recommended, in order to find an acceptable solutioncase by case.

Aerators can be placed without applying the given minimum values, but theinstallations often have to be especially modified. An excessive density cancause heavy turbulence in the basin with subsequent movement of themachines.

UNIFORM DISTRIBUTION

AERATORS DEVIDED TO SEPARATE BASINS

TAPERED SYSTEM

SELECTOR IN THE MIDDLE,AERATION BASIN AROUND IT




8.4.2 Lifting Cable

When delivered to the clients, O.K.I. aerators are fitted with a lifting cable.The steel loop at the upper end of the cable is fixed at a distance of aboutfour meters from the upper flange of the process air hose. In this way, rubbingbetween process air hose and lifting cable can be avoided. In round basins,where air distribution pipes are placed in the middle of the basin this wouldbe impossible. The lifting cables can then be attached to the outer edge ofthe basin, for example. In a case like this Nopon must be contacted.

8.4.3 Electric Cables and Protection Air Hose Attachments

All O.K.I. aerators are equipped with power and signal cables. O.K.I: 100 and200 series aerators are additionally equipped with protection air hose, whichis used to pressurise the hood of the O.K.I. aerator. Reliable attachments andsupport for the cables and hoses have to be prepared.

The cables and the hoses are bound into a bundle to the side of the processair hose by using special rubber straps (provided by Nopon Oy). With thestraps bound it is easy to carry out.

When defining the length of the cables and hoses it is important to take intoaccount that the cables and hoses are tied up all the way to the process airhose. Further, they still have to reach the terminal box, which is notnecessarily near to connection flange of the process air hose.

Protection air hose have to be cut to a right length in order to avoid thesituation where condensed water is gathered in the hose. Condensed watercould block the reliable work of pressurising valves. Too long cables andhoses must be cut to a right length on installation site.

8.4.4 Process Air Hose

Defining the length of the process air hose is an important part of the layoutdesign of any aeration system with NOPOL® O.K.I. aerators. The design ofthe air distribution pipelines of the plant is closely connected with thisdefinition. The main rule is that the process air hose is designed as short aspossible, i.e. air distribution system is to be taken as close as possible toeach aerator. In addition, a regulation valve is to be installed between eachprocess air hose and the air distribution pipe in order to enable eventualcounter pressure regulation. Sometimes a throttle valve is required.




In basins where the O.K.I. aerators are installed at more than 8 meters’depth, a special heavy-duty hose is used instead of the standard hose.Compressing the air deeper causes heating. High waste water temperatureand/or high ambient temperature also contribute to the heating. The heavyduty process air hose tolerates temperatures up to 130 °C for a long time.Both types resist temperatures down to –40 °C. In extreme situations NoponOy has to be contacted.

When the position of the connecting point and the location of the O.K.I. in thebasin have been defined, the length of the process air hose is determined.The hose must have free space to move in sewage whirls. There must not beany obstacles like the basin wall or other hoses or lifting wires in closeproximity to hose. Table 2 can be used as aid for determining the hoselength.




Table 2: Determination of process air hose length

15 16 17 18 19 20 20,5 21,5 22,5 23,5 24,5 25,5 26,5 27,5 28,5 29,5 30,5 31,5

14 15 16 17 18 19 20 20,5 21,5 22,5 23,5 24,5 25,5 26,5 27,5 28,5 29,5 30,5

13 14 15 16 17 18 19 20 20,5 21,5 22,5 23,5 24,5 25,5 26,5 27,5 28,5 29,5

12 13 14 15 16 17 18 19 20 20,5 21,5 22,5 23,5 24,5 25,5 26,5 27,5 28,5

11 12 13 14 15 16 17 18 19 20 20,5 21,5 22,5 23,5 24,5 25,5 26,5 27,5

WATERDEPTH

10 11 12 13 14 15 16 17 18 19 20 20,5 21,5 22,5 23,5 24,5 25,5 26,5

(Y) 9 10 11 12 13 14 15 16 17 18 19 20 20,5 21,5 22,5 23,5 24,5 25,5

8 9,5 10 11 12 13 14 15 16 17 18 19 20 20,5 21,5 22,5 23,5 24,5

7 8,5 9,5 10 11 12 13 14 15 16 17 18 19 20 20,5 21,5 22,5 23,5

6 7,5 8,5 9,5 10 11 12 13 14 15 16 17 18 19 20 20,5 21,5 22,5

5 6,5 7,5 8,5 9,5 10 11 12 13 14 15 16 17 18 19 20 20,5 21,5

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

DISTANCE FROM AIR CONNECTION FLANGE (X)




8.4.5 Hose Flanges

Alignment and attachments of the hose flanges and couplings should beincluded in the overall designing of the pipework. It is recommended that thehose flanges point directly towards the O.K.I. aerators This way most of thestrain and tension affecting the hose and the cables can be avoided.

O.K.I. aerator is delivered with two different kinds of hose flanges. By usingthem optimising the process air hose length and the alignment of hoseflanges can be done locally.

Figure 6: Recommended installation of hose flanges

8.4.6 Frequency Converter Control

One way to control O.K.I. aeration system is to use frequency converter.Frequency converters can be used together with aerators or with plant airblowers. In the control system of the process, the dissolved oxygen ismeasured in the basin. It is advisable to use more than one probe, since theprobes are rather sensitive for malfunction. The information from dissolvedoxygen probe is taken to processor, which controls the speed of the rotationof the aerator rotor or the speed of blower.

The simplest way to use frequency converter is to use it for adjusting thespeed of blower. By doing this the air feed is controlled all around the basin.

A more complicated and more expensive solution is to control the speed ofthe O.K.I. rotor rotation. The installation of a frequency converter is advisableonly in the case that the aerator really needs the speed adjustment. The mainrule for controlling the speed of the O.K.I. rotor rotation is to only reduce it in




order to avoid exceeding the maximum power allowed of the particular O.K.I.It is also possible to increase the speed of rotor rotation. To do this firstconsult the local aerator supplier or Nopon Oy.

When designing the process where the O.K.I. aerator is used through afrequency converter, it has to be noted that the oxygen transfer is reducedwhen reducing the speed of rotation. A 5 % decrease in speed reduces thepower consumption of O.K.I. aerator by 15 % and the oxygen transfercapacity by 5 - 15 % depending on the water depth and air flow rate.

In case the plant has more than one blower connected to plant air manifold, itis beneficial to control the process by switching the blower on/off and byusing one of the blowers through a frequency converter. Furthermore, if theO.K.I. aerators are also operated through the frequency converter, the speedof rotation can be controlled as well.

If wished, a one-speed aerator can be used without process air if a frequencyconverter is used

Use of frequency converters can also be motivated when regulation /optimisation of aeration is required.

8.4.7 Blower Air Delivery Control

In the previous section controlling the process by frequency converter wasdescribed. A more inexpensive, easy and rough way to control O.K.I. aerationsystem is in case of several blowers to control the number of blowers inoperation. The feedback from dissolved oxygen probes controls the blowersso that the right number of blowers is in operation. In addition, some of theO.K.I. can be stopped when air flow rate is reduced. It has to be noted thatthe process might need all the mixing capacity of the aerators.

8.4.8 Air Distribution Control

The control system of the process can be implemented manually orautomatically via feed back from the dissolved oxygen probes. The control ofthe air flow rate to each O.K.I. is done by means of the process air valves inline. Stepless valve is the most suitable for this purpose.




8.5 Electric System Design

The purpose of this chapter is to clarify those points of the electric plan thatare important to know already in the design phase.

O.K.I. aerator is delivered to customer without any connecting plugs due tovarying national standards and wishes of the customers. Cable-ends aresealed with plastic foil. Cable terminal boxes are not included in the delivery.

8.5.1 Over Current Relay

The electric system to which the O.K.I. aerators are connected has to containan excess current relay, which is regulated to go off at a level of 90 % currentof the nominal effect of the gear motor.

The task of the over current relay is to protect the electric motor of the O.K.I.aerator from overload and eventual damage caused by overload. The O.K.I.installation, operation and maintenance manual gives the limit values for theexcess current relay separately for each aerator type.

8.5.2 Motor Protection

O.K.I. aerators are equipped with winding protection devices embedded ineach winding. They are connected in series. 100 and 200 series models areprotected with thermistors and 1000 series models with thermal units.

These devices as such do not protect the electric motor of the O.K.I. aerator,but they have to be connected by means of a secondary contactor to the maincontactor, connected to the electric feed of the electric motor of the aerator.

8.5.2.1 Thermistors

Excessive temperature in any of the thermistors causes a notable increase ofthe total resistance in the thermistors. When the temperature exceeds thedetermined limit temperature and the thermistors are connected in thesystem, increase of the resistance makes, via the main contactor, the aeratorstop.




8.5.2.2 Thermal Units

These are mechanical switches, which operate exactly in the same way asthermistors, but they are based on mechanical function. They operate with aconstant pressure up to the switching point. Then at the present switchingpoint the mechanically and electrically unstressed bimetallic disc snaps overand via the main contactor makes the aerator stop.

8.5.3 Starting Current

When designing the electric system it is to be noted that the current requiredwhen starting an O.K.I. aerator is about 6 to 7 times more than the nominalcurrent of the electric motor of the aerator. This imposes certain requirementsfor example for the choice of the fuses.

8.5.4 Electro-magnetic Disturbances

During the operation of the O.K.I. aerators it is to be ensured that the aeratordoes not cause electro-magnetic disturbances in the ambient or the ambientfor the operation of O.K.I. Environmental standards determine the tolerances.

8.6 Air Distribution Design

8.6.1 Process Air

One or several blowers can produce the required process air. It is advisableto design the system so that more than one blower is used. Then the systemis more reliable against defects on blowers. When using only one big blowerthe efficiency of air production is higher compared to two smaller units at fullload.

By using at least two blowers a very simple control system can be achieved.

8.6.1.1 Flow Rate

Each type of O.K.I. aerator has its own air flow range. The range isdependent on the depth of the basin. When designing the O.K.I. aerationsystem the air flow ranges have to be taken into account both in normaloperation and in maintenance.




O.K.I. aeration system should not be designed by using the allowed maximumair flow rate per aerator. In the following the obvious problems have beendescribed, if the maximum air flow rate has been used in design work.

Very often in order to control air distribution between the aerators, process airvalves are used. When using valve controlled process air distribution themaximum air flow rates can be easily exceeded. On the other hand inaeration system, where only few aerators are installed, the maximum allowedair flow rate is exceeded during the maintenance work of O.K.I. aerator. If themaximum air flow rate has been used in designing the process, the control ofthe process is difficult, and at high waste water load can result in air flowincrease. As the form of the SOTR curve indicates, the required oxygen levelis not reached by increasing the air flow. The upper limit of air flow rate ineach O.K.I. is at the level where increase of air does not significantly affectthe oxygen transfer rate. The increase of air cause only more vibration andthe turbulence on the water surface will increase. This will in turn increase theforces against process air hoses.

8.6.1.2 Valves

Very common and simple way to control the oxygen level in different parts ofthe basin is to open and close the process air valves connected to eachO.K.I. aerator. When choosing the valve it has to be noted that the valve hasto be stepless.

Figure 7: Typical ball valve characteristics

Flow Range

100%

100%Opening Position




8.6.2 Hood Protection Air (100 and 200 series O.K.I. aerators)

O.K.I. aerators of the 100 and 200 series, when submerged, need in additionto process air, protection air for the hood. The protection air is needed tokeep the pressure in the hood just above the surrounding hydrostaticpressure. For protection air supply there are two different possibilities.

1. Protection air is taken from the plant air manifold, the same pipeline whereprocess air is flowing. The required pressure is always at the right level.This solution is suitable for the aeration systems where process air iscontinuously on. The hood has protection air supply all the time.

2. Protection air can be produced by a separate compressor. The air isdistributed through separate pipelines to each O.K.I. aerator. Theadvantage of the system is that when stopping the process air hose the airflow inside the hood is always secured. In this alternative the pressurereduction valve is needed in case the supply pressure might exceed thesurrounding hydrostatic pressure by 0,1 bar.

8.6.2.1 Valves

Protection air supply joint to each O.K.I. is advised to be equipped with shutoff valve. The control of the protection air flow can be done by the valve,especially when using separate protection air supply. If protection air is takenfrom the plant air manifold the control of the air flow is not needed. There is,however, an advantage in using the valve in this case also, since the air feedto one individual O.K.I. can be shut off for the maintenance work.

8.6.2.2 Flow Rate and Flow Meters

Since the purpose of protection air is only to maintain the pressure in thehood just above the surrounding pressure, the air amount needed is not high.Protection air is also cooling the air inside the hood.

Too high an air flow rate might damage the non-return valve in the hood andcause air to escape the hood in case the protection air is temporarily not inuse. In normal use the required protection air feed is about 10 l/min peraerator.

Depending on the application, it can be advisable to equip each protection airjoint with a flow meter. Especially the systems where separate protection air




supply is used, the protection air flow might exceed the recommended air flowrate to aerator and therefore flow meters are needed.

8.7 Lifting System

When designing the sewage plant the space for lifting system have to betaken into consideration. In practice the size and the depth of the basindefines the size of the crane needed.

Lifting height is defined by the lifting cable and by the height of the outerbasin wall. In addition, the height of the O.K.I. has to take into account. Themost common way to lift the O.K.I. is to use mobile crane. The greater liftingrange is in question the heavier lifting equipment is needed. The weight andthe size of the crane are issues, which have to be included in designingmaintenance roads and lifting areas around the basin.

It is possible to use stationary crane above the basin. In that case adequatespace for lifting the O.K.I. over the basin wall and the site where to lower theO.K.I. outside the basin must be provided.

When transport distances in the basin area are long, it is possible to useremovable float (Nopon Oy will not provide). The O.K.I. is first lifted onto afloat, which is used to transport the O.K.I. to point where it can be lifted out ofthe basin with a lighter crane.

Points to be taken into consideration when lifting the O.K.I.:

n Lifting capacity of the crane has to exceed the total weight of anO.K.I. (2,5 t is adequate for every case)

n When designing the lifting procedure, note that the cables and thehoses must not be subject for chafing nor tension caused bypipework and structures around the basin area.

n Lifting range of the crane, so that all aerators can be placed intotheir positions in basin.

n The space which an O.K.I. needs as a temporary standing placeoutside the basin.

n If possible, the crane should be placed so that as many aerators aspossible could be placed from one crane position.




8.8 Work Safety

It is necessary to pay attention to safety aspects when designing the basinstructures. Aerator control and maintenance procedures must be carriedthrough without any safety risks.

Basin structures must be designed so that maintenance personnel does nothave to hang above the aeration basin in operation when carrying outinstallation or maintenance procedures, especially not when they are doingelectric and hose connections. Hose flanges or electric connections shouldnever be located outside a service bridge.

When upgrading the aeration system with O.K.I. aerators, it sometimesrequires renovations of the service bridges or the pipework, etc. Pleasecontact Nopon Oy for further information if necessary.

National regulations with the electrical installations must be respected.

8.9 Air Filtration

Cleanliness of the air supplied to the aerators is affected by cleanliness of thesuction air, filtration of air and cleanliness of the air pipes. Suction air shouldbe free from particles such as dust, leaves, etc. and drops of liquids.

The blowers need certain cleanliness of the air they are transmitting.Filtration according to EU5 standards are enough for most of the blower types(this has to be checked from the manufacture of each blower used).

The same filtration rate is enough for NOPOL O.K.I. aerator mixers’ processair. In process air, particles smaller than 1 mm in diameter do not cause anyproblem. The process air has to be free of abrasive material causing erosionof the piping and components of the aerator.

The requirements for the protection air of O.K.I. 100 and 200 series arestricter. Foreign particles may stick into the non-return valves of theprotection air system. Untight non-return valves may risk the aerator to leakwater into the hood. For protection air filtration rate of EU5 is required.

With capacities of up to about 10.000 m3/h it is often appropriate to provideeach blower with a separate filter-equipped air intake directly from the outsideair. At large plants a special air intake room should be constructed andequipped with soundproofing.




8.10 Water Separators

Protection air fed into the pressurising system of a NOPOL O.K.I. aerator ofaerator mixer must not contain condensed water. Water may cause excessivecorrosion inside the unit.

Condensed water has to be removed from protection air before connection ofthe protection air hose. If protection air is taken directly from the process airpipe protection air connection should be upwards. If the process airconnections are downwards, condensed water is removed with process air.

Figure 8: Protection air taken from process air pipe

In case there is a separate pipe for the protection air, a water separatorshould be installed. The separator must be situated before the protection airconnections of the aerators. A water trap is recommended to collect thecondensed water of the protection air.

Figure 9: Water separator installed to water trap of separate protection airpipe

Process air containing condensed water does not affect the operation of aNOPOL O.K.I. aerator.




8.11 Installation, Operation and Maintenance

8.11.1 General

The NOPOL® O.K.I. aerator is easy and fast to install. When done correctlyand when using correct equipment a small crew can install several aeratorsper day. This also makes the installation less costly.

The NOPOL® O.K.I. aerators can be installed in basins with a variety ofconstructions. As the units are not mounted to the bottom, the installation ispossible also in basins with bottom made of gravel or other porous material.However, there are requirements on sturdiness, thickness, etc. The correctinstallation of all aeration equipment is essential.

Whereas the airtightness is the crucial question in most other installationsand therefore careful tests have to be performed, in the case of O.K.I.aerators this has been pre-tested already at the factory before the aeratorhas been shipped. Therefore the NOPOL® O.K.I. is ready for use as shipped.

8.11.2 Manuals

Detailed instructions for installation, operation and maintenance of theNOPOL® O.K.I. aerators are presented in the manuals, which are deliveredwith every delivery of NOPOL® O.K.I. aerators. Also a video describing theinstallation is delivered with every delivery. Following these instructions anyorganisation, familiar with the kind of environment that a treatment plant is,can make the installation safely and correctly. If the operation manual isadhered to, even first time operators of a treatment plant can start theoperation. In many cases it is though advisable to arrange training. Nopon Oycan make a proposal for training program and offer for its possible part ifrequested to. Maintenance of a plant equipped with NOPOL®O.K.I. aeratorsis likewise relatively easy. Without emptying the basin hoisting the machinesfrom the basin and following the service instructions the aerators can beserviced without interrupting the process.

8.11.3 Installation Supervision

In order to secure that the installation has been done properly Nopon hascreated easy to use forms which the installation crew has to fill in during theinstallation. Following these instructions and filling in these forms safe andcorrect installation is secured. In some cases it wise to buy installation




supervision from Nopon Oy. The supervision is an additional security that theinstallation is done properly and that the operation can be started safely.Sometimes performance tests are also done. It is normally cost-efficient toperform the installation supervision and the performance tests during sametime especially if tests are purchased from outside source.

8.12 Guarantees

According to the General Guarantee Terms for NOPOL® Aeration Systems,the Seller’s responsibility covers defects occurring within two years from thetaking into operation of the aeration system, however, not later than after 30months from the delivery.

It is the Purchaser’s responsibility to take care that the installation andoperation of the equipment is carried out in accordance with the Seller’sinstructions. Guarantee terms are valid only under the following conditions

1. The evenness of air distribution has to be checked. If unevenness occursnecessary measures have to be undertaken to secure even distribution.

2. The guarantee is valid only if the installation and operation of theequipment is carried out according to the Seller's instructions and theInstallation Supervision Document which is included in the installationinstructions, has been returned to the Seller, duly signed by the person incharge of the assembly.

3. After the equipment has been taken into operation, a data log has to bemaintained showing the operational conditions for the full operatingperiod. The data log shall include at least following: quantity of air used,pressure loss of the aeration system and quantification of failures in theair supply. The Seller may at reasonable time and forewarning withoutcausing unreasonable work for Purchaser review the data records.

The aeration efficiency of O.K.I. aerators is guaranteed in clean water onstandard conditions. Standard temperature for water is 10 °C and for processair 20 °C. Standard pressure for process air is 101,3 kPa. SOTR values arecalculated according to ASCE (1984) standard. SOTR guarantee is valid onlyif installation has been done according the instructions of the manufacturer.



Written by: MR Inspected by: Accepted by:


9.1 Design of the Activated Sludge Plant for the Pulp and PaperIndustry

9.2 Waste Water Treatment in Pulp and Paper Industry

arto


NOPON OY Design of the activated sludge plant for the pulp and paper industry Date: 09.07.1998

Written by: MR Inspected by: Accepted by: Page: 1 (23)

DESIGN OF THE ACTIVATED SLUDGE PLANTFOR THE PULP AND PAPER INDUSTRY

ENVIRO DATA OYEngineering & Consultants



1 INTRODUCTION

The pulp and paper production affects the environment in a number of ways.Pollutants are discharged into the air and water, noise is generated and themill buildings and stacks constitute a feature of the landscape.

The air and water pollutants originating from the wood components and fromthe escaping process chemicals may be formed process discharges.

Most of the solid water pollutants from pulp and paper mills consist of fibersfrom the process. There are also inorganic salts, bark and wood fragments,mineral fillers etc. these suspended solids are removed from the effluentsusing sedimentation flotation and filtration processes. The common name forthem is the primary treatment.

The dissolved water pollutants from the process can be classified into easilyand slowly biodegradable materials. There are also some colouredcompounds, toxic material and salts.

Part of the material emitted from pulp and paper mills is rapidly degraded bythe micro-organisms in the water. In the biological degradation of organicmaterial dissolved oxygen is consumed. In extremely cases this can lead tooxygen depletion, which severely affects aquatic life.

Biological destruction of the dissolved organic compounds is also widely usedfor the purification of the pulp and paper mill effluents.

There are several biological methods and probably the most widely used ofthem are:

- aerated lagoons- trickling filters and biodisc filters- activated sludge- several anaerobic methods

The fundamental characteristics of the different biological treatment methodsare approximately the same. However, the basic design and facilities used fordifferent methods differ greatly. In this paper the activated sludge process isdiscussed in details. This method utilises an active mass of flocculent micro-organisms to convert organic matter aerobically into cellular material. It can beefficiently separated from its suspending liquid by physical processes. Wastewater and micro-organisms are aerated in an aeration basin using nowadaysmostly diffused aeration.

The activated sludge process is widely used for many waste waters. However,there are remarkable differences in design when the method is used fordifferent kind of effluents. The design of the activated sludge plant for the pulpand paper mill effluents are discussed in this paper.



2 EFFLUENT LOADINGS

The main raw material for pulp is normally plant fibres. A constantly increasingpart of these fibres is recycled. Most of the pulp is still produced fromtemperature zone softwoods but the utilisation of hardwoods has increasedrapidly over the last decade. Today approximately 40 % of total amount ofwood used for industrial purposes is hardwood. Another source for rawmaterials is recycled paper which in 1985 made up about 30 % of the totalfiber consumption. Since that the consumption of recycled paper hasincreased little faster than the consumption of virgin fibres.

Non wood fibers (bagasse, bamboo, straw etc) in 1985 supplied less than 7 %of the total fibre consumption. However, in certain regions non-wood fibresconstitute an important raw material for the pulp and paper industry.

2.1 Chemical Composition of Fibrous Raw Materials

Wood substance is essentially composed of carbon, hydrogen and oxygen.There are also various amounts of inorganic constituents (ash). Theelementary composition of the organic part of dry wood, with small variations,is about:

- carbon 50 %- hydrogen 6 %- oxygen 44 %

The composition of annual plants (grass etc) is very similar, the carboncontent being a little lower and the oxygen content a little higher than thevalues shown above.

The wood and plant fibres consist of the following main components:cellulose, hemicelluloses, lignin, extractives and inorganics (see Table 2.1).

The distribution of these components varies between plant and wood species.There are also differences within the same species because of variation in thecomposition of the soil.



Table 2.1 Chemical character and content in woods plants of fibre components 1.

Fibre component Approximate compo- Approximate degree Degree of Aproximate content of :sition of base of polymerization DP crystallization soft- hard- annual plants

molecules woods % woods% ( straw, bagasse ) %

cellulose 1000-10 000 crystalline 35-45 40-50 30-40

hemicellulose 50 - 500 amorphous 25-30 20-40 20-35

holocellulose - - - 65-70 60-80 70-80

lignin 5 - 100 amorphous 26-33 19-40 12-20

extractives terpenes 1 - 3 - 2-4 1-5 1-3

resin acids

fatty acids

phytosterols

inorganics alkali carbonates silica - - 0.2-0.6 0.2-4 2-18

( ashcontent )

C H O6 12 6

C H O6 12 6

C H O (OCH )9 9 2 3

C H O5 10 5

(C H O etc )18 32 2

(C H O etc )29 50

(C H ) ,etc5 8 n

(C H O etc )20 30 2



If the species are classified into three groups: Softwoods, hardwoods and annualplants, it is found that the quantitative distribution of five component groups fallswithin approximate limits indicated in Table 2.2.

Table 2.2 Chemical composition of fibrous raw material1.

Group Raw cellulose lingnin hot water alcoholbenzen ash

material soluble soluble

Grasses Cerealstraw 34 - 40 16 - 20 9 - 15 2 - 5 4 - 11ricestraw 28 - 41 10 - 17 13 - 17 1 - 7 14 - 22bamboo 35 - 47 22 - 30 16 - 21 3 - 6 1 - 5

soft- Norwegian

woods spruce 43 29 2.3 1.8 0.4Scotspine 43 29 3.9 4.8 0.4Southernpines 41 - 44 25 - 28 2.5 - 2.4 2.5 - 3.6 0.2

hard- Americanwoods aspen 52 19 2.1 2.9 0.4

Eucalyptusglobulus 47 20 2.4 1.5 0.4Birch 41 20 1.5 2.8 0.4Gmelinaarborea 46 na 6 4 0.8

Before making paper and board, the fibrous raw material must be pulped. There areseveral different types of pulping methods used for making different kind of pulps.

Chemical and dissolving pulps are both of the chemical type, which means that thefibres are liberated by chemical dissolution of the lignin middle lamella. Thus nochemical action in refinors or beaters is required for fibre separation.

Semichemical pulps are treated by chemical pulping process followed by a treatmentin mechanical fiberising equipment. When the cooking yield of the semichemical pulpis as high as 85 - 95 %, the term chemimechanical pulp is used.

It is also possible to make pulps using only mechanical means.

There are no well-defined yield limits between various groups of pulps. The followingtable gives a rough indication of the yields for the main pulp groups.



Table 2.3 The yield for the main pulp groups1.

Pulp group Pulp yield%

Processexamples

mechanical 90 - 97 GWD, TMP, RMPchemimechanical 85 - 95 CTMP, Cold sodasemimechanical 60 - 85 NSSCchemical 40 - 60 kraft, sulphitedissolving 30 - 45 kraft, sulphite

For the NSSC, Kraft and sulphite pulping methods the chemical recovery is commonpractice.

2.2 Effluent Loadings of Some Pulping and Paper Making Methods

The aim of this study is to describe the design of the activated sludge plant for thepulp and paper industry.

This biological treatment of wastewater is primarily to reduce the content of solubleorganic compounds.

The content of soluble organic compounds is expressed using effluent parameterslike BOD (biological oxygen demand) and COD (chemical oxygen demand). The useof these parameters is based on the great importance of oxygen level in the receivingwater.

The effluent loadings from different pulping and paper making processes greatly rawmaterial, the process type and the equipment used. For the design of the activatedsludge process a very important part of the work is the gathering of the effluentloading data. The following kind information will be needed:

- monthly averages- weekly averages- daily averages- daily minimum and maximum valuesfor flow data also hourly maximum and minimum values

This kind of information is possible to collect from existing plants but it is also neededfor the Greenfield mills. Many serious mistakes have been made especiallyconcerning the short time minimum and maximum values.

The major part of the suspended solids (SS) in the mill effluents usually consists offibres or fibre particles.

Fibres are detrimental because they tend to settle in the receiving water forming fibrebanks in which fermentation may occur. This may cause oxygen depletion for the



decomposing. Mechanical pulp fibres decompose slowly while chemical fibresdecompose more rapidly.

The pulp mill effluents also contain suspended solids like lime mud and the paper milleffluents various kind of fillers like china clay, talc etc.

The contents of easily biodegradable compounds are usually measured by the BODtest. A considerable part of the wood components dissolved in the pulping andbelching processes is easily biodegradable. Examples of such compounds are lowmolecular (molecular weight less than 700 daltons) hemicelluloses, methanol, aceticacid, formic acid, sugars etc.

Slowly biodegradable compounds in the mill effluents mainly consist of highmolecular substances (molecular weight over 1000 daltons) of lignin andcarbohydrate origin the amount of such compounds can be estimated by measuringthe COD-value and subtracting the BOD-value from it. Living organisms can slowlydegrade and also absorb them. The may also cause to living organisms biologicalchanges.

There has been also noticed some toxic effects of the pulp and paper mill effluents.The most well known toxic compound is resin acids and also some unsaturated fattyacids. Much work has also been done to evaluate the toxic effects of the bleach planteffluents.

The discharges from the pulp industry may have pH-changing effects in the receivingwaters.

Dissolved inorganic salts from the pulp and paper processes are usually harmless toaquatic life. However the salts containing nitrogen and phosphorus act as fertilizers inthe recipient. They must also be added during the activated sludge treatment.

The pulp and paper making processes and the effluent loadings from the subprocesses will not be discussed in details. For giving an idea and summarising someloading example are given there is also the Table 2.4 concerning the present totaleffluent loadings of some pulp and paper making processes.

Before cooking or refining the fibrous raw material it is prepared in a section of themill called the wood room, barking house etc. The effluent loadings from the dry andwet debarking of Scandinavian softwoods and hardwoods are given in Table 2.4

Table 2.4. Effluent loadings from dry and wet debarking of Scandinavian soft- andhardwoods. Figures kg / tonne dry wood.

Debarking BOD COD SS

Softwoods ( dry ) 0-2 0-5 0-5Softwoods ( wet ) 1-5 5-10 2-5Hardwoods ( wet ) 1-5 7-15 2-7



Figures are for the effluents after good mechanical screening. Sedimentation cangreatly decrease these figures.

The environmental effects from mechanical pulping are mainly limited to effects ofdissolved organic substance and fibres. There is a certain BOD-value and theeffluents are normally also toxic to fish.

The bleaching of the mechanical pulp especially with the peroxide greatly increasesthe effluent loading.

Table 2.5 gives a summary of discharges from ground-wood and TMP process lines.

Table 2.5 Effluent loadings from ground-wood and TMP- processes. Figures are inkg / tonne air dry pulp.

Process Rawmaterial

BOD COD SS

Groundwood spruce 10 - 20 20 - 50 10 - 30TMP spruce 15 - 30 25 - 60 10 - 30

Sedimentation decreases dissolved and solid materials.

The loading of the mechanical pulps greatly also depends on the yield.The type of fibrous raw material has only little influence on the COD and also on theBOD values (see Figure 1).



Figure 1. Chemical oxygen demand as a function of pulp yield.

In chemical pulping the fibers are liberated by breaking down and dissolving thelignin by chemical reactions semichemical pulping (NSSC) is characterised by acombination of chemical and mechanical attack.

In mechanical and semimechanical pulping a recovery system for the chemicals isnormally used. Most of the effluent loading is coming from the bleaching of the pulp.

A number of in-plant technologies are available to reduce the discharge to water fromthe cooking-washing-screening section of the mills. Likewise the pollution due tocondensates and accidental spills can be reduced efficiently. When these measureshave been carried out the bleach plant effluent is the dominating source of pollution(Table 2.6).

Rel

ease

d C

OD

kg

/ dry

pul

p

Microscale cooking - cold soda Alder Beech Eucalyptus Persimmon Spruce

Laboratory - scale Gmelina - cold soda Mixed hardwoods - bisulfite

Adjustment to a straight line produces at all pointsCOD = 1.365 * TS - 9.4 r = 0.997For microscale cooking alone(the line in the figure):COD = 1.296 * TS + 0.36 r = 0.9991For labrotary-scale cooking aloneCOD = 1.369 * TS - 7.7 r = 0.994



Table 2.6 The discharges (in kg / t air dry pulp) to the water from different milldepartments. Softwood sulphate pulping.

Source of discharges BOD

42163

Cooking-washing-screeningCondensatesBleachingAccidental spills

25

There are, however, available measures for the decreasing the effluent loading fromthe bleach. Basically there are three ways to reduce the bleach plant discharge:

- using of the more effective delignifying methods like cooking and theoxygen bleaching

- external treatment of the bleach plant effluents (ultrafiltration, ligninremoval process LRP etc.)

- use of bleaching chemicals which give rise to less polluting material

Some of these methods are also widely used (especially belonging to the firstcategory) and the effluent loading from the bleaching have been greatly reduced.

Wastepaper is an increasingly important raw material for production of newsprint,tissues and some printing and writing papers.

The greatest portion of wastepaper is not chemically treated but is pulpedmechanically. Processing with chemicals (deinking) is, however, necessary fornewsprint and writing paper.

The important distinction between these systems is, that in deinking some of thefillers ink particles, hot melts etc. are removed. The effluent loading is then alsohigher. If bleaching is used for these fibers the effluent loading will increase more(see Table 2.7).

Table 2.7 Approximate BOD, COD and SS ( in kg / ton air dry pulp ) in pulping ofwaste paper including deinking and bleaching.

TYPES OF TREATMENT BOD COD SS

Mechanical pulping only 15 40 50

Pulping including deinking 20-40 50-90 150-200

Bleaching 10-20 20-40 -



Papermaking needs large quantities of water. However, the volume of pollutedeffluent varies from mill to mill with the degree of white water system closure. Thepollutants consists of suspended solids (fibers, inorganic fillers) as well as dissolvedsubstances (dissolved wood components, papermaking additives etc.). Generally themain part of the dissolved wood origin substances is generated in the pulp mill andfollows the pulp to the paper mill.

Pulp drying is less water consuming and less polluting than paper making.

For summarising the BOD-loading the Table 2.8 has been made.

Table 2.8 The typical water consumption and effluent loading for some pulp andpaper mills. Figures kg or m3 / tone air dry products before mechanical treatment.

EffluentVolume

SS BOD

Kraft pulp, unbleached 40 - 60 10 - 20 8 - 20Kraft pulp, bleached 50 - 80 10-40 20-40Kraft pulp, bleached ( O2 –delignification ) 50 – 80 10 - 40 12 - 18Sulphite pulp, unbleached ( Ca ) 80 - 100 20 - 50 30 - 70Sulphite pulp, bleached ( Ca ) 150 - 180 20 - 60 45 - 85Groundwood 10 - 15 10 - 30 8 - 14RPM 8 - 15 10 - 40 12 - 18TMP, unbleached 10 - 30 10 - 40 15 - 25TMP, bleached 10 - 30 10 - 40 20 - 30Chemimechanical (NSSC) 30 - 60 15 - 50 10 - 25Newsprint 20 - 30 8 - 20 2 - 4Magazine paper 20 - 30 10 - 20 2 - 4Woodfree printing paper 30 - 50 12 - 25 3 - 6Kraft paper 10 - 20 8 - 15 1 - 3Folding board 20 - 30 2 - 8 2 - 5Liner board 10 - 20 10 - 25 1 - 3



3 COMPREHENSIVE ACTIVATED SLUDGE PROCESS DESIGN

The activated sludge process is a continuous system in which aerobic biologicalgrowth are mixed with wastewater, then separated and concentrated in a gravityclarifier. The relationships that govern the activated sludge process are2 ,3.

cellsOrganics + a´O2 + N +P new cells + CO2 + H2O +

non-degradable soluble residue

Cells + b´O2 CO2 + H2O + N + P + Non degradable cellular residue

The parameter necessary to generate a process design are the fraction of organicsremoved oxidised for energy, denoted by the coefficient a´, the fraction of organicsand synthesised into biomass, denoted by the coefficient a, a reaction rate coefficientK, and the rate of endogenous oxidation b. These coefficients are obtained fromliterature, from experience elsewhere, or from laboratory or pilot plant studies onspecific wastewaters. The design parameters employed in this chapter are shown inFigure 2.

Flow = Q AERATION BASIN FLOW = Q+R FLOW = Q VOLUME = V

BOD = oS MLVSS = XV BOD = Se

MLSS = X VSS = Xe

RECYCLE FLOW = R VSS = ∆ XV

Figure 2. Design parameters

There are several design methods but the basic features are very similar.

The following system is originally expressed by Adams et al.

Organic removal characteristics

The overall reaction batch oxidation conditions can frequently be expressed as anexponential of the form:

Se / S° = e –KXv

t/s° ( 3 -1)

CLARIFIER



where:

Se = Influent total BOD, COD or TOC, mg/l

oS = Effluent soluble BOD, COD or TOC, mg/lXv = Average MLVSS concentration, mg/lt = Aeration time, daysK = organic removal rate coefficient, day –1

There is an increasing use of the completely mixed activated sludge system,particularly in the treatment of the industrial wastes. In this case the soluble organicconcentration in the effluent is equal to that in the aeration tank. Organic removal in acompletely-mixed reactor under steady state conditions is defined by the relationship:

)()(

ySKtX

SSSe

v

e −=−oo ( 3 – 2 )

In cases where COD or TOC is used and non-biodegradable organics are present, yrepresents the concentration of none-biodegradable organics in mg/l. For BOD data,y is usually equal to zero. Equation 3 – 2 implies that as the concentration of organicsremaining in the reactor decreases, the rate removal also degreases since theorganics remaining are progressively more difficult to remove. The reaction ratecoefficient, K has been found to be temperature dependent and can be corrected fortemperature by a coefficient, θ, as follows:

K2 = K1 θ ( T2-T

1)

Where :K2 organic removal rate coefficient at temperature, T2, Co ,day-1

K1 organic removal rate coefficient at temperature, T1, Co ,day-1

θ tempereture coefficient

Although θ usually varies from 1,02 to 1,09 for activated sludge systems, it isrecommended that θ should be experimentally defined for the specific waste water inquestion.

The performance of the activated sludge process and the characteristics of thesludge are related to the organic loading (F/M) and to the sludge age (G). Highloadings (low sludge ages) can lead to dispersed or filamentous sludges with poorsettling properties. Low loadings (high sludge ages) can result in floc oxidation anddispersion.

The organic loading is defined by the relationship:

F / M = Só / Xv t

And the sludge age by the relationship:



Where:

F / M food to micro-organism ratio, kg organics applied / kg MLVSS- dayS0 influent total BOD, COD or TOC, kg / dayXv average MLVSS in the aeration basin, kg / dayG sludge age, days

vX∆ Excess biological volatile sludge production, kg VSS/ day

The biomass solids generated in the process are composed of approximately 50 – 70percent biodegradable organic material and 30 – 50 percent non-biodegradableorganic material.

As the sludge age in the process is increased, the non-biodegradable accumulatesand the biodegradable fraction of the volatile suspended solids, x, decreases. Thebiodegradable fraction can be following relationship:

v

rvvrvr

bX

aSbXXaSbXaSx

2

)7,0)(4()( 2 −+−+= ( 3-5 )

where :X biodegradable fraction of MLVSSa sludge synthesis coefficient, kg VSS produced / kg organics removed, normal

values between 0,3 – 0,5Sr organics ( BOD, COD or TOC ) removed, kg / dayb sludge auto-oxidation coefficient, kg VSS oxidiSed /day-kg MLVSS in the

aeration basin, normal values between 0,05-0,2Xv average MLVSS in the aeration basin kg.

Nutrient requirements

The growth of the biomass and sludge settleability can be adversely affected ifnitrogen and phosphorus are not supplied in supplied in sufficient amounts. Thebiomass (volatile solids) generated in the activated sludge process usually containsapproximately 12 - 14 % nitrogen and 2,5 - 3 % phosphorus. However, as the organicloading degreases (the sludge age increases) and the biomass becomes moreendogenous, the nitrogen content will approach 5 - 7 % of the total weight of biomassand the phosphorus content, 0,8 - 1,0 %.

The nitrogen and phosphorus requirements can be estimated by the nutrient losseswith the excess bio-sludge and the effluent.

GX

Xv

v

=∆



Oxygen requirements

It has been shown previously that the total oxygen requirements in a biologicalsystem are related to the oxygen consumed to supply energy for synthesis and theoxygen consumed for endogenous respiration. This assumes that oxygen must besupplied to the system in order to:

1. provide oxygen for biological organic removal ( a´Sr );2. provide oxygen for endogenous respiration where cells lyse and release soluble

oxidizable organic compounds ( b´xXv );3. provide oxygen requirement for chemical oxidation as measured by the immediate

oxygen demand test ( Rc );4. provide oxygen for the oxidation of ammonia to nitrate ( Rn ).

This expression is : Rr = a´Sr + b´xXv + Rc + Rn

Where :

Rr total oxygen utilisation, kg O2/daya´ oxygen utilisation coefficient for synthesis, kg O2 utilisation/kg organics

removed ( normal values 0,5 - 0,9 )b´ oxygen utilisation coefficient for endogenous activities, kg O2 utilised / day –

kg MLVSS ( normal values 0,05 - 0,2 )Rc chemical oxygen demand as measured by the immediate oxygen demand

test, kg O2/day ( normal value 0,5 – 5% from the total COD )Rn oxygen utilised in the oxidation of ammonia to nitrate, kg O2/dayXv average MLVSS in the aeration basin, kg

Excess sludge production

In the activated sludge system, excess sludge must be periodically wasted. Wastedis sludge is usually digested and dewatered before final disposal. The mathematicalrelationship used to compute sludge accumulation includes the followingcomponents:

1. increase in sludge attributable to influent SS which are not degraded in theprocess ( fX1 );

2. increase in biological volatile sludge due to cellular synthesis ( aSr );3. decrease in biological volatile sludge due to cellular oxidation or endogenous

respiration ( bxXv );4. decrease in sludge due to suspended solids lost in the effluent ( Xe )

The expression for computing excess biological volatile sludge production, vX∆ , is :

vr bxXaSX −=∆ ( 3 – 10 )

The expression for computing total sludge production, X∆ , is :



ev

vx X

f

XfXX

∆=∆

where:X∆ total excess sludge production, kg SS/day

f non-biodegradable fraction of the influent suspended solidsX1 influent suspended solids, kg SS/dayfv volatile fraction of MLSS in aeration basin, MLSSXe effluent suspended solids, kg SS/day

vX∆ excess biological volatile sludge production, kg VSS/dayXv average MLVSS in the aeration basin.

It must be once again pointed out that the required design parameters, especially thekinetic parameters, must if possible be developed from comprehensive bench andsometimes also pilot scale studies. For calculating the oxygen transfer see also theappendix 1.



4 DESCRIPTION OF FACILITIES

The activated sludge plant consists of several sub-processes (bioreactor,sedimentation etc.). It also necessary to have some preliminary treatment beforepassing to the main treatment plant. The mill wastewater treatment includes normallythe following unit process and operations:

- Screening- sand trap- primary clarification or flotation- emergency spill pond- cooling system- equalization pond- nutrient addition facilities- aerated bioreactors- secondary clarification- primary and secondary sludge collection and pumping- sludge thickening and dewatering facilities- treated effluent pipeline, foam control facilities and submerged

receiving water outfall and diffuser

The screening equipment are normally medium coarse screens with spacings from10 - 25 mm. Sand removal is normally designed for sand particles with diameterbigger than 0,3 mm.

The design of the primary clarification is normally based on the surface loading andretention time. The maximum loadings are measured with the proper bench scaletestes.

The volume of emergency spill pond is normally 4 - 8 hours and it is designed as bigas possible, taking into the consideration the site layout.

The volume of the equalization pond is based on the variation of COD and BOD ofthe influent.

The design of the activated sludge plant is described more in details chapter 3.

The calculations necessary for the aeration are expressed in appendix 1. It ispossible to use surface aerators, but in many cases it is more economical and alsofrom the technical point of view more practical to use submerged aerators.

The construction materials normally used for the pulp and paper effluent are selectedaccording the construction, corrosion and economical reasons.



5 INSTRUMENTATION

The effluent treatment plant will be monitored very often by the mill distributed controlsystem (DCS) from the mill control room or from the water treatment plant controlroom. The control is often also divided so that the sludge handling will be controlledfrom the boiler house and the other parts of the plant from the effluent plant controlroom the present trend is also to develop and use “expert systems“. The kind of workhas started some years ago and the results are encouraging.

Process variables such as pH, conductivity, dissolved oxygen and temperature aremonitored throughout the plant and displayed on the DCS. A DCS l/O rack is locatedin the electrical substation.

Primary sludge, waste activated sludge to the thickening, the adding of nutrients andpolymers are all flow controlled.

The cooling system is temperature controlled.

Important control parameter for the waste activated sludge removal is also the sludgeage. A new parameter for the same purpose is also the rotation number.

The DCS provides the operator interface to remotely START / STOP pumps and toremotely OPEN / CLOSE sluice gates.

All kind of reports are also distributed through the DCS system.



6 COST ESTIMATES

The investment cost for the activated sludge plant depends on the effluent flow,BOD-loading, the necessary pre-treatment, mill site conditions etc. The variation isgreat. One example of the distribution main activities for the investment costs isfollowing:

%Civil works and underground piping 33Main equipment and piping 33Instrumentation and electrification 15

Design and project managments 9

Contingencies 10 Total 100

Figure 4 Investment costs for the activated sludge plants constructed in Finland in1980´s, the price level 1990.

To the Figure 4 has collected the investment costs for the activated sludge plants ofsome Finnish pulp and paper mills. The prices are revised with the construction indexand correspond to the price level of 1990.

10 20 30 40 50 60 70

50

100

150

200

250

Effluent loading, BOD t/d

Inve

stm

ent,

10 m

k

7



The operating and maintenance costs also depend greatly on the local conditions.One example of the percent age distribution of the operating costs is the following:

%

Energy 30Chemical 35Manpower 10

Monitoring 5Sludge transport 10Maintenance cost (1,5-2,5% from theinvestment)

10

Total 100

Operating cost for some pulp and paper industry activated sludge plants has beenshown in Figure 5.

Figure 5 The operating cost mk/t BOD7 removed. The information is based on theFinnish experience. The price level is 1990.

10 20 30 40 50 60

400

500

600

700

800

900

Ope

ratin

g co

st, m

k/t B

OD

-re

d

Effluent loading, BOD t/d7



7 TRAINING AND START-UP

The training and the start-up periods are important for the successful operating of theactivated sludge plant. It is necessary to give different kind of training for the all millpersonality. Especially effective it is naturally for the plant operators.

It has been noticed that it is most easy to learn the operating of all equipment. Moredifficult is to understand the bioprocesses and control of the plant. This part of thetraining needs special attention.

The start-up period is always difficult and it should be reserved time enough (3-4months) for this part of work. It should also be noted the limitations given by the coldseason.



Appendix 1

OXYGEN REQUIREMENTS AND THE AERATION SYSTEM

1 OXYGEN REQUIREMENTS

The oxygen requirements are normally calculated for the average influent BODmultiplied with the factor 1,1 - 1,3 and for the summer conditions.

For calculating the oxygen consumption it is possible to use following formula:

This expression is: Rr = a´Sr + b´xXv + Rc + Rn

Where:

Rr total oxygen utilisation, kg O2/daya´ oxygen utilisation coefficient for synthesis, kg O2 utilisation/kg organics

removed (normal values 0,5 - 0,9)b´ oxygen utilisation coefficient for endogenous activities, kg O2 utilised / day –

kg MLVSS (normal values 0,05 - 0,2)Rc chemical oxygen demand as measured by the immediate oxygen demand

test, kg O2/day (normal value 0,5 – 5 % from the total COD)Rn oxygen utilised in the oxidation of ammonia to nitrate, kg O2/dayXv average MLVSS in the aeration basin, kg

2 OXYGEN TRANSFER EFFICIENCY OF AERATORS AT FIELD CONDITIONS

Manufactures rate their equipment in tap water at standard atmospheric pressure,zero dissolved oxygen and 20 Co . The following equation is used to correct thestated transfer capacity for actual design conditions.

( )[ ] wTsLsw CCCNN

20

/−

−= αθβo

α ratio of oxygen transfer rate in waste water to that in clean waterβ ratio to dissolved oxygen concentration at saturation in waste water to that in

clean water (usually 0,9 to 0,95 for most waste waters)θ temperature correction coefficient: for diffused air system between 1,02-1,03Tw effluent temperatureCsw saturation oxygen concentration, temperature Tw

Cs oxygen concentration, temperature 20 Co , pressure 1 barN° oxygen transfer capacity kg O2 /h or kg O2/kW at standard conditionsN oxygen transfer capacity (temperature Tw and waste water)



It is also possible to use nomogram or other graphic calculation methods for thetransformation.

3 OXYGEN DEMAND DISTRIBUTION AND MIXING

If a plug-flow or a selector type aeration tank is used it is important to check andcalculate the oxygen demand distribution along the length of the tank. Thesecalculations should be based on the oxygen utilisation rate measurements madeduring the pilot tests. It is also possible to get valuable information from the existingfull size plants.

Very often 40 – 50 % of the total oxygen input is needed in the first 1/3 of the tankvolume. In the last 1/3 the oxygen consumption will be only 15 - 20 % of the totaloxygen consumption.

In a “real” plug-flow tank the length to width ratio (L:W) is over 1:8 often 1:10 - 1:20.

It is also important to remember the mixing requirements. Normally 10 - 30 W/m3

effect input is needed for keeping the aeration tank mixed.

1 UNEP manual Environmental Management in the Pulp and Paper Industry, volumes 1 and 22 Jörgensen E, Gromiec M (editors): Mathematical Models in Biological Waste Water Treatment,Development in Environmental Modelling, Elsevier, Amsterdam 19853 Adams C E, Eckenfelder W, Jr.: Process Design Techniques for Industrial Waste Treatment, EnviroPress, Nashville 1974

NOPON OY Waste Water Treatment in Pulp and Paper Industry Date: 09.08.1997


WASTE WATER TREATMENTIN PULP AND PAPER INDUSTRY

PREPARED FOR NOPON OY BY HEIKKI SIITONEN, DUOPLAN OY



TABLE OF CONTENTS

1 SHORT DESCRIPTION OF MAIN PULP AND PAPER MAKINGPROCESSES ................................................................................................. 1

1.1 GENERAL .............................................................................................. 11.2 PULPING PROCESSES ............................................................................. 1

1.2.1 General...................................................................................... 11.2.2 Sulphate Pulping ........................................................................ 21.2.3 Sulphite Pulping ......................................................................... 5

1.3 PAPERMAKING PROCESSES ..................................................................... 71.3.1 General...................................................................................... 71.3.2 Newsprint................................................................................... 81.3.3 LWC .......................................................................................... 91.3.4 Other Paper Grades................................................................... 91.3.5 Board Manufacturing.................................................................. 91.3.6 Effluent Loads from Paper and Board Making Processes ......... 10

2 EFFLUENT DISCHARGE REGULATIONS FOR PULP AND PAPERINDUSTRY IN EUROPE, NORTH AMERICA AND FAR EAST..................... 11

2.1 GENERAL ............................................................................................ 112.2 EUROPE .............................................................................................. 11

2.2.1 Finland..................................................................................... 112.2.2 Sweden.................................................................................... 122.2.3 Norway .................................................................................... 132.2.4 Germany.................................................................................. 132.2.5 France ..................................................................................... 152.2.6 United Kingdom........................................................................ 172.2.7 Italy.......................................................................................... 172.2.8 Spain ....................................................................................... 18

2.3 NORTH AMERICA .................................................................................. 192.3.1 The USA .................................................................................. 192.3.2 Canada .................................................................................... 21

2.4 INTERNATIONAL CONVENTIONS .............................................................. 212.4.1 The Helsinki Convention (HELCOM) ........................................ 212.4.2 Proposals of the Nordic Council of Ministers............................. 24

2.5 FAR EAST............................................................................................ 252.5.1 Japan....................................................................................... 252.5.2 China ....................................................................................... 272.5.3 Indonesia ................................................................................. 34

3 MAIN PRINCIPLES OF BIOLOGICAL OXIDATION................................... 35

3.1 GENERAL ............................................................................................ 353.2 MAJOR PHASES OF ORGANIC MATERIAL BIO-OXIDATION ........................... 353.3 MOLECULAR OXYGEN DEMAND .............................................................. 35

3.3.1 General.................................................................................... 353.3.2 Determination of AOR .............................................................. 363.3.3 Determination of SOTR............................................................ 37

ABBREVIATIONS



AD Air Dry PulpAOR actual oxygen requirementAOX adsorbable organic halidesBAT Best Available TechnologyBCT Best Conventional Pollutant Control TechnologyBMP Best Management PracticesBOD7 Biological oxygen demandBPT Best Practicable Control TechnologyCODCr chemical oxygen demandCMP chemi-mechanical pulpCTMP chemi-thermo-mechanical pulpECF elemental chlorine freeEPA Environment Protection AgencyHELCOM Helsinki ConventionISO brightnessLWC light weight coatedMLVSS mixed liquor volatile suspended solidsNSPS New Source Performance StandardsNSSC neutral sulphite semi-chemicalP.E. population equivalentPSES Pre-treatment Standards for Existing SourcesPSNS Pre-treatment Standards for New SourcesRCF recycled fiberSOTR standard oxygen transfer rateSS suspended solidsTCF total chlorine chemical freeTMP thermo-mechanical pulpTSS total suspended solidsVSS volatile suspended solids



1 SHORT DESCRIPTION OF MAIN PULP AND PAPER MAKINGPROCESSES

1.1 General

In the following, an overall description of significant current pulp and papermaking processes is presented. Also, effluent release and loads will bespecified in each process modification.

Main pulping processes consist of sulphate (or Kraft), sulphite and semi-chemical (NSSC) processes.

In a world-wide scale, main paper making processes are those producingnewsprint, fine papers (e.g. LWC, uncoated printing and writing papers) andpaperboards (e.g. packaging boards, liner and fluting). In this context, onlybasic main production phases are discussed, as the actual paper machineinstallations contain various details depending on e.g. the machine supplier.

1.2 Pulping Processes

1.2.1 General

The aim of all pulping processes is to separate or dissolve lignin derivedmaterial from raw material, wood (e.g. spruce, pine, birch, aspen, oak) or non-wood species (e.g. straw, bagasse, hemp), and release valuable cellulosefibers to be recovered and processed for further use. Various pulpingmodifications are in use and the processes are chosen mainly depending onthe type of the raw material and the quality requirements of the end product.All pulping methods consist of the following subprocesses (note. small millscan lack some operations, e.g. bleaching or recovery of cooking chemicals):

- debarking (or raw material handling)- chipping- cooking (or digesting)- washing and screening- bleaching and screening- drying- recovery of cooking chemicals

Common mill operations for all pulping modifications are briefly outlined in thefollowing.



In debarking, bark and other impurities like sand, are removed. Cutted woodlogs are fed to a rotating debarking drum, where bark is separated by thefriction of the logs and the drum wall. Fresh or recirculated water can beintroduced into the debarking drum and the process is called wet debarking.Wet debarking is needed when high quality end products, e.g. viscose pulpare produced. In dry debarking, the drum operates without water, and thedebarked logs are washed at the outlet of the debarking drum. Dry debarkingis widely used in modern pulp mills world-wide. Effluent loads from wet anddry debarking are shown in Table 1.

Table 1: Effluent loads from debarkingLoad Dry debarking Wet debarkingFlow, m3/ADt 0 - 1 1 - 3TSS, kg/ADt 0.5 - 3 2 - 5BOD7, kg/ADt 0 - 2 1 - 5CODCr, kg/ADt 0 - 5 5 - 10

In chipping, the debarked logs are chipped to small particles to be fed tocooking. Chipping can take place directly after debarking or after log storage.No process effluent exist from chipping.

Drying of pulp can be carried out by a cylinder or a fourdrinier type machine.Dry section can be steam heated cylinder or air float dryer type. Flash dryingafter mechanical dewatering, using hot gases from a separate burner systemis also a common application. Effluent amount from drying varies dependingon the type of the machinery, typical values being 2-4 m3/ADt in modern mills.

Other more specific mill operations are described in each process conceptbelow.

1.2.2 Sulphate Pulping

In sulphate (Kraft) processes, major part of organic lignin material present inraw material is separated from cellulose in heated pressurised vessels, usingalkaline sulphur containing liquor, so-called white liquor. This process iscalled cooking. Current cooking modifications consist of batch and continuoussystems. In modern mills, no process effluent exist from cooking, except floordrains.

After cooking, the pulp (or brown stock) is washed and screened. Thepurpose of washing is to clean the pulp prior to bleaching and also to recovercooking chemicals. Washing machinery consists of drums or presses inseries, also so-called diffusers are used. The aim of screening is to separate



impurities (knots, bark etc.) from pulp to maintain high quality of the endproduct. Common screening equipment are pressure screens andhydrocyclones. No effluent exist from modern washing and screening plant,except accidental floor drains.

Bleaching and post screening follows washing, when e.g. market pulps areproduced. Bleaching can be understood as a continuation stage aftercooking, where the rest of the lignin material is removed from pulp. At thesame time, chromophoric compounds, which cause the dark colour of thebrown stock, are dissolved or turned to non-colour compounds. Often inmodern Kraft mills, so called oxygen bleaching is used as a pre-bleachingstage before final bleaching. Oxygen bleaching occurs in pressurised alkalinecircumstances with molecular oxygen gas. Oxygen stage is incorporated inthe black liquor recovery cycle and thus no effluent is released from oxygenbleaching.

In modern mills, major part of a Kraft mill's effluent loads is originated fromfinal bleaching. Bleaching applications include systems which use chlorinechemicals (chlorine gas, chlorine dioxide and hypochlorites) and non-chlorinechemicals like hydrogen peroxide, ozone, peracetic acid or mixture of these.Today, bleaching systems are divided into ECF (elemental chlorine free) andTCF (total chlorine chemical free) bleaching plants. ECF bleaching meansbleaching without chlorine gas using chlorine dioxide. TCF bleachingsequence includes e.g. ozone and peroxide stages. Chlorine gas use isdecreasing dramatically e.g. in Europe, but in North American mills gaseouschlorine is still a relative common bleaching chemical. When gaseouschlorine is used in bleaching, so-called chlorinated organics are released inthe effluent. These compounds can be toxic to aquatic life depending on theirmolecular weight. Total amount of chlorinated compounds are measuredusing AOX (adsorbable organic halides) test method. Chlorinated compoundsare decomposed to some extent in aerobic waste water treatment, e.g. inactivated sludge plants. In Table 2, ECF and TCF bleaching effluents arecompared.

Table 2: Effluent loads from ECF and TCF bleachingLoad ECF1 TCF2

Flow, m3/ADt 20 - 30 10 - 15TSS, kg/ADt 2 - 5 1 - 3BOD7, kg/ADt 10 - 15 10 - 15CODCr, kg/ADt 30 - 40 20 - 30AOX, kg/ADt < 0.8 0

1 Bleaching: softwood, kappa(in) 13 - 15, sequence D-EOP-D-D, brightness 90 % ISO2 Bleaching: softwood, kappa(in) 8 - 10, sequence AZ-EOP-P, brightness 85 - 90 % ISO, norecovery of effluents



Chlorinated phenols, kg/ADt < 0.1 0Screening after bleaching typically consists of pressure and centrifugalscreens.

Recovery of cooking chemicals begins in pulp washing where the reactedlignin-containing liquor, so-called black liquor, is separated from pulp. Blackliquor is collected into storage tanks from which it is pumped to evaporationfor water removal and increasing dry solids content. After evaporation, thestrong black liquor is incinerated in the recovery boiler. The recovery boilerproduces energy for the pulp mill and recovers also the inorganic cookingchemicals (sodium and sulphur) as smelt. The smelt is dissolved in weakwhite liquor (recycled from causticizing plant) and the formed green liquor isfed to causticizing. In the causticizing department, burnt lime is added to thegreen liquor and the end product is cooking liquor, so-called white liquor.White liquor is a mixture of sodium sulphide and sodium hydroxide solution.In causticizing, burnt lime reacts with sodium carbonate in green liquor andcalcium carbonate, or lime mud precipitation occurs. Lime mud is washed,dewatered and led to lime kiln for recovery of lime. In modern Kraft mills, nocontinuous process effluent exist from the cooking chemicals recovery cycle.Momentary spills as black liquor and white liquor leakages can occur.Currently these spills are collected in a storage tank and recycled to process.

In Table 3, typical effluent loads from Kraft mill operations are shown.

Table 3: Typical effluent loads from Kraft pulping (softwood, ECF bleaching)Mill department Flow BOD7 CODCr AOX

m3/ADt kg/ADt kg/ADt kg/ADtDebarking 2 2 5 -Bleaching3 30 10 30 1Condensates4 2 2 4 -Spills 5 3 6 -TOTAL 39 17 45 1

In Figure 1, fresh water and effluent flows of a typical ECF Kraft mill arepresented.

3 Includes brown stock washing loss and drying machine4 From cooking and evaporation



Figure 1: Water recycling in bleached Kraft pulp mill

1.2.3 Sulphite Pulping

In sulphite processes, delignification or cooking takes place typically in acidiccircumstances, with sulphite and bisulphite ions present and calcium,magnesium, ammonium or sodium as base options. Also, alkaline sulphitepulping methods using e.g. antraquinone as accelerator are in use. Also,high-yield semi-chemical processes are in use, e.g. NSSC (neutral sulphitesemi-chemical) modifications.

After cooking, the pulp is washed and screened. In modern sulphite mills,washing and screening equipment are similar to those used in Kraft mills. Noprocess effluent is released from cooking, washing and screening.

Common bleaching methods of sulphite pulp are ECF (chlorine dioxide) andTCF processes. Sulphite pulp is generally easier to be bleached than Kraftpulp and modern mills use 100 % TCF bleaching sequences to reachbleached market pulp brightness. In the past, when producing e.g. viscosepulp, effluent flows and loads were huge from bleaching plant. Today,



pollution loads from sulphite bleacheries are close to the loads of respectiveKraft bleaching plants. In case of TCF bleaching, effluent loads are practicallyequal to the Kraft pulp TCF bleaching.

Recovery of cooking chemicals is possible if the base is sodium ormagnesium. The recovery systems consist of several phases and cooking liquoris prepared by absorbing sulphur dioxide to alkaline base solution. Waste pulpingliquor is evaporated and incinerated in a furnace. For different bases, possiblerecovery systems are shown in Figure 2. As an example, in case of magnesiumbase, recovery block diagram is presented in Figure 3.

Figure 2: Recovery systems



Figure 3: Recovery block diagram

Typical effluent loads from sulphite pulping are specified in Table 4.

Table 4: Typical effluent loads from modern sulphite pulping (waste liquorrecovery degree over 97 %)

Mill department Flow BOD7 CODCr AOXm3/ADt kg/ADt kg/ADt kg/ADt

Debarking 2 2 5 -Bleaching5 30 - 40 10 - 15 20 - 40 < 0.8Condensates6 4 - 8 20 - 30 40 - 60 -Spills 5 5 10 -TOTAL 41 - 55 37 - 52 75 - 115 < 0.8

1.3 Papermaking Processes

1.3.1 General

In the following, main principles of producing of various paper grades arediscussed.

Main typical waste water flows from a paper machine can be summarised asfollows:

Rejects originated from screening:Rejects contain various impurities depending on the stock preparation andpulp quality (bark, sticks, sand, inorganic suspended solids). Rejects arereleased to main sewer or dewatered separately.Excess white water:

5 Modern ECF bleaching includes washing loss. If TCF bleaching is used, effluent amount isat the same level as in Kraft pulping, 10 - 15 m3/ADt.6 BOD load caused mainly by evaporator condensates consisting of acetic acid.



In paper machine circuits, white water is needed to transport pulp andadditives to paper machine wet end. At first, white water and pulp mixtureenters the head box, from where it is distributed to the wire section. Major partof the white water is removed at the wire and final water removal occurs in thepress section. After pressing, rest of the water is evaporated from the finalproduct in the drying section.

Major part of the white water is recycled as dilution water and as cleaningshower water in the paper machine operations. Part of the shower water isfresh water, and respective amount of excess white water will be dischargedthrough a fibre recovery unit to the mill's effluent treatment system. Use offresh water, or specific waste water amount (m3/ADt), determines the degreeof closure of the white water system. The smaller the figure, the better is theclosure degree. Overflow of excess white water is responsible for the maincontinuous effluent load from a paper mill.

Spills:Temporary discharges consist of overflows from pulp and white water storageand pump tanks and floor drains. The key design principle in modern papermills is that volumes of white water and broke (low-quality paper/pulp duringpaper machine disturbances and shut-downs) storages are equal.

Main paper and board making processes are briefly described in thefollowing. Also, a summary table consisting of main pollutants is presented.

1.3.2 Newsprint

In newsprint production, raw materials are typically unbleached mechanicalpulp, semibleached Kraft pulp or unbleached sulphite or recycled fiber (RCF).

Stock preparation consists of two pulp lines with necessary fresh and whitewater systems. Mechanical pulp line includes typically pulp storage,equalising tank and post-refiner unit. Broke line has various pulpers, a brokestorage, a thickener, a screen and a deflaker.

Typical newsprint machine has wire, press and dryer sections, calender stackand reeler for finished product.

White water system is composed of dilution circuits in stock preparation and afiber recovery filter (disc filter) for all fibercontaining effluents. Continuouseffluent from a newsprint mill originates from disc filter, so-called clear filtrate.Clear filtrate contains 10 - 50 mg/l suspended solids, mainly fiber, anddissolved organic compounds.



1.3.3 LWC

Raw materials for LWC (light weight coated) paper are bleached mechanicalpulp, bleached sofwood Kraft pulp and coating chemicals (pigment, latex,starch).

Stock preparation contains two pulp lines, a broke line, necessary fresh andwhite water systems and dosing of coating chemicals.

Typical LWC machine has wire, press and dryer sections followed bycalender and reeler. Coating can be part of the LWC machine (so-called on-machine coater) or a separate unit (off-machine coater).

Coating chemicals are prepared in a coating kitchen. Coating clay and filler isdelivered as bulk material and mixed with water. The coating solution is madein batch runs. Starch is converted and dosed to the mixing tank with otheradditives.

1.3.4 Other Paper Grades

Other significant paper grades currently are uncoated printing and writingpapers (SC and MF papers), packaging papers (sack papers) and tissuegrades.

1.3.5 Board Manufacturing

Liner

Liner board is typically made of two layers, the top layer is conventional Kraftpulp and the bottom layer is high-yield Kraft pulp or recycled fiber (paper orboard).

Stock preparation includes two pulp lines, a broke line and fresh and whitewater systems. White water system consists of white water storage, dilutionwater distribution and fiber recovery unit for effluent to be released to mainsewer.

The board machine has two head boxes, top and bottom wires, press anddryer sections followed by calender and reeler.Fluting

Fluting, or corrugated medium board is made of unbleached NSSC pulp or ofrecycled paper.



Stock preparation is composed of NSSC pulp and recycled paper lines, brokeline and fresh and white water systems. White water system consists ofdilution water distribution equipment and fiber recovery system for the excessfiber-containing effluent.

The board machine has conventional wire section, press and dryer sections,machine calender and reeler.

Other board grades

Other common board grades are packaging paperboards (e.g. foldingboxboard).

1.3.6 Effluent Loads from Paper and Board Making Processes

In Table 5, typical effluent loads from paper and board mills are presented.

Table 5: Effluent loads from modern paper and board mills and recycled fibermills (note: figures include loads from mechanical pulping and necessary rawmaterial processing in each mill case)

Mill type Effluent SS BOD7 CODCr

m3/ADt kg/ADt kg/ADt kg/ADtNewsprint (TMP) 10 - 25 20 - 30 15 - 18 30 - 40LWC 10 - 40 10 - 40 10 - 15 10 - 30Liner and fluting 5 - 15 10 - 50 10 - 30 20 - 60



2 EFFLUENT DISCHARGE REGULATIONS FOR PULP ANDPAPER INDUSTRY IN EUROPE, NORTH AMERICA AND FAREAST

2.1 General

In Europe, the current waste water regulations can differ considerablybetween one country and another. In the near future, the regulations can beexpected to be uniform as various European Union's pollution controldirectives will come into force in the member countries by the end of 1990s.

In the USA and Canada, the effluent regulations will be revised after theCluster Rule regulations will be promulgated, most probably by the end of1997. The Cluster Rule standards will be applied in the USA, but later theyalso may have certain effects on the federal and provincial regulations inCanada.

In the Far East, Japan is the key country as to the development ofenvironmental regulations. The pulp and paper industry is currently in a rapidevolving phase in the Asean region, and at the same time also pollutioncontrol regulations will be developed and tightened.

2.2 Europe

2.2.1 Finland

A basic law for water pollution control (the Water Act) came into force in1961.

With respect to pulp and paper industry, the basic law includes rules andregulations for granting a waste water discharge permit. The Water Act doesnot include any general standards for pollution loads of pulp and paperindustry waste waters. Instead, the Water Act demands specifications (e.g.allowable pollution loads, measures to decrease water pollution andmonitoring programme to follow the effects of the pollutants on the receivingwater course) to be made in the operating licence of every individual wastewater discharge permit granted by a regional Water Rights Court. Mostcommonly, restrictions for pollutants like CODCr,(BOD7), AOX and phosphorusare demanded.



The basic law allows discharge permits for certain periods only and a newapplication must be prepared before the expiry of the existing permit.

For pulp and paper industry in Finland, also Helsinki Convention (HELCOM)recommendations and Nordic Council of Ministers recommendations areimportant to be followed (see Chapters 2.4.1 and 2.4.2).

2.2.2 Sweden

No general effluent standards or norms are presently applied. In case ofindividual discharge permits the authorities usually introduce two types ofallowable discharge values: so-called limit values and guide values. The limitvalue is not allowed to be exceeded. The guide value may be exceeded, butin these conditions necessary measures have to be put in use to avoid furtherexceeding. Limit and guide values are typically defined as annual or monthlyaverage absolute loads. Water pollutants which are usually required to bespecified are CODCr, BOD7, AOX, P-tot and N-tot. In Table 6, frequency andanalysing standards are presented.

Table 6: Effluent monitoring requirements and standards in SwedenPollutant Sample type and frequency Analysing method

CODCr Alternative 1: SS 02 81 424 daily and one weekend sample perweek. Analyses of COD value of eachsample (non-settled and non- filtered)

Alternative 2:4 daily and one weekend sample perweek. Analyses of COD value of eachsample after removing coarse particlesand fibers

Filtering according toSS 02 81 38filter fabric 70 µm

in addition:one weekly sample per week. Analyses ofCOD of non-settled and non-filteredsample

P-tot one weekly sample per week SS 02 81 02

N-tot one weekly sample per week SS 02 81 01

BOD7 one monthly sample per month SS 02 81 43

AOX one weekly sample per week in bleachingplants which use chlorine chemicals

SS 02 81 04

one weekly sample per quarter in paperproduction



In some cases, also suspended solids and chlorate loads are required to bemeasured and reported.

2.2.3 Norway

No general limits are applied. The restrictions for each individual mill aredetermined based on the conditions of the receiving water course and theavailable technology. The regulated water pollutants are suspended solids,CODCr, phosphorus and AOX.

2.2.4 Germany

The current federal minimum requirements for pulp and paper industryeffluent discharge standards are according to a separate regulation shown inTable 7.

Table 7: Effluent discharge limit values for pulp and paper mills in Germany



Production TSS2) CODCr BOD5 N3) Ptot AOXbasis mg/l kg/t mg/l /kg/t mg/l mg/l kg/t

Chemical pulp(sulphite) - 70 -/5 104) 25) 1

Paper andboard1)

- class 1 506) 3 25/1 104) 25) 0.047)

- class 2 506) 6 25/2 104) 25) 0.047)

- class 3 506) 9 25/3 104) 25) 0.047)

- class 4 506) 12 -/6 - 25) 0.0257)

- class 5 - 2 25/- 104) 25) 0.027)

- class 6 - 3(5)8) 25/- 104) 25) 0.017)

- class 7 - 5 259)/- 104) 25) 0.0127)

1)Paper and board mills are divided into the following groups:1 Woodfree unsized2 Woodfree sized3 Woodfree, highly refined and special paper (with more than one quality

change per working day as annual average)4 Pergament5 Woodfree, coated, more than 10 g coating/m2 (integrated)6 Woodcontaining (integrated with mechanical pulping), end product

predominantly not from recycled fibre7 Recycled paper, mainly based on recycled fibre

2)filterable solids

3)ammonium-, nitrate- and nitrite-N

4)when effluent amount exceeds 500 m3/d

5)when effluent amount exceeds 1000 m3/d

6)in cases where effluent is subjected to biological treatment

7)at specific conditions regarding the use of chlorohydrine-containing wet-strength papersthe limit value is 0.12 kg/t or 0.2 kg/t

8)5 kg/t when over 50% of the pulp is TMP or when a substantial part of the pulp is bleachedwith hydrogen peroxide

9)if effluent amount is below 10 m3/t, the limit value is 50 mg/l and the specific limit value0.25 kg/t, respectively

According to Waste Water Charges Act amendment, industrial plants areliable to pay discharge fees, if certain limit values are exceeded. The fees arebased on damage units (Schade einheit) as presented in Table 8.



Table 8: Damage units for determination of discharge fees, applicable to pulpand paper mills in Germany (1991)Pollutant Damage unit Limit value (as concentration

or annual load)

CODCr 50 kg 20 mg/l, 250 kg/a

AOX 2 kg 100 µg/l, 10 kg/a

Phosphorus, as P 3 kg 0.1 mg/l, 15 kg/a

Nitrogen, as N 25 kg 5 mg/l, 125 kg/a

Acute toxicity to fish- effluent amount 3000 m3 GF

1) = 2

Metals and their compounds- Hg 20 g 1 µg/l, 100 g/a- Cd 100 g 5 µg/l, 500 g/a- Cr 500 g 50 µg/l, 2.5 kg/a- Ni 500 g 50 µg/l, 2.5 kg/a- Pb 500 g 50 µg/l, 2.5 kg/a- Cu 1000 g 100 µg/l, 5 kg/a

1) GF is the dilution factor by which the waste water turns to non-toxicto fish, according to a specified test method.

The present fee (since January 1, 1997) per damage unit is DEM 80. From1999 the fee will be increased to DEM 90.

2.2.5 France

In the 1994 decree, some general requirements for the quality of the effluentsto be released to the watercourses are specified. Main requirements are thefollowing:

• pH to be held between 5.5 - 8.5• temperature inferior to 30 oC (35 oC if anaerobic waste water treatment is

used)• colour inferior to 100 mg Pt/l• phenolic type substances 0.3 mg/l or 3 g/d• phenols 0.1 mg/l or 1 g/d• AOX 5 mg/l or 30 g/d• hydrocarbons totally 10 mg/l or 100 g/d• additional specific requirements for toxic and bioaccumulative substances

listed in the Annex 4 of the decree (several organic and inorganiccompounds)



For pulp mills, the current regulations are shown in Table 9.

Table 9: Effluent discharge limit values for pulp mills in France. The values areexpressed as kg/ADt, monthly average max. values.

new mills existing millsMill type TSS BOD5 CODCr TSS BOD5 CODCr

MECHANICAL- unbleached 0.7 0.7 1.5 0.9 0.9 2.0- bleached 0.7 0.7 3.0 0.9 0.9 3.9TMP- unbleached 0.7 0.7 4.5 0.9 0.9 5.9- bleached 0.7 0.7 6.0 0.9 0.9 7.8CTMP- unbleached 0.7 3.0 12.0 0.9 3.9 15.6- bleached 0.7 4.0 16.0 0.9 5.2 20.8KRAFT (hardwood)- unbleached 5.0 1.5 15.0 6.5 2.0 19.5- bleached 5.0 2.0 25.0 6.5 2.6 32.5KRAFT (softwood)- unbleached 5.0 2.0 20.0 6.5 2.6 26.0- bleached 5.0 3.0 50.0 6.5 3.9 65.0BISULPHITE 5.0 5.0 35.0 6.5 6.5 45.5WASTE PAPERS (deinking) 0.7 0.7 4.0 0.9 0.9 5.2

AOX restriction for bleached pulp mills is 1 kg/ADt.

The current restrictions for paper mills are presented in Table 10.



Table 10: Effluent discharge limit values for paper mills in France. The valuesare expressed as kg/ADt, monthly average max. values

New mills Existing millsEnd product/capacity TSS BOD5 CODCr TSS BOD5 CODCr

Capacity inferior to 60 Adt/d 2.0 4.0 8.0Paper with more than 90%virgin fibre, without fillers

0.7 0.7 2.5 1.5 1.0 4.0

Paper with more than 90%virgin fibre, with fillers orcoating

0.7 0.7 3.0 1.5 1.5 6.0

Paper with more than 90%virgin fibre with fillers andcoating

0.7 0.7 3.0 1.5 2.0 8.0

Paper with more than 90%waste paper, without fillers

0.7 0.7 3.0 1.5 1.5 6.0

Paper with more than 90%waste paper, with fillers orcoating

0.7 0.7 4.0 1.5 2.0 8.0

Paper with more than 90%waste paper, with fillers andcoating

0.7 0.7 4.0 1.5 2.0 8.0

Fluting 1.9 1.9 8.0

In each case the daily maximum value can be twice as high as the monthlyaverage maximum value in the above-mentioned tables.

2.2.6 United Kingdom

Emission limits for point sources, e.g. industrial plants, are determined tomaintain the quality objectives of the receiving water, taking into account allothers discharges to the same water course as well. Typically, regulations areset for waste water flow, TSS, BOD, COD and pH. These requirements varyto some extent depending on the type of the receiving water (e.g. river, inlandlake, estuary).

Discharges of waste water to public sewers are controlled by the WaterIndustry Act and the Trade Effluents (Prescribed Processes and Substances)Regulations 1989. A consent is also required, when waste water is releasedto a common sewer.

2.2.7 Italy

According to the Law 1976, waste water standards are divided into threecategories:



• Tabella A Industrial waste waters when discharged directly to water course

• Tabella B Industrial waste waters when discharged to municipal sewage treatment plant

• Tabella C Municipal sewage (over 50 P.E.)

In Table 11, limit values according to Tabella A are presented.

Table 11: Industrial effluent discharge limits in Italy according to Tabella A(direct release to water course)Object Limit value RemarkspH 5.5 - 9.5 After dispersion, 50 m from the discharge point

6.5 - 8.5Total suspendedsolids (TSS)

80 mg/l Filter openings 0.45 µm

Settleable solids 0.5 ml/l Imhoff cone, 2 hoursBOD5 40 mg/l For certain industrial effluents, the limit may be

equal to 70% of the total BOD5 releaseCODCr 160 mg/lTemperature increment < 3 oC In rivers, after the dispersion zoneColour non-visible When dilution is 1:20Total phosphorus 10 mg/l In certain lakes and dams 0.5 mg/l

2.2.8 Spain

General limit values for industrial effluent discharges are given in the Law1985. These limit values also form a basis for a discharge fee (tax) system.The annual fee (F) is determined by the following equation:

F = C x P, where C = annual contaminating unitsP = contaminating unit price

C = K x Q, where C = annual contaminating unitsK = quality factor, dependent on the pollutants in

the effluentQ = annual effluent flow, m3 (Note. so called clean

waters, e.g. from cooling units, can be excluded)

The discharge tax system is applied for mills which release their effluents to ariver or a lake.

In Table 12, limit values divided into three categories, are presented.



Table 12: Effluent discharge limit values for calculation of discharge fees inSpainSubject Max. limit value

Category 1 Category 2 Category 3pH 1) 5.5 - 9.5 5.5 - 9.5 5.5 - 9.5Suspended solids, mg/l 2) 300 150 80Settleable solids, ml/l 3) 2 1 0.5Coarse solids none none noneBOD5, mg/l 4) 300 60 40CODCr, mg/l 500 200 160Temperature increment, oC 5) 3 3 3Colour, non-visible 6) 1:40 1:30 1:20Total phosphorus, mg/l 7) 20 20 10

1) After dispersion, 50 from the discharge point resp. level 6.5-8.52) Filter openings 0.45 µm3) Imhoff cone, 2 h settling4) For certain industrial effluents, the limit may be equal to 70% of the total

BOD5 release5) In rivers after dispersion zone. In lakes, max. effluent temperature is 20 oC6) Colour to be determined through 10 cm of diluted effluent in each category7) In certain lakes and dams the limit value is 0.5 mg/l

The authorities determine the quality factors for each categories. The mostpolluting substance as specified in Table 12 determines the category andresp. quality factor to be used.

The general minimum requirements have been issued primarily to be able tobe used in the discharge tax system. Additional regulations can be set up e.g.based on the type and use of the receiving water.

2.3 North America

2.3.1 The USA

The EPA issues technology-based effluent limit values guidelines on afederal level for effluent discharges. These are mainly adopted as such in thestate legislation.

Concerning the current effluent discharge regulations, various technologicalconcepts are determined as a basis for the requirements. In the following,these definitions are presented:

• BPT (Best Practicable Control Technology) effluent guidelines apply todischarges of conventional pollutants (BOD5, TSS, pH, fecal coliforms, oiland grease)



• BCT (Best Conventional Pollutant Control Technology) presently is equalto BPT

• BAT (Best Available Technology) standards define for the direct dischargelimits of toxic and non-conventional pollutants. Presently, the BAT concepthas limited significance as it only concerns mills which use chlorophenol-based biocides.

• NSPS (New Source Performance Standards) are applied to new or rebuiltmills discharging their effluents directly to watercourses. NSPS regulationsinclude conventional, non-conventional and toxic pollutants.

• PSES and PSNS (Pre-treatment standards for existing and new sources)are specific standards for discharges from existing and new mills topublicly-owned sewage treatment works.

In addition, the Cluster Rule includes the following definition for all mills:

• BMP (Best Management Practices) for spent pulping liquor managementand spill prevention and control in chemical pulp mills

The current effluent discharge restrictions were adopted in 1982 and theywere determined for 24 mill types. In Table 13, an example of effluentrestrictions for major production groups are presented.

Table 13: Effective EPA effluent restrictions for six production groups,expressed as annual averages, kg/ADt, in USAMill type/Group BPT NSPS

BOD5 TSS BOD5 TSSA Unbleached kraft and paper 2.8 6.0 1.8 3.0B Bleached market kraft pulp 8.05 16.4 5.5 9.5H Bleached kraft pulp and fine paper 5.5 11.9 3.1 4.8J Bleached sulphite pulp and paper 16.5 23.5 2.36 3.03L TMP and paper 5.55 8.35 2.5 4.6N Groundwood pulp and newsprint 3.9 6.85 2.5 3.8

Allowed pH range of treated effluent is 5 - 9. In wet debarking, additional BODload of 1.2 kg/ADt and TSS load of approximately 3.1 kg/ADt is allowed. TheNSPS restrictions include also loads from debarking plant.

State regulations can be tighter and also may include other parameters (e.g.AOX, colour, dioxins/furans, heavy metals).



In the Cluster Rule proposals, the production groups are planned to bedecreased to 12 and new restrictions for all production groups are to bedetermined.

2.3.2 Canada

The federal regulations 1992 define the maximum BOD and TSS dischargelevels for pulp and paper mills. Also acute toxicity of the effluent is regulatedby the Federal Government.

In Table 14, the current waste water discharge limits according to the Federalregulations are summarised.

Table 14: Effluent discharge limits (expressed as kg/ADt) by FederalRegulations in CanadaMill type BOD TSS

Daily Monthly Daily MonthlyDissolving sulphite mills 45.0 27.0 62.5 37.5Other pulp and papermills

12.5(24.75) 1)

7.5(14.85) 1)

18.75 11.25

1) Maximum BOD that can be authorised for old mills, built before 1970

The provinces may set their own more stringent regulations.

2.4 International Conventions

2.4.1 The Helsinki Convention (HELCOM)

The basic Convention on the protection of the marine environment of theBaltic Sea area was made at Finland's initiative in March 1974 in Helsinki.

The current amendment of the Convention includes also the definitions ofBAT and BEP. The parties agree to promote the use of BAT and BEP. BAT isto be used for point polluting sources and BEP for all pollution sources.

Concerning pulp and paper industry, the following HELCOM recommen-dations are adopted as revised in March 1996:

• HELCOM recommendation 17/8 (Reduction of discharges from the Kraftpulp industry)

• HELCOM recommendation 17/9 (Reduction of discharges from the sulphitepulp industry



Discharge limits according to HELCOM 17/8 are presented in Table 15.

Table 15: Annual average discharge limit values for the Kraft pulp industryaccording to HELCOM recommendation 17/8The following annual average discharge limit values in kg per tonne of Air Dry Pulp (kg/tADP) produced are not exceeded from 1 January 2000 for any mill which has started tooperate before 1 January 1997:

Pulping process CODCr AOX Tot-P Tot-N

Bleached pulp 30 0.4 0.04 0.4

Unbleached pulp 15 - 0.02 0.3

In countries in transition, the following annual average discharge limit values (kg/t ADP)produced are not exceeded from 1 January 2005 for any mill which has started to operatebefore 1 January 1997:




For any mill, starting to operate or considerably increasing its capacity (by more than 50%)after 1 January 1997, the following annual discharge limit values (kg/t ADP) exist:




In Attachment 1, HELCOM 17/8 determines BAT for the Kraft pulp industry,1995:

1. Dry debarking with minor waste water discharges2. Closed screening3. Stripping of most concentrated condensates and reuse of most

condensates in the process4. Systems which enable the recovery of almost all spillages5. Extended delignification in the digester followed by oxygen delignification6. Efficient washing before the pulp leaves the closed part of the process7. At least secondary treatment for waste water discharges8. Partial closure of bleach plant. The main part of the discharge from the

bleach plant is piped to the recovery system9. Use of environmentally sound chemicals in the process, for example use of

biodegradable chelating agents wherever possible



In Attachment 2, analysing methods to be applied for AOX, CODCr, Tot-P andTot-N are presented (all analyses should be made on unsettled, unfilteredsamples).

Discharge limits according to HELCOM 17/9 are presented in Table 16.

Table 16: Annual average discharge limit values for the sulphite pulp industryaccording to HELCOM recommendation 17/9The following annual average discharge limit values (kg/t ADP) produced are notexceeded from 1 January 2000 (for countries in transition from 1 January 2005) for anymill which has started to operate before 1 January 1997:




For any mill, starting to operate or considerably increasing its capacity (by more than 50%)after 1 January 1997, the following annual discharge limit values (kg/t ADP) exist:




In Attachment 1 of HELCOM 17/9, BAT for the sulphite industry 1995 isdetermined:

1. Dry debarking with minor waste water discharges2. Closed screening3. Neutralising of weak liquor before evaporation followed by re-use of the

main part of condensates in process4. Systems which enable the recovery of almost all organic substances

dissolved in the cook (a total U-value of about 98 % is achievable)5. No discharge from the bleach plant when the sodium based processes are

being used6. At least secondary treatment for waste water discharges7. Partial closure of the bleach plant when another process than sodium

based is used8. Use of environmentally sound chemicals in the process, for example use of

biodegradable chelating agents wherever possible



2.4.2 Proposals of the Nordic Council of Ministers

The Nordic Council of Ministers established a working group comprisingexperts from Sweden, Finland, Norway and Denmark to evaluate the impactsof the pulp and paper industry on the environment and to compile a report onthese impacts by 1993.

Based on this report, among others, the following proposals related toenvironmental protection and pollution control were presented:

• Effluent loads, as shown in Table 17, expressed as annual averages,should by the end of this century not to be exceeded for any mill. Forintegrated mills producing mechanical or recycled fibres pulp the figuresare kg per tonne of product, whereas for the other types of mills the figuresare kg per tonne of air dry pulp.

Table 17: Annual average limit values of effluent loads (kg/ADt) according to aproposal by the Nordic Council of Ministers 1993. Limit values to be reachedby the end of this centuryType of mill AOX CODCr Tot-P Tot-N1)

Bleached kraft 0.4 30 0.04 0.2Unbleached kraft - 15 0.02 0.2Bleached sulphite 0.3 70 0.08 0.6CTMP - 30 0.02 0.2Mechanical2) - 10 0.01 0.2Recycled fiber - 10 0.01 0.2

1) Any nitrogen discharge associated with the use of complexing agents should be added to the figure for tot-N given above

2) "Mechanical" means integrated mills producing newsprint or magazine paper

• In case of any new or considerably enlarged (in the order of 30 %) mill,the following levels, as shown in Table 18, should not be exceeded asannual averages.



Table 18: Annual average limit values of effluent loads according to aproposal by the Nordic Council of Ministers 1993. New and enlarged millsType of mill AOX CODCr Tot-P Tot-N1)

Bleached kraft 0.2 15 0.02 0.15Unbleached kraft - 8 0.01 0.15Bleached sulphite 0.1 35 0.04 0.3CTMP - 15 0.01 0.1Mechanical2) - 5 0.005 0.1Recycled fibre - 5 0.005 0.1

1) Any nitrogen discharge associated with the use of complexing agents should beadded to the figure for tot-N given above

2) "Mechanical" means integrated mills producing newsprint or magazine paper

2.5 Far East

2.5.1 Japan

Based on the historical background and the development of theenvironmental legislation and policy, Japan has been the key country in theregion since the 1960s.

The latest revision of the Water Pollution Control Law took place in June1996. The main effort in Japan in water pollution control has put on theprotection of three water areas, namely: Tokyo Bay, Ise Bay and Seto InlandSea. COD restrictions announced by the Government to reduce COD loadsfrom industry and municipalities.

General water quality standards can be divided into two categories:environmental quality standards for protection of human health and thestandards for the conservation of living environment. Depending on the typeand use of the watercourse, different regulations are issued (e.g. for fishery,agricultural, water supply purposes of natural lakes, rivers and artificialreservoirs).

Similarly, waste water standards are also applied for the protection of humanhealth and for the protection of the living environment.

For the protection of human health, the permissible limit values are shown inTable 19.



Table 19: Effluent limit values for the protection of human health in JapanPollutant Permissible limit, mg/l

Cadmium and its compounds 0.1Cyanide compounds 1Organic phosphorus compounds 1Lead and its compounds 1Hexavalent chromium compounds 0.5Arsenic and its compounds 0.5Total mercury 0.05Alkyl mercury compounds not detectable1)

PCB 0.03Trichloroethylene 0.3Perchloroethylene 0.1

1) According to the analysing method issued by the Director General of the EnvironmentAgency.

For all industrial facilities, the general waste water standards are shown inTable 20.

Table 20: Effluent limit values related to the protection of living environment inJapanPollutant Unit Limit valuepH 5.8 - 8.6 (release to other than coastal

waters)5 - 9 (release to coastal waters)

BOD, COD mg/l 160 (daily average 120)SS mg/l 200 (daily average 150)N-hexane mg/l 5 (mineral oil)

mg/l 30 (animal fat and vegetable oil)Phenols mg/l 5Copper mg/l 3Zinc mg/l 5Dissolved Fe mg/l 10Dissolved Mn mg/l 10Chromium mg/l 2Fluorine mg/l 15Coliforms pcs/cc 3000 (daily average)Nitrogen mg/l 120 (daily average 60)Phosphorus mg/l 16 (daily average 8)

Note: Prefectures may, by decree, set more stringent standards locally

It must be emphasised, that in individual mill cases, the limit values as toBOD and COD often are more stringent than those presented in Table 20. Asan example of very tight current Agreement category restrictions for a pulpand paper mill can be presented:



• BOD: max. 10 mg/l (daily average 8 mg/l)• COD: max. 30 mg/l (daily average 18 mg/l)• SS: max. 15 mg/l (daily average 10 mg/l)

In the end of 1980s, the release of dioxins in pulp mill effluents becameconcerned as a major environmental hazard, related to the use of chlorinegas in bleaching. In December 1990, the industry issued a voluntary target forAOX release being 1.5 kg/ADt to be reached by the end of 1993. The actualAOX discharge in 1993 from 31 bleached Kraft mills and from 2 dissolvingpulp mills was on the average 0.8 kg/ADt (min. 0.4 and max. 1.3 kg/ADt).

2.5.2 China

The recent key effluent regulations are the following:

• Regulations on prevention and cure of water pollution for paper industry(issued by Environmental Protection Committee of State Council, Ministryof Light Industry, Ministry of Agriculture and Ministry of Finance onDecember 20, 1988)

• Standards of discharge of water pollutants for paper industry, GB 3544-92(in force after July 1, 1992)

The 1988 regulations include, among others, the following definitions andrequirements for pulp and paper industry:

Classification of pulp and paper mills:

• The so-called large enterprise means a mill with pulp production more than30 000 ADt/a. The middle enterprise has a production between 10 000 to30 000 ADt/a and a small enterprise produces less than 10 000 Adt/a.

• Before 1995, large and middle size enterprises using alkaline pulpingprocesses, should have a chemical recovery system. The efficiency of thechemical (black liquor) recovery must reach the following percentages:

wood pulping > 90%bamboo, reed, silver grass, bagasse > 80%straw > 75%

• Before 1995, the small chemical pulping enterprises with alkali recoveryunit, must reach the following chemical recovery efficiency levels:

production from 7000 to 10000 ADt/a > 70%



production from 4000 to 7000 ADt/a > 60%production from 3000 to 4000 ADt/a > 50%

• Before 1995, the small chemical pulping enterprises without alkali recoveryunit, should control and decrease the black liquor discharges with respectto COD and BOD5 loads as follows:

production from 5000 to 10000 ADt/a > 60% reductionproduction from 3000 to 5000 ADt/a > 50% reductionproduction less than 3000 ADt/a > 30% reduction

• Enterprises that are based on chemi-mechanical, alkaline half-chemicalpulping or lime straw pulping, or alkaline hemp/cotton pulping (includingviscose pulping), should have waste water treatment and these millsshould decrease organic pollution loads more than 50 % before 1995.

• The enterprises that are based on acid sulphite pulping process must use

acid recovery process or waste water must meet the standards regulatedby the state or local governments before 1995.

• Waste water originating from neutral sulphite pulping process should be

used in agriculture or should have other utilisation. Before 1995, theutilisation degree should be 60 % or more.

• Any enterprise that has not reached the 1988 regulation standards before

the end of 1995, must propose a plan consisting of optional measures todecrease effluent loads to acceptable levels. The plan will be checked,accepted and executed by local people's government.

GB 3544-92 standard determines the characteristics of effluents from pulpand paper mills. The acceptable pollution loads depend on the type of the milland the watercourse the effluent is released. The water quality standards,applied in the GB 3544-92, are:

GB 3097 Quality Standards of Sea WaterGB 3838 Environmental Quality Standard of Surface WaterGB 6920 Water Quality. Determination of pH Value. Glass

Electrode MethodGB 7488 Water Quality. Determination of Biochemical Oxygen

Demand for 5 Days (BOD5). Dilution and Inoculation MethodsGB 11901 Water Quality. Determination of Suspended Substance.

Gravimetry.GB 11914 Water Quality. Determination of Chemical Oxygen Demand

(COD). Dichromate Method.Waste water quality standards are divided into three categories:



• 1st class The first class standards are applied to waste waters dischargedto III grade water area specified in GB 3838 (except protection areas ofwater bodies) and to II grade marine area according to GB 3097.

• 2nd class The second class standards are applied to waste watersdischarged to IV and V grade water area in GB 3838 and to III grademarine area in GB 3097

• 3rd class The third class standards are applied to waste watersdischarged to sewers of cities or towns having a waste water (sewage)treatment plant.

The standard values are given as the allowable maximum concentrations andspecific effluent amounts.

In Table 21, Table 22 and Table 23, waste water restrictions for pulp andpaper mills implemented before January 1, 1989 are shown.

Table 21: First class limit values for effluent discharges from pulp and papermills implemented before January 1, 1989 in ChinaMill type I Class limit values

Effluent amount BOD5 CODCr SSm3/ADt mg/l mg/l mg/l

Integrated pulp and paper mills:

Wood-basedUnbleached 220 150 350 200Bleached 320 150 350 200

Non-wood-basedUnbleached 270 150 350 200Bleached 370 150 350 200

Non-integrated paper mills:

Paper and paperboard 80 60 150 100

Note. Allowable pH range for all types of waste waters is 6 - 9



Table 22: Second class limit values for effluent discharges from pulp andpaper mills implemented before January 1, 1989 in ChinaMill type II Class limit values








Table 23: Third class limit values for effluent discharges from pulp and papermills implemented before January 1, 1989 in ChinaMill type III Class limit values







Note. Allowable pH range for all types of waste waters is 6-9

Respective waste water restrictions for the pulp and paper mills implementedbetween January 1, 1989 and June 30, 1992 are presented in Table 24,Table 25 and Table 26.



Table 24: First class limit values for effluent discharges from pulp and papermills implemented between January 1, 1989 and June 30, 1992 in ChinaMill type I Class limit values








Table 25: Second class limit values for effluent discharges from pulp andpaper mills implemented between January 1, 1989 and June 30, 1992 inChinaMill type II Class limit values










Table 26: Third class limit values for effluent discharges from pulp and papermills implemented between January 1, 1989 and June 30, 1992 in ChinaMill type III Class limit values








For new mills and mills built after July 1, 1992, the waste water restrictionsare specified in Table 27, Table 28 and Table 29.

Table 27: First class limit values for effluent discharges from new pulp andpaper mills implemented after July 1, 1992 in ChinaMill type I class limit values

Effluentflow

BOD5 CODCr Suspendedsolids

AOX

m3/ADt kg/t mg/l kg/t mg/l kg/t mg/l kg/t mg/lIntegrated pulpand paper mils:Wood-basedunbleached 150 4,5 30 15 100 10,5 70bleached 240 7,2 30 24 100 16,8 70 1,5 8Non-woodbasedunbleached 190 5,7 30 19 100 13,3 70bleached 290 8,7 30 29 100 20,3 70 1,5 7Non-integratedpaper mills:paper andpaperboard

60 1,8 30 6 100 4,2 70

Note: Allowable pH range for all types of waste waters is 6 - 9



Table 28: Second class limit values for effluent discharges from new pulp andpaper mills implemented after July 1, 1992 in ChinaMill type II Class limit values

Effluentflow


AOX

m3/ADt kg/t mg/l kg/t mg/l kg/t mg/l kg/t mg/lIntegrated pulpand paper mills:Wood-basedUnbleached 150 15 100 52.5 350 30 200Bleached 240 28.8 120 84 350 48 200 2.5 10Non-wood-basedUnbleached 190 28.5 150 85.5 450 38 200Bleached 290 43.5 150 130.

5450 58 200 2.5 9

Non-integratedpaper mills:Paper andpaperboard

60 3.6 60 9 150 6 100


Table 29: Third class limit values for effluent discharges from new pulp andpaper mills implemented after July 1, 1992 in ChinaMill type III Class limit values

Effluentflow


m3/ADt kg/t mg/l kg/t mg/l kg/t mg/lIntegrated pulpand paper mills:Wood-basedUnbleached 150 75 500 120 800 60 400Bleached 240 120 500 192 800 96 400Non-wood-basedUnbleached 190 114 600 171 900 76 400Bleached 290 174 600 261 900 116 400Non-integratedpaper mills:Paper andpaperboard

60 400 500 400




2.5.3 Indonesia

Since 1991, effluent restrictions applicable to currently operating pulp andpaper mills using Best Practicable Technology (BPT) have been in force.These regulations should be applied and reached by all mills in 1995, seeTable 30 below.

Table 30: Effluent restrictions for pulp and paper industry in Indonesia in 1995.Figures represent maximum values which are not to be exceeded.Load Pulp mills Paper mills Integrated mills

mg/l kg/ADt mg/l kg/ADt mg/l kg/ADt

BOD5 150 15 125 10 150 25.5

COD 350 35 250 20 350 59.5

TSS 200 20 125 10 150 25.5

pH 6 - 9 6 - 9 6 - 9 6 - 9 6 - 9 6 - 9

Effl. flow 100 m3/ADt 80 m3/ADt 170 m3/ADt

Regulations based on Best Available Technology (BAT) have been issued fordesign of new or rebuild pulp and paper mills to be in force from 1995. TheseBAT regulations are planned to be applied to all mills by the year 2000.These limit values are shown in Table 31.

Table 31: Effluent restrictions for pulp and paper industry in Indonesia in 2000.Figures represent maximum values which are not to be exceeded.Process Flow BOD5 COD TSS

m3/ADt mg/l kg/ADt mg/l kg/ADt mg/l kg/ADtPULPKraft unbl. 50 75 3.75 200 10 60 3Kraft bl. 85 100 8.5 350 29.75 100 8.5Dissolving 95 100 9.5 300 28.5 100 9.5CMP andgroundwood

60 50 3 120 7.2 75 4.5

Semi-chem. 70 100 7 200 14 100 7Soda pulp 80 100 8 300 24 100 8De-inked 60 100 6 300 18 100 6PAPERFine bl. 50 100 5 200 10 100 5Coarse 40 90 3.6 175 7 80 3.2Cigarrette 175 60 10.5 100 17.5 45 7.8Other bl. 35 75 2.6 160 5.6 80 2.8

Note. pH in all process effluents should be between 6 - 9



3 MAIN PRINCIPLES OF BIOLOGICAL OXIDATION

3.1 General

The following summary concerns mainly biological oxidation of organiccompounds present in pulp and paper industry effluents. Pulp and paperindustry effluents differ e.g. from municipal sewage treatment, as the qualityof the effluents can vary a lot and in general, complete nitrogen removalneeds not to be included in the aeration design (except in case of ammoniumsulphite process effluents).

3.2 Major Phases of Organic Material Bio-oxidation

In biological treatment, microbes degrade and remove organic materialpresent in mill effluents in the following ways:1. capturing of organic suspended matter by biosludge2. adsorption of colloidal substances on the bioflocs3. biosorption of soluble organic compounds by microbes

Also sorption of non-degradable organic compounds on biosludge can occur.Sorptive properties depend on the treatment method, e.g. plug-flow systemshave better sorptive biosludge than completely mixed systems.

It is clear, based on the above-mentioned phenomena, that removal oforganic matter from waste waters is a complex multi-stage process. Molecularoxygen supplied by the aeration equipment, is needed mainly to satisfy 3.organic matter removal stage. The biosorption rate is directly proportional tothe concentration of biosludge, the sludge age and the characteristics of thesoluble organic compounds. Degradation of compounds in stages 1. and 2.can also occur and in this way additional soluble organic material formetabolism of microbes can be generated.

3.3 Molecular Oxygen Demand

3.3.1 General

Much discussion takes place today about the reliable basis of the designcriteria for aeration equipment (BOD, COD or TOC). The two essential stagesin the design procedure of an aeration system are:

• Determination of actual oxygen requirement (AOR)• Determination of standard oxygen transfer rate (SOTR)



In this context, main emphasis is laid on the AOR procedure. SOTR value,and the respective aeration system, depends on many specific items, such asthe shape and depth of the aeration basin and the type of the aerationequipment. SOTR value must be determined and tested by the aerationequipment supplier in each case.

3.3.2 Determination of AOR

In case of pulp and paper mill effluents, AOR value includes dissolvedmolecular oxygen needed in aerobic biological processes for:

• assimilative respiration• endogenous respiration• chemical oxidation of some inorganic compounds, e.g. sulphides• biological oxidation of ammonia to nitrate (if present)

AOR requirements vary considerably depending on the efficiency degree ofthe process, sludge and organic volume loadings, detention time of theeffluent in the aeration basin, sludge age etc. Determination of correct AORvalue for each pulp and paper mill effluent type requires a profound long termfield experience of similar effluent treatment plants. The aeration equipmentsupplier should ask for the AOR value from the client to be able to give properguarantees for the oxygenation capacity of the aeration equipment.

The general equation for oxygen demand is:

AOR = a' x BODrem + b' x MLVSSaer + k x CODCr + 4,6 x NH4(rem)

where a' oxygen consumed in assimilative respiration (pulp and paper mill effluents typically 0,5 - 0,8)

BODrem BOD removal in treatment, kg/d

b' oxygen consumed in endogenous respiration (range 0,05 - 0,2)

MLVSSaer active biomass in aeration

k fraction of COD which requires molecular oxygen capable to oxidise certain inorganic compounds (typically 0,01 - 0,1)

NH4(rem) ammonium nitrogen removal, kg/d



Denitrification is not needed in the treatment of pulp and paper mill effluentsand thus anoxic stage is not required. However, in case of ammoniumsulphite mills, nitrogen restrictions may require also denitrification. Presently,no experience of total nitrogen removal from pulp mill effluents is available.

As indicated earlier, coefficients a' and b' are specific for each type of pulpand paper mill effluent. Even within a certain effluent category, e.g. newsprintmill effluent, the coefficients can vary to some extent mill by mill. If pilot trialscan be carried out, the coefficients a' and b' can be determined as follows:

• equation between specific BOD removal (kg BOD/kg VSS,d) and oxygenconsumption (kg O2/kg VSS,d) will be made based on the pilot tests indifferent loading conditions. Typically, the function will be linear, and thecoefficient a' can be obtained as the angle coefficient and b' can beobtained when the specific oxygen consumption = 0 (ordinate value).

No clear categories of a' and b' for e.g. for pulp mill effluents and paper milleffluents can be shown. If no pilot trials are possible to be made, as a firstassumption for both pulp and paper mill effluents, a' = 0,6 and b' = 0,1 valuescan be used.

For safety reasons, specially in case of pulp mill effluents, coefficient k = 0,1is recommended to be used.

3.3.3 Determination of SOTR

The following equation between AOR and SOTR exists:

AOR = SOTR x (( cT x β - cL)/cS) x α x θT-20)

where cT oxygen saturation concentration in temperature T

β actual O2 saturation conc. in effluent/clean water O2 conc.

cL dissolved oxygen concentration in aeration

cS saturated O2 concentration in clean water at temp. T

α mass transfer coefficient, Kla(effl.) / KLa(tap water)

θ temperature coefficient for mass transfer

T temperature in aeration


10 Glossary Page: 1.1 (18)


10 Glossary10.1 Symbols ........................................................................................10.110.2 Terms............................................................................................10.410.3 Conversion Factors.....................................................................10.17

arto

10 Glossary




10 GLOSSARY

10.1 Symbols

a Total mass transfer area per volumetric unita* Substrate respiration ratea’ Coefficient caused by pressureA Area of air/water boundary surface m2

AOR Actual Oxygen Requirement kg O2/db Endogenous respiration rateB Quantity of removed BOD kg/dBOD Biological Oxygen Demand mg/lBR Volumetric load kg BOD/m3 • dC∞* Steady state dissolved oxygen (DO) saturation

concentration attained at infinite time at watertemperature T and field atmospheric pressure.The value can be estimated as followsC∞* = CST • { 1+0.035 (h-0.25)} mg O2/lwhere:CST = the table value for dissolved oxygen (DO)at the temperature T at surface level mg O2/l

C*10 Saturation dissolved oxygen concentrationat 10 °C g O2/m

3

C∞*20 Steady state dissolved oxygen (DO) saturationconcentration attained at infinite time at watertemperature 20 °C and standard atmosphericpressure (101.3 kPa).The value can be estimated as follows:C∞*20 = CST,20 • (1+0.035 h) mg O2/lwhere:CST,20 = the table value for dissolved oxygen (DO)at the temperature 20 °C at surface level = 9,07 mg O2/l

CL Actual oxygen concentration in aeration tank g O2/m3

Ct Dissolved oxygen concentration at time t g O2/m3

D Diameter of the pipe mDD Area density of diffusers m2/m2

DL Molecular diffusion of oxygen through boundaryfluid film

DO Dissolved oxygen mg O2/le Standard oxygen transfer efficiency at chosen

aeration depth %e2 Oxygen absorption given by the curves %E Process efficiency %EPDM ethylene-propylene-dienef Coefficient of BOD conversionfr Temperature correction coefficientF/M Food-to-micro-organism loading

(sludge loading) kgBOD/kgMLSS • dF/V Volumetric load kgBOD/m3 • dG Velocity gradient 1/sh Submersion depth of diffusers mh Depth of the oxygen absorption curves measured m




H Henry’s constant (mg/l)/(kN/m2)HDPE hihg density polyethylenek Constant factor for blowers 1.395k’ Ammonium oxygenation coefficientk1 Flow rate correction factorkdim Hourly variation factorKL Mass transfer coefficient of boundary fluid filmKLa Apparent volumetric mass transfer coefficient

in clean water at temperature T 1/dl Length of aeration tank mL Imaginary thickness of boundary fluid film mMLSS Mixed liquor suspended solids kg/m3

MLVSS Mixed liquor volatile suspended solids kg/m3

n exponentN’ Number of diffusersN, ND Total nitrogen concentration of effluent kg N/m3

No Total nitrogen concentration of influent kg N/m3

NH4,o Ammonium concentration of influent kg N/m3

NH4 Ammonium concentration of effluent kg N/m3

NH4, NH Ammonium reduction kg N/m3

NR, NT Total nitrogen reduction kg N/dN(NO3)A Nitrate concentration of effluent kg N/m3

N(ges)Z Total nitrogen concentration of influent kg N/m3

N(NH4)A Ammonium concentration of effluent kg N/m3

N(org)A Organic nitrogen of effluent kg N/m3

Nüs Nitrogen bonded to excess sludge kg N/m3

OC Oxygenation capacity kg O2/hOVR Actual oxygen requirement kg O2/m

3 • hp Atmospheric pressure kPap’ Water pressure above the diffuser kPa, mmHgp2 Blower discharge pressure kPapa Head loss in diffusers kPapb Head loss in inlet valves and filters kPaP.E. population equivalentPE polyethyleneph Hydrostatic pressure at the diffuser kPapi Atmospheric pressure at the altitude of the plant kPapp Head loss in pipework including control valves kPaPP polypropyleneps Atmospheric pressure at standard conditions kPaP Required power kWPX Net production of biomass kg MLVSS/dqa Air flow m3/hqa’ Blower air flow in real conditions m3/hqa,d Air flow/diffuser m3/hqdim Hourly design flow m3/hqi, q Influent flow m3/hQd Domestic sewage flow m3/dQdim Daily design flow m3/dQe Effluent flow m3/dQe, Q Influent flow m3/dQI Industrial sewage flow m3/dQL Leakages m3/dQmax Maximum daily flow m3/dQr Return sludge flow m3/d




Qw Excess sludge flow m3/dR Actual oxygen requirement kg O2/dS Substrate (BOD) reduction kg/m3

Se, S Effluent substrate (BOD) concentration kg/m3

Si, So Influent substrate (BOD) concentration kg/m3

SOTE Standard Oxygen Transfer Efficiency %SOTR Standard Oxygen Transfer Rate kg/hSVI Sludge volume index ml/gt time min, htd Hours of domestic sewage flow per day htI Hours of industrial sewage flows per day hT Temperature of clean or process water °C, KTi Maximum intake air temperature during

summertime KTp Average temperature in distribution pipes KTs Air temperature in standard conditions KTSR Concentration of suspended solids in the

aeration basin kg MLSS/m3

v Air flow velocity in pipe m/sV Aeration volume m3

w Width of aeration tank mx Proportion of active biomassX Concentration of suspended solids in the

aeration tank kg MLSS/m3

Xo Mol fraction of oxygen in aeration airXO2, Ci Oxygen content in the air kg O2/m

3

Xe Concentration of suspended solids in theeffluent kg SS/m3

Xr Return sludge suspended solids kg MLSS/m3

Xw Concentration of suspended solids inexcess sludge kg MLSS/m3

α Proportion of the total oxygen transfercoefficient measured in sewage and in clean water

β Proportion of the DO saturation coefficientsmeasured in sewage and in clean water

δ Specific weight of water kg/dm3

µ Dynamic viscosity Ns/m2

θ Temperature correction coefficientθc Mean cell residence time (sludge age) dθh Hydraulic retention time hη Total efficiency of blower %




10.2 Terms

acid clean a general expression for cleaning DDS systems by volatile acid (HCI,HCOOH) during operation

acidity the capacity of a solution to react with hydroxyl ions. Acidity ismeasured by titration with a standard alkaline solution (base) to aspecified end point. Typically, it is measured in milligrams of calciumcarbonate per litre.

activated sludge Sludge withdrawn from a secondary clarifier following the activatedsludge process. Activated sludge consists mostly of biomass, withsome inorganic settleable solids. Return sludge is recycled to thehead of the process; waste (excess) sludge is removed forconditioning.

activated sludge loading The kilograms (pounds) of biochemical oxygen demand (BOD) in theapplied liquid per unit volume of aeration capacity or per kilogram(pound) of activated sludge per day.

activated sludge process A biological waste water treatment process by which a mixture ofwaste water and activated sludge is agitated and aerated. Theactivated sludge is subsequently separated from the treated wastewater (mixed liquor) by sedimentation and wasted or returned to theprocess as needed.

advanced waste watertreatment

Any physical, chemical, or biological treatment process used toaccomplish a degree of treatment greater than that achieved bysecondary treatment (see also tertiary treatment).

aeration The initiation of contact between air and liquid by one or more of thefollowing methods: (a) spraying the liquid in the air; (b) bubbling airthrough the liquid; (c) agitating the liquid to promote surfaceabsorption of air.

aeration group a grid of pipes with diffusers installed forming a closed pipeworkconnected by one flange to the dropleg air feed pipe

aeration method a method of dissolving oxygen into water, e.g. bottom aeration, finebubble aeration, surface aeration, jet aeration, etc.

aeration period The time, usually expressed in hours, during which mixed liquor issubjected to aeration in an aeration basin.

aeration system a combination of aeration equipment (aeration groups or aerators)designed to dissolve oxygen into water (activated sludge)

aeration tank A tank in which waste water or other liquids are aerated (also calledaeration basin).

aerator aeration equipment, typically used for mechanical aeration equipment;like O.K.I. aerators, surface aerators

aerobes Organisms that live only in aerobic conditions.




aerobic Living or occurring in an environment containing oxygen.

aerobic respiration The breakdown of organic substances by aerobes in the presence ofoxygen.

air lift A device for raising liquid by injecting air in and near the bottom of ariser pipe submerged in the liquid to be raised.

air lift pump A pump used for lifting activated sludge from the aeration basin orclarifier to waste or return activated sludge. Fine-pressured airbubbles are discharged to the water at the bottom, and the densersurrounding water pushes up in the discharge pipe to the outlet (alsocalled air-lift or air-lift returns).

algae Photosynthetic, microscopic plants that can seriously deplete oxygenin the presence of sunlight.

alpha α ratio of mass transfer coefficient in waste water and clean water

ammonia A chemical combination of hydrogen (H) and nitrogen (N) occurringextensively in nature and expressed as NH3.

ammonia-nitrogen Quantity of elemental nitrogen present in the form of ammonia (NH3).

amoeba Small, one-celled organism using pseudopodic (false feet) formovement (see Sarcodina).

amperometric titration The electronic detection of the equivalence point in a titration, throughobservation of the change in diffusion current at a suitable appliedvoltage as a function of the volume of titrating solution.

anaerobes Organisms that live in the absence of oxygen.

anaerobic A condition in which no oxygen is available in the environment (forexample, a septic clarifier).

anaerobic respiration The breakdown of organic substances in the absence of oxygen.

AOR Actual Oxygen Requirement (in waste water)

bacteria A group of universally distributed, rigid, essentially unicellularmicroscopic organisms lacking chlorophyll. Bacteria perform a varietyof biological treatment processes, including biological oxidation,nitrification, and denitrification.

bacterial analysis The examination of wastewater to determine the presence, number,and identity of bacteria (also called bacterial examination).

bacterial examination Examination of waste water to determine the presence, number, andidentity of bacteria. Also called bacterial analysis.

bacteriological count A means for quantifying numbers of organisms.

beta β ratio of oxygen saturation value in waste water and clean water




biochemical oxygen demand(BOD)

(1) The quantity of oxygen used in the biochemical oxidation oforganic matter in a specified time, at a specified temperature, andunder specified conditions. (2) A standard test used in assessingwaste water strength.

biochemical oxygen demand(BOD) load

The BOD content, usually expressed in kilograms (pounds) per unit oftime, of wastewater passing into a waste treatment system or to abody of water.

biodegradable The destruction of organic materials by organisms and waste watertreatment systems.

biomass The amount (usually measured in kilograms or pounds) of biologicalmaterial contained in the treatment system.

bottom mounting bracket device for fixing and levelling an aeration system pipework on basinbottom

centrifuge Mechanical device used to separate solids from water using acentrifugal force (commonly called spin test when used as a processcontrol test).

chemical oxygen demand(COD)

A quantitative measure of the amount of oxygen required for thechemical oxidation of carbonaceous (organic) material in waste water,using inorganic bichromate or permanganate salts as oxidants in a 2hour test.

ciliated protozoa Small, one-celled organisms possessing cilia (hairlike projections usedfor movement).

clarification Any process or combination of processes, the primary purpose ofwhich is to reduce the concentration of suspended matter in a liquid.The term was formerly used as a synonym for settling orsedimentation. In recent years, the latter terms are preferable whendescribing the settling process.

clarified waste water Waste water from which most of the settleable solids have beenremoved by sedimentation (also called settled waste water).

complete-mix Activated sludge process whereby waste water is rapidly and evenlydistributed throughout the aeration tank, unlike the conventionalaeration process (plug flow).

concentration (1) The amount of a given substance dissolved in a unit volume ofsolution or applied to a unit weight of solids. (2) The process ofincreasing the suspended solids per unit volume of sludge as bysedimentation.

connection for waterdrainage

a cross junction in a water collection pipe or in zone header forconnecting drainage hose (pipe)

connection sleeve part for joining pipes of the diffuser aeration system

contact stabilisation A modification of the activated sludge process using a short contacttime for adsorption of BOD followed by a long contact time for




synthesis or stabilisation by bacteria.

contact time The period of time a substance remains in a basin (see detentiontime).

conventional aeration Process design configuration whereby the organic loading in theaeration tank is higher at the influentend than at the effluent end. Theflow passes through a serpentine system of tanks, typically side-by-side, before passing on to the clarifier (also called plug flow).

core sampler A long, slender pole with a foot valve at the bottom end that allows thedepth of the sludge blanket to be measured (also called sludge judge).

cross junction X or T type coupling for connecting pipes of the same or different sizein straight angle

DDS disc diffuser system

declining growth phase Period of time between the log-growth phase and endogenous phase,where the amount of food is in short supply, leading to ever-slowingbacterial growth rates.

denitrifcation The anaerobic biological reduction of nitrate nitrogen to nitrogen gas.Also, removal of total nitrogen from a system (see also nitrification).

depth of blanket (DOB) The level of sludge, typically measured in metres (feet), in the bottomof the clarifier (see also sludge blanket).

design flow Engineering guidelines that typically specify the amount of influentflow that can be expected on a daily basis over the course of a year.Other design flows can be set for monthly and peak flows.

detention time The period of time a waste water flow is retained in a basin for storageor completion of physical, chemical, or biological reaction (see alsocontact time).

Diffuser aeration device forming air into small bubbles, used for fine bubblebottom aeration systems

dissolved oxygen (DO) The oxygen dissolved in waste water, usually expressed in milligramsper litre, or percent of saturation.

dissolved solids Solids in solution that cannot be removed by filtration; for example,NaCI and other salts that must be determined by evaporation (seealso total dissolved solids).

distribution header air distribution pipe from main header to dropleg pipes

drainage connection, junction Part(s) in water collection pipe for connecting purge hose (pipe)

drainage coupling A coupling for condensate purge hose (pipe) in water drainageconnection

dropleg, dropleg pipe a pipe connecting zone header of an aeration group and distributionheader (a pipe coming from blower)




dynamic equilibrium See population dynamics.

effluent Waste water partially or completely treated, flowing out of a basin,treatment plant, or industrial treatment plant.

effluent quality The physical, biological, and chemical characteristics of waste wateror other liquid flowing out of a basin, pipe, or treatment plant.

effluent standard Specification of the allowable concentration or mass of a constituentthat may be discharged.

effluent stream A stream of treated waste water.

element, diffuser element a pipe (of max. 5 m length) with assembled diffusers mounted on

endogenous phase See endogenous respiration.

endogenous respiration The internal digestion of stored food within the organism occurringwhen the external food sources are limited.

EPDM elastomer material used in membrane diffusers, ethylene-propylene-diene

excess sludge The sludge produced in an activated sludge treatment process, or anyother process that requires sludge recirculation, that is not needed tomaintain the process and is withdrawn from circulation (also calledwaste sludge or waste activated sludge WAS).

extended aeration A modification of the activated sludge process that provides foraerobic sludge digestion within the aeration system. The processincludes the stabilisation of organic matter under aerobic conditions.Effluent contains finely divided suspended matter and soluble matter.

extended aeration process A modification of the activated sludge process using long aerationperiods to promote aerobic digestion of the biological mass byendogenous respiration.

facultative The ability of an organism to live in aerobic or anaerobic conditions.

filamentous growth Intertwined, threadlike biological growths, characteristic of somespecies of bacteria, fungi, and algae. Such growths reduce sludgesettleability and dewaterability.

filamentous organisms Bacterial, fungal, and algal species that grow in thread-like colonies,resulting in a biological mass that will not settle and may interfere withdrainage through a filter.

filamentous sludge Activated sludge characterised by excessive growth of filamentousbacteria, resulting in poor sludge settling.

flange drilled plate for joining pipes, e.g. dropleg and zone header

floc Collections of smaller particles agglomerated into larger, more easilysettleable particles through chemical, physical, or biological treatment(see also flocculation).




flocculation In water and waste water treatment, the agglomeration of colloidal andfinely divided suspended matter after coagulation by gentlemechanical or hydraulic stirring. In biological waste water treatmentwhere coagulation is not used, agglomeration may be accomplishedbiologically.

flow The movement of water or other fluids from place to place.

flow rate Q Volume of liquid that passes through a cross-section of conduit in agiven time; measured in such units as kg/h, m3/s, l/d or gallons perday.

flow recording Documentation of the quantity of rate of flow.

food to micro-organism ratioFM

In the activated sludge process, the loading rate expressed as kgBOD5 / kg mixed liquor or mixed liquor volatile suspended solids /d.

foot part of bottom mounting bracket, fixed on tank bottom

free-swimming ciliates Mobile, one-celled organisms using cilia (hairlike projections) formovement.

fungi Small non-chlorophyll-bearing plants lacking roots, stems, or leaves.Fungi occur in, among other places, water, waste water, or wastewater effluents and grow best in the absence of light.

header (pipe) diffuser row consisting of diffuser elements where diffusers aremounted, connected to zone header

high-purity oxygen A modification of the activated sludge process using relatively pureoxygen and covered aeration basins in conventional flowarrangement.

high-rate aeration A modification of the activated sludge process whereby the mixedliquor suspended solids loadings are kept high, allowing high food tomicro-organism FM ratios and shorter detention times.

holder pipe support, part of TPK bottom mounting bracket, syn. clamp

hydraulic loading Waste water amount applied to treatment process, usually expressedas volume / unit time, or volume / unit time / unit surface area.

hydraulic retention time a ratio: total aeration basin volume / influent flow (m3/h)

influent Waste water flowing into a basin, treatment plant, or treatmentprocess (see antonym effluent).

inorganic compounds All those combinations of elements that do not include organic carbon.

inorganic matter Mineral-type compounds that are generally nonvolatile,noncombustible, and nonbiodegradable. Most inorganictypecompounds, or reactions, are ionic in nature; therefore, rapid reactionsare characteristic.

Kjeldahl nitrogen test A standard analytical method used to determine the concentration of




organically bound ammonia nitrogen state.

KLa Overall mass transfer coefficient of aeration system (of oxygen)

layout drawing of the placement of diffusers/aeration groups in basin bottom

log growth phase The initial stage of bacterial growth, during which there is a plentifulsupply of food, causing bacteria to grow at the maximum rate.

main body a saddle like part of diffuser for fixing diffuser to a header pipe;diffuser disc is fixed to main body

main header air distribution pipe from the blower(s)

maximum flow The greatest volume of influent to a treatment plant within a giventime period (see peak flow).

mean cell residence timeMCRT

Average time a given unit of cell mass stays in the activated sludgeaeration tank. Mean cell residence time is typically calculated as ratioof total mixed liquor suspended solids in aeration tank to that of wastewater.

mean flow The arithmetic average of the discharge at a given point or station onthe line of flow for some specified period of time (see design flow).

mechanical aeration (1) The mixing, by mechanical means, of wastewater and activatedsludge in the aeration tank of the activated sludge process to bringfresh surfaces of liquid into contact with the atmosphere. (2) Theintroduction of atmospheric oxygen into a liquid by the mechanicalaction of a paddle, paddle wheel, spray, or turbine mechanism.

mechanical aerator A mechanical device used for introducing atmospheric oxygen into aliquid (see also mechanical aeration).

membrane diffuser diffuser of elastic (rubber) disc or tube used as a bubble formingdevice

metazoa Group of animals having bodies composed of cells differentiated intotissues and organs and usually a digestive cavity lined withspecialised cells.

micro-organisms Microscopic organisms, either plant or animal, that are invisible orbarely visible to the naked eye. Examples are algae, bacteria,fungi/protozoa, and viruses.

microbial activity The activities of micro-organisms resulting in chemical or physicalchanges.

microbiology The study of microscopic organisms of living matter and theirprocesses.

microscopic examination (1) The examination of wastewater to determine the presence andamount of plant and animal life such as bacteria, algae, and protozoa.(2) The examination of wastewater to determine the presence ofmicroscopic solids. (3) The examination of microbiota in processwater, such as the mixed liquor in an activated sludge plant.




minimum flow (1) Flow occurring in a stream during the driest period of the year(also called low flow). a) The lowest quantity of influent to a treatmentplant or within a sewer within a given time period (see antonym peakflow).

mixed liquor Mixture of raw or settled waste water and the activated sludgeprocess.

mixed liquor suspendedsolids MLSS

The concentration of suspended solids in activated sludge mixedliquor, expressed in milligrams per litre.

mixed liquor volatilesuspended solids MLVSS

That fraction of suspended solids in activated sludge mixed liquor thatcan be driven off by combustion at 550 °C (1022 °F); indicates theconcentration of active micro-organisms available for biologicaloxidation.

moving average A tool used in trend analysis for determining patterns or changes intreatment processes. For example, a 7-day moving average would bethe sum of the datum points for 7 days divided by 7.

National Pollutant DischargeElimination System permit

NPDES, Permit that is the basis for the monthly monitoring reportsrequired by most states in the U.S.

nematodes Any of a phylum (Nematoda) of elongated cylindrical worms parasiticin animals or plants or free-living in soil or water.

nitrate An oxygenated form of nitrogen, typically written NO3 (see nitrogen).

nitrification The oxidation of ammonia nitrogen to nitrate nitrogen in wastewaterby biological or chemical reactions (see also denitrification).

nitrite An intermediate oxygenated form of nitrogen typically written (NO2).

nitrogen An essential nutrient often present in waste water as ammonia, nitrate,nitrite, and organic nitrogen. The concentrations of each form and thesum, total nitrogen, are expressed as milligrams per litre elementalnitrogen. Nitrogen is also present in some ground water as nitrate andin some polluted ground water in other forms.

NOPOL® registered trademark of the NOPOL® DDS and NOPOL® O.K.I.Aeration Systems (use capital letters)

NOPOL® CLEAN a method for cleaning diffusers during operation by spraying formicacid (HCOOH) into the aeration air

NOPOL® O.K.I. submersible aerator mixer

organic Volatile, combustible, and sometimes biodegradable chemicalcompounds containing carbon atoms (carbonaceous) bonded togetherand with other elements. The principal groups of organic substancesfound in waste water are proteins, carbohydrates, and fats and oils(see antonyms inorganic compounds aunt inorganic matter).

organic loading The amount of organic material, typically measured as BOD5, appliedto a given treatment process; expressed as weight per unit time per




unit surface area or unit weight.

organic matter Chemical substances of animal or vegetable origin or, more correctly,containing carbon and hydrogen.

overflow rate The settling velocity of particles removed in an ideal basin if theyenter at the surface; one of the criteria for the design of settling tanksin treatment plants. Overflow rate is expressed as volume of flow perunit water surface area of the tank (see also surface overflow rate).

oxidation ditch A secondary waste water treatment facility that uses an oval channelwith a rotor placed across it to provide aeration and circulation. Thescreened waste water in the ditch is aerated by the rotor andcirculated at about 0.3 to 0.6 m/s (I to 2 ft/sec) (see also secondarytreatment).

oxygen demand The quantity of oxygen used in the oxidation of substances in aspecified time, at a specified temperature, and under specifiedconditions.

oxygen uptake rate OUR The oxygen used during biochemical oxidation, typically expressed asmilligrams O2 per litre per hour in the activated sludge process.

peak flow The maximum rate of influent flow a treatment plant expects toreceive during a specified time period (for example, peak hourly, peakdaily, peak monthly).

PEHD high density polyethylene, used in sintered diffuser discs

pH A measure of the hydrogen-ion concentration in a solution. On the pHscale (0 to 14), a value of 7 at 25 °C (77 °F) represents a neutralcondition. Decreasing values, below 7, indicate an increasinghydrogen-ion concentration (acidity); increasing values, above 7,indicate a decreasing hydrogen-ion concentration (alkalinity).

pin floc Small floc particles that settle poorly.

plug flow See conventional aeration.

population dynamics The ever-changing numbers of microscopic organisms within theactivated sludge process (also called dynamic equilibrium).

positive displacement pump A type of pump in which the water is induced to flow from the sourceof supply through an inlet pipe and inlet valve. Water is brought intothe pump chamber by a vacuum created by the withdrawal of a pistonor pistonlike device which, on its return, displaces a certain volume ofthe water contained in the chamber and forces it to flow through thedischarge valves and discharge pipes.

primary effluent The liquid portion of waste water leaving primary treatment.

primary sludge Sludge obtained from a primary settling tank.

primary treatment (1) The first major treatment in a waste water treatment facility,usually sedimentation but not biological oxidation. (2) The removal of




a substantial amount of suspended matter but little or no colloidal anddissolved matter. (3) Waste water treatment processes usuallyconsisting of clarification with or without chemical treatment toaccomplish solids-liquid separation.

protozoa Small animals, including amoebae, ciliates, and flagellants

publicly owned treatmentworks POTW

In general, another name for waste water treatment plants.

Purge hose, purge pipe hose or pipe for conducting condense water out of water drainagepipe, to be fixed to a drainage coupling

raw influent Waste water before it receives any treatment.

receiving water A river, lake, ocean, or other watercourse to which waste water ortreated effluent is discharged

respiration The intake of oxygen and discharge of carbon dioxide during theprocess of bacterial decomposition of organic materials

respiration rate See specific oxygen uptake rate SOUR

return sludge Biomass produced in the activated sludge process that is recycled tothe head of the process to promote more complete biologicaloxidation (also called return activated sludge RAS).

rotifers Minute, multicelled aquatic animals possessing a circular set or sets ofciliate resembling wheels

Sarcodina Species of amoebae found in waste water

screw-on ring ring shape part for fixing diffuser disc to main body of diffuser,threaded or bayonet type

secchi disk A visual inspection tool to measure the clarity or turbidity of theeffluent

secondary effluent (1) The liquid portion of waste water leaving secondary treatment (2)An effluent that contains not more than 30 mg/l each of BOD5 andsuspended solids.

secondary treatment (1) Typically, a level of treatment that produces removal efficienciesof 85 % for biochemical oxygen demand (BOD) and suspended solids(2) Sometimes used interchangeably with the concept of biologicalwaste water treatment, particularly activated sludge process. Thisterm is commonly applied to treatment that consists of clarificationfollowed by a biological process, with separate sludge collection andhandling.

septic See anaerobic

settleability test Determination of the settleability of solids in suspension by measuringthe volume of solids settled out of a measured sample over aspecified interval of time; typically reported in ml/l (see settleometer).




settleometer A 2-litre or larger beaker used to conduct the settleability test

sludge (1) Accumulated solids separated from waste water during processing.(2) The removed material resulting from flocculation, sedimentation,and/or biological oxidation of waste water (see also activated sludge).

sludge age A ratio: excess sludge / total MLSS, the average residence time ofsuspended solids in a biological waste treatment system, equal to thetotal weight of suspended solids in the system divided by the totalweight of suspended solids leaving the system per unit of time(typically per day)

sludge blanket Accumulation of sludge hydrodynamically suspended within anenclosed body of waste water (see depth of blanket)

sludge judge See core sampler

sludge load a ratio: total BOD load (kg/d) / total MLSS

sludge return ratio a ratio: volumetric return sludge flow / influent flow; expressed in %

sludge solids Dissolved and suspended solids in sludge

sludge volume index SVI The ratio of the volume in millilitres (cubic inches) of sludge settledfrom a 1000 ml (60 cu in.) sample in 30 minutes to the concentrationof mixed liquor in milligrams per litre multiplied by 1000.

solids In waste water treatment, any dissolved, suspended, or volatilesubstance contained in or removed from waste water.

solids inventory The amount of sludge in the treatment system typically expressed inkilograms (tons). The inventory of plant solids can be tracked throughuse of a mass balance set of calculations.

solids loading Amount of solids applied to a treatment process per unit time per unitvolume

solids retention time SRT The average time of retention of suspended solids in a biologicalwaste treatment system, equal to the total weight of suspended solidsleaving the system per unit of time (typically per day).

SOTE Standard Oxygen Transfer Efficiency (%), amount of oxygendissolved into water per total amount of oxygen in the air fed into thesystem

SOTR Standard Oxygen Transfer Rate, oxygenation capacity in clean waterin standard conditions

specific oxygen uptake rateSOUR

Measures the microbial activity in the biological system. It is typicallyexpressed as milligrams O2 per hour per gram of volatile suspendedsolids (VSS) (also called respiration rate).

spin test See centrifuge

stabilisation A process used to equalise waste water flow composition beforeregulated discharge




stalked ciliates Small, one-celled organisms possessing cilia (hairlike projections usedfor feeding) but that are not mobile.

step feed A procedure for adding increments of settled waste water along theline of flow in the aeration tanks of an activated sludge plant (alsocalled step feed aeration).

straggler floc Large (6 mm [0.25 in.] or larger) floc particles that have poor settlingcharacteristics

submersion depth water depth minus diffuser level from basin bottom

suctoreans Ciliates that are stalked in the adult stage and have rigid tentacles tocatch prey.

supernatant The liquid remaining above a sediment or precipitate aftersedimentation

surface overflow rate Design criterion used in sizing clarifiers, typically expressed asvolume of flow per unit amount of clarifier surface area (m3/m2 d,gpd/sq It).

suspended solids SS (1) Insoluble solids that either float on the surface of, or are insuspension in, water, waste water, or other liquids. (2) Solid organic orinorganic particles (colloidal, dispersed, coagulated, flocculated)physically held in suspension by agitation or flow.

temperature (1) Thermal state of a substance with respect to its ability to transmitheat to its environment. a) The measure of the thermal state on somearbitrarily chosen numerical scale such as Celsius or Fahrenheit.

tertiary treatment The treatment of waste water beyond the secondary or biologicalstage. Tertiary treatment normally implies the removal of nutrients,such as phosphorus and nitrogen, and of a high percentage ofsuspended solids (see also advanced waste treatment).

total carbon TC A quantitative measure of both total inorganic (TIC) carbon and totalorganic (TOC) carbon, in milligrams per litre, in water or waste water,as determined instrumentally by chemical oxidation to CO2, andsubsequent infrared detection in a carbon analyser.

total dissolved solids TDS The sum of all dissolved solids (volatile and non-volatile) in wastewater.

total organic carbon TOC The amount of carbon bound in organic compounds in a sample.Because all organic compounds have carbon as the commonelement, total organic carbon measurements provide a fundamentalmeans of assessing the degree of organic pollution.

total oxygen demand TOD A quantitative measure of all oxidisable material in a sample of wastewater as determined instrumentally by measuring the depletion ofoxygen after high-temperature combustion.

total solids TS The sum of dissolved and suspended solids in waste water.




total suspended solids TSS The amount of insoluble solids floating and in suspension in the wastewater. It is referred to as total nonfilterable residue.

toxicity The adverse effect on living organisms by some agent (for example,heavy metals or pesticides)

trace nutrients Substances vital to bacterial growth. Trace nutrients are defined inthis text as nitrogen, phosphorus, and iron.

trend analysis The use of data and statistical tools to study patterns and changes inwaste water treatment processes. Computer software programs aid inthe speed and scope of this type of analysis.

tube diffuser tube, cylindrical shape diffuser

turbidity (1) A condition in water or waste water caused by the presence ofsuspended matter, resulting in the scattering and absorption of light.a) Any suspended solids imparting a visible haze or cloudiness towater, which can be removed by filtration. (3) Analytical quantitydetermined by measurements of light scattering and typically reportedin turbidity units (Formazin turbidity units (FTU) or Jackson turbidityunits (JTU)).

ultimate biochemical oxygendemand BODU

(1) Commonly, the total quantity of oxygen required to satisfycompletely the first-stage biochemical oxygen demand (BOD). (2)More strictly, the quantity of oxygen required to satisfy completelyboth the first- and second-stage BODs.

uPVC unplastised polyvinyl chloridevirus The smallest lifeform capable of producing infection and disease in

humans or other large species.

volatile solids VS Material, generally organic, that can be removed from a sample byheating, typically to 550 °C (1022 °F); non-volatile inorganic solids(ash) remain.

volatile suspended solidsVSS

That fraction of suspended solids, including organic matter andvolatile inorganic salts, that will ignite and burn when placed in anelectric muffle furnace at 550 °C (1022 °F) for 60 minutes.

volumetric load a ratio: BOD load (kg/d) / total aerated volume

volumetric loading The amount of flow applied to a treatment process per unit time perunit volume of the basin or clarifier.

waste sludge Biological sludge that is drawn off to be conditioned for ultimatedisposal (also called waste activated sludge WAS; see also excesssludge and return sludge).

water collection pipe a pipe in the end of the diffuser rows connecting these and forming aclosed loop pipe work

wedge piece part for fixing diffuser main body mechanically to pipe




weir (1) Diversion dam (2) Device that has a crest and some sidecontainment of known geometric shape, such as a V, trapezoid, orrectangle, and is used to measure flow of liquid. Liquid surface isexposed to the atmosphere. Flow is related to upstream height ofwater above the crest, position of crest with respect to downstreamwater surface, and geometry of the weir opening.

weir overflow rate The amount of flow applied to a treatment process (typically aclarifier) per linear measure of weir (m3/m d).

washout Condition whereby excessive influent flows (typically at peak flowconditions) cause solids in aeration basin and / or clarifier to becarried over into downstream processes or discharged to receivingstream.

zone header the pipe with branches for connecting diffuser elements (rows) andone flange to connect dropleg pipe

10.3 Conversion Factors

To convert, multiply in direction shown by arrows

U.S. units ⇒ ⇐ SI units

acre(mgal/d) 0.1069 9.3536 ha/(103m3/d)

Btu 1.0551 0.9478 kJBtu/lb 2.3241 0.4303 kJ/kg

Btu/ft2. Fo.h

5.6735 0.1763W/m2. Co

bu/acre-yr 2.4711 0.4047 bu/ha.yrft/h 0.3048 3.2808 m/hft/min 18.2880 0.0547 m/hft2/capita 0.0929 10.7639 m2/capitaft3/capita 0.0283 35.3147 m3/capitaft3gal 7.4805 0.1337 m3/m3

ft3/ft.min 0.0929 10.7639 m3/m.minft3/lb 0.0624 16.0185 m3/kgft3/Mgal 7.04805x10-3 133.6805 m3/103m3

ft2/Mgal.d 407.4611 0.0025 m2/103m3.dft3/ft2.h 0.3048 3.2808 m3/m2.hft3/103gal.min 7.04805x10-3 133.6805 m3/m3

ft3/min 1.6990 0.5886 m3/hft3/s 2.8317x10-2 35.3145 m3sft3/103ft3.min 0.001 1.000.0 m3/m3.mingal 3.7854 0.2642 Lgal/acre.d 0.0094 106.9064 m3/ha.d




gal/ft.d 0.0124 80.5196 m3/m.dgal/ft2.d 0.0407 24.5424 m3/m2.dgal/ft2.d 0.0017 589.0173 m3/m2.hgal/ft2.d 0.0283 35.3420 L/m2.mingal/ft2.d 40.7458 2.4542x10-2 L/m2.mingal/ft2.min 2.4448 0.4090 m/hgal/ft2.min 40.7458 0.0245 L/m2.mingal/ft2.min 58.6740 0.0170 m3/m2.dgal/min.ft 12.4193 8.052x10-2 L/min.mhp/103gal 0.1970 5.0763 kW/m3

hp/103ft3 26.3342 0.0380 kW/103m3

in 25.4 3.9370x10-2 mmin Hg(60°F) 3.3768 0.2961 kPaHg(60°F)lb 0.4536 2.2046 kglb/acre 1.1209 0.8922 kg/halb/103gal0.1198 0.1198 8.3452 kg/m3

lb/hp.h 0.6083 1.6440 kg/kw.hlb/Mgal 0.1198 8.3454 g/m3

lb/Mgal 1.1963x10-4 8345.4 kg/m3

lb/ft2 4.8824 0.2048 kg/m2

lb/in2(gage) 6.8948 0.1450 kPa(gage)lb/ft3.h 16.0185 0.0624 kg/m3.hlb/103ft3.d 0.0160 62.4280 kg/m3.dlb/ton 0.5000 2.0000 kg/toneMgalacre.d 0.9354 1.0691 m3/m2.dMgal/d 3.7854x103 0.264x10-3 m3/dMgal/d 4.3813x103 22.8245 m3/smin/in 3.9370 0.2540 min/102mmtons/acre 2.2417 0.4461 Mg/hayd3 0.7646 1.3079 m3

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