Activated Sludge Bioselector Processes

Source: 1987 Annual Report, Davenport Wastewater Treatment Facility, Davenport, Iowa. (Used with permission)

History ● Benefits ● Arrangement ● Design● Experiences ● Troubleshooting ● Research Needs

Orris E. AlbertsonEnviro Enterprises, Inc.

LaBarge, WY

May 2005

TABLE OF CONTENTS

LIST OF TABLES............................................................................................................. iLIST OF FIGURES .........................................................................................................iiiDEFINITIONS/ACRONYMS.......................................................................................... vEXECUTIVE SUMMARY .......................................................................................... E-1

1.0 PROCESS DESCRIPTION ...............................................................................1-11.1 Definitions/Objectives ...........................................................................1-11.2 History of Bioselector Development .....................................................1-31.3 Theory of Bioselection.........................................................................1-121.4 Filamentous vs. Non-filamentous Bulking ..........................................1-151.5 Measurement of the Sludge Volume Index .........................................1-201.6 Dilute Sludge Volume Index Procedure ..............................................1-261.7 Conversion of uSVI to DSVI or SSVI.................................................1-28

2.0 BENEFITS OF BIOSELECTION .....................................................................2-1

3.0 GENERAL DESIGN ARRANGEMENT OF BIOSELECTORS .....................3-13.1 F/M Cascade Design ..............................................................................3-13.2 Contact Loading Analysis....................................................................3-113.3 Sludge Age...........................................................................................3-123.4 Design Features....................................................................................3-16

4.0 TYPES OR DESIGNS OF BIOSELECTORS ..................................................4-14.1 Aerated, High DO Bioselectors (SXAH).................................................4-74.2 Aerated, Low DO Bioselectors (SXAL)................................................4-124.3 Anoxic Bioselectors (SXAXM).............................................................4-184.4 Anaerobic Bioselectors(SXANM).........................................................4-12

5.0 PROCESS EXPERIENCES WITH BIOSELECTORS.....................................5-15.1 Davenport, IA – Aerated, Low DO (SXAL) and Anoxic (SXAXM)........5-15.2 Columbus Southerly, OH – Aerated, Low DO (SXAL)..........................5-35.3 Columbus Jackson Pike, OH – Aerated, Low DO (SXAL) ....................5-95.4 Santa Fe, NM – Aerated, Low DO (SXAL) ..........................................5-135.5 Gig Harbor, WA – Aerated, Low DO (SXAL) .....................................5-165.6 Phoenix 23rd Ave, AZ – Anoxic, Aerated, Low DO (SXAXAL)...........5-185.7 Phoenix 91st Ave, AZ – Anoxic, Aerated, Low DO (SXAXAL) ...........5-245.8 Tri City, Clackamas County, OR – Anaerobic (SXANM)

and Anoxic (SXAXM and SXAXAL)......................................................5-285.9 Upper Occoquan Sewage Authority, VA –

Aerated, High DO (SXAH) ...................................................................5-335.10 Hamilton, OH – Anoxic (SXAXAL and SXAXM) .................................5-365.11 Middletown, OH – Anaerobic (SXANM) .............................................5-385.12 Star Valley Cheese Coop, Thayne, WY – Anaerobic (SXANM)..........5-39

5.13 Tree Top, Selah, WA–Anaerobic (SXANM) andAerated, High DO (SXAH) ...................................................................5-39

5.14 Fibra, America, Brazil – Anaerobic (SXANM).....................................5-40

6.0 TROUBLESHOOTING BIOSELECTORS ......................................................6-16.1 Low F/M in the ICZ...............................................................................6-26.2 High F/M in the ICZ ..............................................................................6-46.3 Air Rate to the Bioselectors ...................................................................6-56.4 Limited Oxygenation Capacity in the Oxic Zones ................................6-66.5 Secondary Bulking (Oxic Zone) ............................................................6-66.6 A Very Low SVI – High Turbidity........................................................6-76.7 Toxic/Inhibitory Compounds.................................................................6-76.8 Limited Nitrogen and Phosphorus Supply/Availability.........................6-86.9 Soluble Organics Breakthrough.............................................................6-86.10 Summary Comments..............................................................................6-9

7.0 RESEARCH NEEDS......................................................................................... 7.17.1 General Introductory Comments............................................................ 7.27.2 Research Target ..................................................................................... 7.47.3 Recommended Areas of Research ......................................................... 7.4

i

List of Tables

1.1 Activated Sludge Solids-Liquid Separation Problems............................................... 1-2

1.2 Comparison of Elements of Early Anaerobic/Aerobic Bioselector Processes ........ 1-12

1.3 Probable Causes of Hydrous Sludge Bulking and Filamentous Bulking ................ 1-16

1.4 Major Occurrences of Viscous/Hydrous Bulking.................................................... 1-19

2.1 Benefits of Bulking Control by Bioselection............................................................. 2-3

2.2 Effectiveness of Bioselectors in Controlling Filamentous Organisms .................... 2-10

3.1 General Design Guidelines for Bioselector Sizing

(after Albertson 1987,1992, 1994)............................................................................. 3-6

3.2 Prior Art Batch and Continuous Flow Experience with Bulking Sludge Control

Concepts and the DO and BOD5 Mass Loading in the ICZ ...................................... 3-7

3.3 Summary of the Design and Operation of Bioselectors

by Daigger and Nicholson (1990).............................................................................. 3-8

3.4 Summary of the Design and Operation of Bioselectors

by Marten and Daigger (1997)................................................................................... 3-9

3.5 Calculation of Baffle Wall Height ........................................................................... 3-19

4.1 Characteristics of Initial Contact Zone of Bioselectors ............................................. 4-3

4.2 Design FS/M Criteria for Anoxic (SXAXAL or SXAXM) Bioselectors

to Account for Daily Peaking Factors........................................................................ 4-4

4.3 Wastewater Characteristics Used in an Equation

to Establish Criteria for the Bioselector Design ........................................................ 4-5

4.4 Technical Parameters and Results of the Bioselector Activated Sludge Process

in the Leopoldsdorf Sugar Mill Obtained During the Campaign in 1984 (Kroiss) ... 4-9

4.5 Design Recommendations Aerated, High DO (SXAH) Bioselectors........................ 4-11

4.6 Results of 1973a Chudoba Laboratory Staged Aeration Studies............................. 4-12

4.7 Design Recommendations for Aerated SXAL and SXAXAL Bioselectors................. 4-18

4.8 Design Recommendations for Mechanically Mixed Anoxic

(SXAXM) Bioselectors.............................................................................................. 4-21

4.9 Deer Island, Boston, MA, Pilot Study Results ........................................................ 4-24

4.10 Phase 3 Secondary Treatment Performance at Deer Island WWTP........................ 4-26

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4.11 Design Recommendations for Three-Stage Anaerobic (SXANM) Bioselectors....... 4-30

5.1 Summary of the Columbus, OH Southerly Operating Results .................................. 5-8

5.2 Summary of the Columbus, OH Jackson Pike Operating Results ........................... 5-12

5.3 Summary of the Santa Fe, NM Operating Results................................................... 5-15

5.4 Summary of the Gig Harbor, WA Operating Results .............................................. 5-19

5.5 Prototype NdeN Performance Data, Monthly Average, Phoenix 91st Ave.............. 5-29

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List of Figures

1.1 Davidson’s Anaerobic-Aerobic Bulking Sludge Control Process .....................1-5

1.2 Relationship of SVI and sCOD in First Compartment of Reactors ...................1-6

1.3 1974 Heide and Pasveer Flowsheet: Bulking Control and TN Removal...........1-8

1.4 1975 The Five Stage Phoredox Process (Modified Bardenpho)........................1-9

1.5 NdeN Process Flowsheets of Ludzak-Ettinger, Drews and Barnard ...............1-10

1.6 Three and Five Stage UCT NdeN Processes ...................................................1-11

1.7 General Response of Activated Sludge to ICZ Food/Mass Ratio....................1-18

1.8 SVI vs Initial MLSS @ SSVI 80-85 mL/g ......................................................1-21

1.9 Settling Tests for SVI – Settlometer ................................................................1-24

1.10 Settling Tests for SVI – Graduated Cylinder...................................................1-25

1.11 Calculated DSVI from SVI Employing Merkel’s (1971) SVI to DSVI

Equation ...........................................................................................................1-30

2.1 The Effect of Extended Filament Length on the MLSS SVI (Palm et al.)........2-2

2.2 Secondary Clarifier Operation Diagram SSVI3.5 ...................................................................... 2-4

2.3 Secondary Clarifier Operating Diagram – DSVI...............................................2-6

2.4 Secondary Clarifier Operating Diagram – SVI (uSVI)......................................2-7

2.5 Secondary Clarifier Operation Diagram – DSVI...............................................2-8

3.1 Semi-Aerobic Process for Bulking Control and Nutrient Removal...................3-2

3.2 General Arrangement of Bioselectors in U.S. Facilities....................................3-4

3.3 Guideline of the Design/Operable Contact Loading in ICZ ............................3-13

3.4 Controlling the Contact Loading with Internal Recycle ..................................3-14

3.4.1 Relationship between Biomass Retention Time, SRT, and Occurrence of

Filamentous Microorganisms (after Wanner, 1994)........................................3-15

3.5 Design of Submergence Depth and Baffle Wall Arrangement........................3-17

3.6 Scum-Free Bioselector Zones with Submerged Baffle Walls .........................3-18

4.1 Effect of ICZ F/M on SVI of MLSS (after Chudoba et al., 1973a 1973b)......4-14

4.2 Relation between SVI and Theoretical Sludge Loading (MLSS = 3.5 g/l)

in the First Aeration Compartment of Plants Included in the Survey

(after Tomlinson, 1976) ...................................................................................4-16

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4.3 Relationship between SVI and Phosphorus to Organic MLSS Ratio ..............4-29

5.1 Davenport, IA – Wastewater Treatment Facility Annual Report, 1987 ............5-2

5.2 Davenport, IA – Historical SVI Data.................................................................5-4

5.3 Columbus, OH, Southerly – General Process Concept/Semi-Aerobic ..............5-6

5.4 Columbus, OH, Southerly – Results ..................................................................5-7

5.5 Columbus, OH Jackson Pike – Modified Aeration Basins

with SXAL Bioselectors ....................................................................................5-10

5.6 Columbus, OH Jackson Pike – Results............................................................5-11

5.7 Santa Fe, NM – Aerated Anoxic Bioselector Modifications (16.5 ft WD) .....5-14

5.8 Gig Harbor, WA – Bioselectors in a Contact-Stabilization Process................5-17

5.9 Phoenix 23rd Ave. – Modified Aeration Basin with Bioselection ...................5-21

5.10 Phoenix 23rd Ave. – Flow, SVI and MLSS......................................................5-22

5.11 Phoenix 23rd Ave. – Bi-weekly Range and Average DSVI .............................5-23

5.12 Phoenix 91st Ave. – Bioselector in the NdeN Aeration Basins........................5-26

5.13 Phoenix 91st Ave. – NdeN Demonstration Data ..............................................5-27

5.14 Tri-City, OR – Configuration of Anoxic Selector System

(Daigger and Nicholson, 1990)........................................................................5-31

5.15 Tri-City, OR – Performance of an Anoxic Selector

Daigger and Nicholson, 1990) .........................................................................5-32

5.16 UOSA, VA – Aeration Basin and Selector System

(Daigger and Nicholson, 1990)........................................................................5-34

5.17 UOSA, VA – Effect of Selector Operation on SVI

(Daigger and Nicholson, 1990)........................................................................5-35

5.18 Hamilton, OH – Monthly Average SVI...........................................................5-37

5.19 Fibra, Brazil – Sludge Volume Index Control .................................................5-42

6.1 True Nature of Hydrous (Viscous) Bulking Organisms ....................................6-3

v

Definitions of Acronyms

alpha F - a characteristic of wastewater, ratio of oxygen transfer to wastewater vs.

clean water

AN - non-selector anaerobic zone for Bio-P removal

AX - non-selector anoxic denitrification zone for nitrate removal

AOTR - actual oxygen transfer rates, mg/L⋅hr, kg/hr

Bio-P - biological removal of phosphorus by a specialized group of organisms.

BOD5 - five-day biochemical oxygen demand, mg/L, kg/d

C - Fteley and Stearns submerged weir coefficient, Table 3.2

C-S - contact-stabilization activated sludge process

CBOD5 - five-day inhibited biochemical oxygen demand, mg/L, kg/d

CL - contact loading, mg sCOD/g TSS (RSS + IR MLSS)

COD - chemical oxygen demand, mg/L, kg/d

CMAS - complete mix activated sludge process

DF - dilution factor for DSVI test procedure

DO - dissolved oxygen, mg/L

DSVI - see SVI

ECP - exo-cellular protoplasm, sometimes improperly generalized as

polysaccharides unless specifically identified as polysaccharides

F/M - food (COD, BOD5)/mass, kg COD/kg MLSS⋅d, kg BOD5/kg MLSS⋅d

F/ΣM - food/ sum of the mass, kg/kg⋅d

FS/M - soluble food/mass, kg sCOD/kg MLSS⋅d, kg sBOD5/kg MLSS⋅d

IR - internal recycle flow from oxic zone to anoxic bioselector and/or anoxic

zone, L/s, m3/hr, ML/d

M - mechanically mixed anoxic or anaerobic bioselector

MCRT - mean cell residence time includes all solids in the process, days

ML - activated sludge mixed liquor, mg/L

MLSS - mixed liquor suspended solids, mg/L or g/L (also megaliter, ML/d)

MLVSS - mixed liquor volatile suspended solids

vi

N - nitrogen, any form

NA - not available

NH4-N - ammonia nitrogen, mg/L, kg/d

NO3-N - nitrate nitrogen, mg/L, kg/d

NO2-N - nitrite nitrogen, mg/L, kg/d

OX - oxic zone with typical DO level of > 2 mg/L

OUR - oxygen uptake rate, mg/L⋅hr, mg/g⋅hr

P - phosphorus, any form

Q, Qi - influent (raw or primary effluent) wastewater, L/s, m3/hr, ML/d

RAS - return activated sludge flow from clarification, L/s, m3/hr, ML/d

RSS - RAS suspended solids concentration, mg/L, g/L

sBOD5 - soluble BOD5, usually filtered at 0.45 or 1.5 µm, mg/L, %BOD5, kg/d

sCOD - soluble COD, usually filtered at 0.45 or 1.5 µm, mg/L, %COD, kg/d

SLR - solids loading rate in the secondary clarifier, kg/m2⋅d

SOTR - standard oxygen transfer rates, kg/m3⋅hr, mg/L⋅hr

SRT - solids retention time includes all solids in the biological reactors, days

SRTOX - aerobic or oxic SRT in zones with target DO > 2 mg/L, days

SVI - sludge volume index, mL/g

DSVI - diluted SVI at 160 to 240 mL/L SSV30, mL/g

SSVI - stirred SVI, mL/g

SSVI2.0 - stirred SVI at 2000 mg/L MLSS, mL/g

SSVI3.5 - stirred SVI at 3500 mg/L MLSS, mL/g

uSVI - undiluted SVI, mL/g

SSV - settled sludge volume at 30 minutes, mL/L

SX - bioselector zone, any type

SXAH - aerated, high DO bioselector

SXAL - aerated, low DO bioselector

SXANM - anaerobic bioselectors with mechanical mixing

SXAXAL - aerated, low DO anoxic bioselector

SXAXM - anoxic bioselectors with mechanical mixing

TKN - total Kjeldahl nitrogen (organic + ammonia), mg/L, kg/d

vii

TN - total nitrogen, mg/L, kg/d

TSS - total suspended solids, mg/L, kg/d

VSS - volatile suspended solids, mg/L, kg/d

WRF - water reclamation facility

WWTP - wastewater treatment plant

ZSV - zone settling velocity, m/hr

ZSV5 - ZSV for first 5 minutes, m/hr

E-1

Executive Summary

It has been over 50 years since the concept of bioselection was first uncovered

(Davidson, 1949) and nearly 30 years since a fundamental approach to bioselection was

published (British Ministry, 1969; Chudoba et al, 1973a, 1973b, 1974; Heide and Pasveer,

1974). The process of bioselection is now widely employed for new and retrofitted activated

sludge facilities. However, there is neither a common nor accepted methodology for bioselector

design, and as a result, the volumetric requirements of individual bioselector basins may vary by

50 to 1100% of the recommendations set forth in this study. While bioselection is successful in

reducing the sludge volume index (SVI) in nearly every case, the level of performance, as

measured by the average and especially the maximum SVIs, can vary widely. Maximum SVIs

govern biological process design – including clarification.

The common U.S. practice is to use the undiluted SVI (uSVI or SVI) to define sludge

settling/compaction characteristics. Unfortunately, uSVI is a very inaccurate method and, in

many cases, will prevent an acceptable comparison of data among plants with and without

bioselectors. This is particularly true when the settled sludge volume (SSV30) exceeds 300

mL/L, typical of facilities operating at >3000 mg/L mixed liquor suspended solids (MLSS) or

when the SVIs are >120 to 150 mL/g. This problem hampered this analysis and will cause

difficulties with any future study. The input data for an objective analysis of the sludge settling

characteristics must be diluted SVI (DSVI) or stirred SVI (SSVI), the European standard

procedure.

There are excellent case histories of multi-staged (three or four zones) bioselectors with

settling/compaction characteristics well within the target area of an average SSVI or DSVI of 60

< 80 mL/g and a maximum DSVI value of 80 < 100 mL/g. When the DSVI is controlled within

this range, wastewater processes are stable, operate at high efficiency, and result in maximum

utilization of aeration and clarifier capacity.

E-2

The body of technical literature on fundamental research and field studies of bioselectors

is large and increasing. There is the aforementioned problem of reducing the data to correlate

design and process factors to the activated sludge settling characteristics, unless DSVI or SSVI

measurements are employed. Currently, there are many designs of bioselectors ranging from one

up to six zones; they may be aerated or not, high DO, low DO, anoxic or anaerobic, and with

food/mass (F/M) ratios of 0.70 to 14.9 kg BOD5/kg MLSS.d in the initial contact zone (ICZ).

The number of independent variables influencing the bioselector performance could exceed 15

separate items of input data/information.

This document presents a portion of the pertinent literature and numerous references on

the control of bulking sludges by the natural biological method. Case histories are included to

support design recommendations. The bioselector design recommendations are provided for:

• Aeration, high DO (SXAH), secondary and advanced wastewater treatment

• Aeration, low DO (SXAL), secondary and advanced wastewater treatment

• Anoxic (SXAXAL and SXAXM), nitrification and denitrification

• Anaerobic (SXANM), secondary and biological nutrient (N & P) removal

The technology has evolved since the first noted ‘sludge bulking’ (Calvert, 1927) – from

Donaldson’s (1932) apt description of filamentous organisms as ‘the weeds of activated sludge’

and Davidson’s (1949) discovery of bioselectors and bulking control with the anaerobic-aerobic

treatment sequence to the 1969-1974 innovators who broke the ground for current bioselector

design. Future clarification of best design and operating modes is expected when the many full-

scale operating systems are scrutinized from a research point of view and their historical data are

fully analyzed.

This document is an initial step toward defining the design and effectiveness of

bioselection to improve the settling rate and compaction characteristics of activated sludge. A

more comprehensive study of the hundreds of bioselectors currently in operation will provide the

data necessary to expand the technology and refine the recommended design criteria.

1-1

Chapter 1.0

Process Description

1.1 Definitions/Objectives

The term bioselection is employed to describe processes where natural (non-chemical)

selection governs or limits the growth of undesirable bacterial species. The specific objective of

bioselectors is to prevent the growth of filamentous organisms, which causes sludge bulking or

high sludge volume indices (SVIs). Improper design of bioselectors can also be the cause of a

different form of sludge bulking, viscous or hydrous bulking, which may or may not be of a

filamentous character.

There are a number of bioselector types employed in activated sludge treatment. The

definitions are qualitative in that they describe the physical environment within the bioselection

zone(s) as opposed to the biological functions that could be taking place. The bioselector zone

(SX) designs presented in this report are aerated – low DO (SXAL) and high DO (SXAH); anoxic

with (SXAXAL)and without aeration (SXAXM); and anaerobic (SXANM). The term oxic (OX) is

not employed for bioselector characterization in this report, although it does occur in the

literature. Oxic conditions would be included under the SXAH mode of operation since the

dissolved oxygen (DO) could range from >0.5 to >2.0 mg/L. The process conditions for each

mode will be further defined in this report.

While the settling properties can only be adequately characterized by stirred SVI (SSVI)

or diluted SVI (DSVI) procedures, SVI is employed in this text for simplicity. Whenever data are

presented, the measurement technique will be noted; SVI is used whenever the procedure

employed is unknown.

1-2

Non-filamentous (hydrous, viscous) bulking can be an operating problem and is

addressed in this review. Conditions that favor hydrous bulking may or may not favor

filamentous growth.

Bulking sludges historically have been the major cause of malfunctioning or inefficient

activated sludge processes. Design and operating practices evolved to compensate for bulking

sludge problems such as larger aeration basins and clarifiers with reduced mixed liquor

suspended solids (MLSS). In the 1960-1980s, chemical toxicants, peroxide and chlorine were

utilized to selectively control filamentous growths of organisms. The use of toxicants controlled

bulking sludges but also reduced effluent quality and could limit nitrification efficiency. The

operational need for higher solids retention times (SRTs) in biological nutrient removal

processes and the economical need to increase MLSS begged for a more practical method to

control bulking and foaming organisms. The bioselector has proven to be the answer for many of

these problems.

The problems of solids-liquid separation in activated sludge are classified in Table 1.1

(IAWQ, 1992a).

Table 1.1 Activated Sludge Solids-Liquid Separation Problems.

Problem Nature of Problem Characterization of Problem

Dispersed growth Flocs are dispersed, Turbid effluent. Poor flocforming only small clumps formation. Lack of zoneor single cells. settling of sludge.

Pin floc Small, compact, dense Low SVI, cloudy, turbidflocs that settle rapidly, effluents. Low quantity ofleaving lighter flocs in filamentous organisms.suspension.

Filamentous bulking An excessive filamentous High SVI, clear supernatant,population, which interferes low RAS concentration.with compaction and Increasing sludge blanketsettling of activated sludge depth. Poor sludge handling.characteristics.

1-3

Problem Nature of Problem Characterization of Problem

Hydrous zoogleal or The sludge flocs become High SVI, lower RASviscous bulking more hydrated and lose concentration. High sludge(non-filamentous) density. blankets.

Rising sludge Gas entrainment and/or Flocs or clumps of sludgegas release caused by rise rapidly to the surface.denitrification/ Surface effervescence.solids carryover.

Foaming and scum Aeration basin and clarifier Lightly colored frothy foamaccumulation of floating forms when aerated.materials, froth and scum Persistent, dark heavy foamof activated sludge. with high solids content.

Excessive sludge blankets: Diffuse sludge blankets Loss of solids and unstableNon-bulking sludge approaching weirs; operation. Solids

excessive solids loading or accumulation in clarifier.transport problems.

Control of bulking sludge as defined by the SVI results in a more stable process, reduces

operational requirements, and will generally produce a better effluent quality. The advantages

gained from SVI control are reflected in an increased capacity in both the aeration basin and

secondary clarifier as well as reduced waste activated sludge (WAS) volumes consistent with

maintaining a higher SRT. WAS processing costs generally decline as the SVI is reduced.

Control of the filamentous organisms can also eliminate most or all of the serious foaming

problems caused by filamentous organisms.

1.2 History of Bioselector Development

While Chudoba et al., (1973a, 1973b, 1974) must be given considerable credit for the

studies that defined environmental conditions that discouraged the growth of bulking organisms,

earlier researchers’ and practitioners’ efforts played an important role. Donaldson (1932a,

1932b), who characterized filaments as the ‘weeds of activated sludge,’ correctly postulated that

backflow mixing of long, rectangular (plug flow) basins contributed to the growth of filaments

and his proposal to baffle the aeration basin into compartments was a valid approach to assist in

the control of filamentous growths.

1-4

Engineers and scientists bent on improving the activated sludge process converted the

original Aldern and Lockett (1914) batch concept to a continuous flow process and with these

modifications introduced problems of bulking sludge. The batch feeding approach provided the

food/mass (F/M) gradient, described much later by Chudoba et al., as necessary to control

filamentous growth. Many problems of bulking occurred in the intervening years due to the

batch-fed lab studies, which led to the design of full-scale continuous flow plants that did not

have an F/M gradient. Sequencing batch reactors (SBRs), which are continuously fed, have had

serious sludge bulking problems.

Davidson (1949) studied several process alternatives to control filament growth in the

treatment of distillery wastewater and found success with an integrated anaerobic-aerobic

flowsheet with the sludge recycle from the aerobic back to the initial anaerobic zone. The unique

process mode, shown in Figure 1.1, demonstrated capability to reduce the undiluted SVI (uSVI)

of 300 to 1000 mL/g to less than 50 mL/g on this difficult-to-treat wastewater. This novel

anaerobic-aerobic concept was patented (Davidson, 1957), but not exploited nor understood by

wastewater researchers and practitioners until more than two decades later.

Bhatla (1967), British Water Pollution Laboratory (1969), Garber (1972) and Ryder

(1973) all noted that low DO (SXAL) in the initial zone of aerated, long, rectangular or staged

basins would provide bulking control. Koller (1966) and Pasveer (1969) rediscovered that

intermittent feeding of batch reactors would limit the growth of filamentous organisms. Chudoba

et al., (1973a, 1973b), in the first two papers of a series, demonstrated that compartmentalization

(staging) of the aerated reactors was a key factor to controlling sludge bulking. This idea was

first postulated by Donaldson (1932a, 1932b). The Czech researchers further recognized that the

organic loading gradient in the initial compartments of the treatment process was the key to

bulking sludge control. The effect of compartmentalization on SVI from their studies is shown in

Figure 1.2. As the number of compartments was expanded in the biological reactor, the

1-5

1-6

1-7

maximum level of soluble chemical oxygen demand (sCOD) in the MLSS increased in the first

compartment. The presence of 70 to 120 mg/L sCOD as a result of an increased F/M in the

smaller initial contact zones (ICZs) was the significant factor in the control of bulking

organisms. Rensink (1974) confirmed the observations of earlier investigators regarding the

benefits of staging the reactor zones to reduce sludge bulking.

The studies of Heide and Pasveer (1973) also revealed that an initial stage of raw

sewage-return sludge (or MLSS) contact at a high F/M ratio was critical to the control of bulking

organisms. They correctly postulated that organic matter, defined by the soluble biochemical

oxygen demand (sBOD5), was removed by faculative bacteria in the absence of an oxygen

source and, in this manner, would limit filamentous growth. The flowsheets representing the

pilot and full-scale operations evaluated by the Dutch researchers are presented in Figure 1.3.

Nicholls (1975) proposed the use of an anaerobic stage ahead of the four-stage

Bardenpho process (Figure 1.4) to enhance the biological phosphorus removal. Spector (1977)

was awarded a U.S. patent for an anaerobic stage as the first contact between the return sludge

MLSS, a process similar to that of Davidsons’ patent (1957) and Nicholls’ studies. Nicholls’

work evolved into the 5-stage Bardenpho and 3-stage Phoredox systems and the Spector

patented process was marketed as the A2O process. Other modifications to the alternating

anaerobic-oxic (AN-OX), anoxic-oxic (AX-OX), and anaerobic-anoxic-oxic (AN-AX-OX ) systems

to remove phosphorus, nitrogen and control bulking sludge were developed in the period before

and after 1975 (Figures 1.5 and 1.6).

Further studies by Lee et al., (1982) and Chudoba et al., (1985) confirmed earlier

conclusions regarding the need for an F/M gradient to control bulking. Albertson (1987, 1992)

summarized the efforts of these earlier innovators and those investigators who followed and

added to the understanding of mechanisms, design and operation of the several types of

bioselectors in use today. The design embodiments of these early innovators shared many

common features. The major difference identified was whether or not the initial zone of the

biological process was mechanically mixed or aerated (Table 1.2).

1-8

1-9

1-10

1-11

1-12

Table 1.2 Comparison of Elements of Early Anaerobic/Aerobic Bioselector Processes

HeideComponent Davidson Garber Chudoba Pasveer Nicholls Barnard Spectoror Mode 1952 1972 1973 1974 1975 1976 1977Mix InitialZone with Air No Yes Yes No No No No

Oxic Zone Yes Yes Yes Yes Yes Yes Yes

Clarifier Yes Yes Yes Yes Yes Yes Yes

Recycle OxicSludge to InitialZone (a) Yes Yes Yes Yes Yes Yes Yes

Mix RawWastewaterWith ReturnSludge Yes Yes Yes Yes Yes Yes Yes

Claim BulkingSludge Control Yes Yes Yes Yes No No Yes

(a) This zone may be either anoxic or anaerobic depending on the plant design temperature,

wastewater characteristics and the definition employed by the author of the publication.

The focus of this discussion is the control of filamentous organisms and hence the

elimination of bulking sludges. Because bulking sludge control and biological phosphorus

removal often occur in a common environment, these processes and data are interwoven in the

literature. Important general references in this regard are Tomlinson and Chambers (1978),

Jenkins et al., (1993) and Wanner (1994). The effectiveness of bioselectors is undisputable, but

there is much to be learned about the process and optimization.

1.3 Theory of Bioselection

The fundamental basis for the natural selection of non-filamentous organisms is the

control of environment during the initial contact of the influent wastewater (or primary effluent),

where most of the soluble substrate (sBOD5, sCOD) is removed from solution to the biomass

1-13

with or without limited oxidation. The key environmental conditions are the presence or level of

DO and the F/M in the selector zone(s). The addition of nitrates via return sludge or an internal

recycle (IR) from the nitrifying zone may also play a positive role in suppressing the growth of

filamentous organisms.

Filamentous organisms, one cause of bulking sludges, are considered to be relatively

primitive. They lack the capability to produce the necessary enzymes to hydrolyze complex

organic compounds into low molecular weight molecules, which they could then oxidize as a

food source. Thus, their primary energy source is small, soluble molecules such as sugars,

organic acids and alcohols, which are in the raw wastewater or products of biological hydrolysis.

In order to metabolize these soluble organics, filamentous organisms require the availability of

molecular oxygen for oxidation and production of cell matter. Combined oxygen, such as nitrate

nitrogen, cannot be efficiently utilized by filamentous organisms, and perhaps most importantly,

they lack the ability to store substrates for later use when molecular oxygen is available.

Heterotrophic organisms are more complex and adaptable to a wide variety of

environmental conditions. These organisms form large, dense flocs, and in the absence of an

excessive number of filaments, settle rapidly and compact well. Bioselection favors those

heterotrophic species that can readily sorb and store or modify soluble substrates in the absence

of molecular or combined oxygen. Bioselection may or may not enrich the MLSS with

organisms that also have the capability to remove and store excess levels of phosphorus (Bio-P)

in the cells. The type of bioselector employed will, in part, determine the level of Bio-P removal

by the system.

Thus, the bioselective mechanism is to contact the return activated sludge (RAS) – and

an internal recycle when employed – with the influent wastewater in an initial contact zone

(ICZ) of the biological reactor. The initial contact zone typically consists of three or four zones

with a high to lower F/M and having limited or no molecular oxygen present. In these zones,

heterotrophs remove the majority (75-90%) of the low molecular weight, soluble substrates from

the wastewater. Since the favored substrate (small, soluble organics/molecules) of the

filamentous bacteria is now limited in the heavily aerated oxic zones following the bioselectors

1-14

and anoxic (denitrification) zones (if employed), their growth – and reproduction – is thus

inhibited. The presence of a limited quantity of filamentous bacteria is generally desirable as

they can help produce a stronger and larger floc structure (Palm et al., 1980), which will readily

settle and compact well in the secondary clarifier.

While bioselectors will cause the removal of the bulk of the simple organics (alcohols,

volatile acids, sugars and amino acids) from solution prior to anoxic or oxic zones, the

hydrolysis of colloidal and particulate organics in the following oxic zones may provide an

opportunity for filamentous organisms to feed and grow. Wanner and Grau (1988) referred to

this cause of bulking as "secondary bulking" and recommended a high to low F/M gradient

(compartmentalization) in the oxic zones. This postulation of Wanner and Grau may be the

reason why bioselectors are very effective when treating highly soluble industrial wastewater

flows. The bulk of the influent COD (BOD5) is removed in the bioselectors and the by-products

of hydrolysis of the low level of particulates in the oxic zone are minimal. Thus, secondary

bulking cannot occur and this could be the reason that very low SVIs (20-50 mL/g) have been

reported (Davidson, 1957; Cranston Print Works, 1992; Tree Top Apple Juice, 1993; Okey,

1997) when treating highly soluble industrial wastewaters containing mostly low molecular

weight organics.

In order to maintain the bioselection capacity of the biomass, it is necessary that the

stored organics be oxidized prior to returning to the initial contact zone. That is, the stored food

must be aerobically processed to carbon dioxide (CO2), water and cell matter by the organism

prior to returning to the bioselector for the process to be effective. If the bacterial storage

capacity is not regenerated in the oxic zone, the soluble substrate can enter the aeration zone and

cause proliferation of filamentous organisms. Also, this condition can cause the development of

viscous sludge bulking (see Section 1.4). Chudoba et al., (1982) correctly referred to the need to

fully oxidize the ‘sorbed’ organics before clarification and sludge recycle as ‘sludge

regeneration.’

The removal of soluble components by the bioselector is a very rapid process. In the

Phoenix 23rd Ave Water Reclamation Facility (WRF) and 91st Ave Wastewater Treatment Plant

1-15

(WWTP), the retention time in the three-stage bioselectors is 5-8 minutes based on the total flow

of primary effluent, return sludge and mixed liquor recycle (Albertson and Hendrix, 1992;

Albertson and Stensel, 1994). After this short contact time, the sCOD in the bioselector effluent

approaches the final effluent sCOD of 25-30 mg/L or over 85% removal of the primary effluent

sCOD defined by 1.2-1.5 µm filtration. These results are typical of other plants with staged

bioselectors.

1.4 Filamentous vs. Non-filamentous Bulking

Historically, the emphasis on bulking sludge has been focused on the study and

identification of the various filamentous organisms. However, when the concept of bioselection

was employed, another cause of bulking sludge surfaced: hydrous or viscous bulking conditions.

It is not uncommon to have both types of bulking present in a wastewater treatment plant

(WWTP).

The first published experiences of viscous bulking began appearing in the 1950s

(Wanner, 1994). The condition was related to the overflowing of slowly settling sludge from the

clarifier and a dilute underflow concentration. Often it has been described by operators and

experienced personnel as a condition where there were two distinct sludge blankets – a light,

fluffy, unstable blanket above a denser blanket. The upper, lighter layer of suspended solids was

susceptible to washout during the higher diurnal or wet weather flow periods, increasing the

effluent TSS to 30 to 150 mg/L during these periods.

Non-filamentous bulking is also known as hydrous, viscous or zoogleal bulking. In this

condition, the biomass accumulates and stores excess food as a slime surrounding the cell or

floc. This storage product, called exo-cellular protoplasm (ECP), results in a lower density,

slower settling floc due to the higher volume of water intimately associated with the floc. The

MLSS and return sludge will feel slick or slippery, and the MLSS will be viscous at a relatively

low solids concentration (Wanner, 1994). Often these hydrous sludges are referred to as having

an excessive level of polysaccharides. However, viscous sludges may also contain loosely bound

organic nitrogen compounds, and thus the term ‘excess polysaccharides’ is not the correct

generalized definition of a non-filamentous bulking condition. Instead exo-cellular protoplasm

1-16

(ECP) is preferred as a proper terminology for excess storage products in hydrous sludges.

Hydrous sludges can be characterized by India ink staining (Jenkins et al., 1993) as the

organically bound water is not stained and is thus visible under microscopic examination.

Jenkins et al., (1993) discussed the relationship of total carbohydrates as glucose, which

acts as an indicator of settling problems, and provided test procedures for staining and

carbohydrate measurements.

There is a lack of experimental documentation on the causes of viscous bulking. It is

probable that some of the environmental conditions that encourage filamentous bulking also

generate hydrous bulking. Hydrous bulking, while unrecognized, has been present as long or

longer than filamentous growths in activated sludge. It can develop in all types of biological

reactors including the batch systems of Aldern and Lockett (1914). However, complete mix

processes are the least susceptible to hydrous bulking, unless toxicity is a problem or the

wastewater is deficient in nutrients, such as nitrogen and phosphorus, as well as oxygen. Some of

the likely causes of non-filamentous bulking and opinion regarding filamentous bulking are set

forth in Table 1.3 (Albertson, 1994).

Table 1.3 Probable Causes of Hydrous Sludge Bulking and Filamentous Bulking

Causative Factor Hydrous Filamentous

Nutrient/Micronutrient Deficiency X X

High Contact Loading X

Inadequate O2 Supply X X

Toxicants X

Uncouplers X

Low F/M Loading X

Soluble Wastewater X X

Excess F/M Loading X X

Excess Storage Products in Return Sludge X X

1-17

The conditions within the bioselectors, such as low DO and high F/M gradient, are also

conducive to generating a hydrous bulking sludge if there is organic overload or the storage

capacity of the return sludge has not been adequately regenerated. Thus, an overloaded

bioselector can be a cause of bulking (filamentous and hydrous) sludges. There is a general

relationship of the F/M (Chudoba et al., 1973a, 1973b) in the initial contact zone to the SVI of

the activated sludge. However, there are too many site-specific aspects of biological growth

kinetics to make this statement in other than a qualitative term as shown in Figure 1.7. The width

of the ‘window’ of low SVI sludge production is not fully developed at this time. However, it

appears to be in an F/M range of 3-6 kg sCOD/kg TSS·d (1.5 to 3.0 kg sBOD5/kg TSS·d) in the

first contact zone of return sludge and influent wastewater. This is an area in need of quantitative

research, both laboratory and field studies. There is also a possible impact of the specific

wastewater components on the selector design criteria that is not understood at this time.

Biological reactors of similar design may or may not have bulking problems with or without a

selector. Operational conditions must also be considered.

Lack of nutrients (N and P) or micronutrients are considered to be a cause of bulking and

the addition of N, P and iron compounds have been found to be beneficial in these cases.

Supplements of magnesium and potassium, which are involved in the Bio-P formation, have also

improved the SVI. High contact loadings occur when the quantity of soluble substrate exceeds

the level of normal bacterial metabolism and storage capacity, and excessive levels of ECP are

produced. These conditions can encourage the growth of filaments. An inadequate supply of

oxygen to support fully aerobic metabolism in the succeeding oxic zone(s) can be a cause of both

types of bulking.

Toxicants and uncouplers can reduce filaments, but may also cause hydrous bulking by

limiting normal phosphate uptake for metabolism. Chlorine, as a toxicant, is used to control

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1-19

filaments by destruction of the filamentous growth. However, this practice, while effective, can

increase turbidity in the effluent and upset sensitive processes. While complete mix activated

sludge (CMAS) systems are more resistant to toxicants, the process favors the growth of some

types of filamentous organisms.

Viscous/hydrous bulking is not limited to the activated sludge processes. Albertson

(1994) reported on a number of facilities with fixed and suspended growth systems that had

severe bulking problems. While most of the facilities in Table 1.4 are industrial, there are

municipal plants processing mostly domestic wastewater that have also experienced hydrous

bulking problems.

Under specific environmental conditions, soluble substrates can be a source of both

hydrous and filamentous bulking. However, bioselection is most effective with soluble

wastewaters provided there is not an excessive F/M loading and there is adequate sludge

regeneration. If the aeration system is incapable of oxidizing the stored substrate in the cells, the

return sludge may be unable to rapidly metabolize the new soluble substrates in the initial

contact zone. This operating deficiency may lead to hydrous and/or filamentous bulking

conditions. Thus, it is necessary to identify the type of bulking, filamentous and/or non-

filamentous, in order to initiate an investigation into the cause(s). While CMAS systems are the

most common cause of filamentous bulking (but not hydrous bulking) in municipal and

industrial wastewater treatment plants, floc overload, aeration capacity, and nutrient levels must

be considered. The ‘cause and effect’ aspect of bulking sludges is still being developed.

Table 1.4 Major Occurrences of Viscous/Hydrous Bulking

Wastewater Location Mode/Operation

Candy Wastes Puebla, MX Sequencing Batch Reactor (SBR)

Bakery Wastes New Jersey Equalization Basin

Pharmaceutical Ireland Trickling Filter

Domestic Phoenix, AZ ICZ Nitrification-Denitrification Activated

Sludge

1-20

Brewery Fulton, NY Mix Box ICZ Activated Sludge

Leachate Seattle, WA Submerged Fixed-Film Reactor (FFR)

Starch Iowa Trickling Filter

Domestic Michigan Trickling Filter

Apple Juice Washington SBR

Domestic & Dairy Iowa Trickling Filter

Wastewater Location Mode/Operation

Brewery Maine Trickling Filter/Activated Sludge

Domestic & Ind Waste Tennessee Trickling Filter/Activated Sludge

Domestic Wastewater Colorado Trickling Filter

Domestic Wastewater Pennsylvania Trickling Filter

1.5 Measurement of the Sludge Volume Index

Due to the lack of a universally accepted procedure to determine the settling

characteristics of activated sludge, it is difficult to fully quantify differences in this property.

The most common measurement in the U.S. of the SVI of the mixed liquor is the unstirred

procedure (uSVI). Unfortunately, it is the most unreliable measurement of the true

settling/compaction characteristics of the mixed liquor in a clarifier.

Rachwal et al., (1982) presented the relationship of unstirred settled sludge volume

(SSV30) of an activated sludge with a stirred sludge volume index (SSVI) of 80-85 mL/g over a

MLSS range of 1000 to 10,000 mg/L. Figure 1.8 displays the results of uSVI vs. SSVI

procedures and reveals that the uSVI method was invalid to define settling characteristics above

approximately 2000 mg/L MLSS. That is, when the MLSS was <1500 to 2000 mg/L, the two

procedures produced approximately the same values. At 4000 mg/L MLSS, the uSVI was about

225 mL/g with an SSVI of 80 mL/g. The SSVI (and the DSVI) tests are a more reliable and

accurate predictors of full-scale settling/compaction characteristics of activated sludge.

The term ‘SVI’ has many different meanings in the international technical community

since data have been reported as uSVI30 (and uSVI60), SSVI30, SVI 2.0, SSVI2.0, SSVI3.5, DSVI30

and perhaps other definitions. The relationships between these various SVI tests are not constant,

1-21

1-22

and plant-to-plant comparisons are difficult or impossible when the uSVI exceeds 100 to 125

mL/g or when the SSV30 exceeds 300 mL/L.

The universal need to standardize the SVI procedure is obvious. The question in debate is

which procedure to use. A brief description and comment on each procedure follows:

(1) uSVI30 (unstirred). The MLSS settling test is conducted in a 1 L graduated cylinder

or a 2 L settlometer for 30 minutes. The recorded 30-minute volume is converted to

SVI as a function of the MLSS. When the settled sludge volume (SSV30) exceeds

250-300 mL/L, the relative value of this test to full-scale settling characteristics is

limited and unpredictable. At SSV30 above 400 mL/L, the values produced may be

of little worth and unpredictable and, worse, often misleading. Sometimes this test,

and the following tests, are continued for 60 minutes and reported as SVI60, etc.

(2) SSVI30 (stirred). This test is conducted in a 1 L graduated cylinder or a 2 L

settlometer. A stirrer operating at about 1 rpm is used to disrupt floc particle

bridging and assist consolidation of the solids. This test will generally produce a

lower 30-minute sludge volume than the uSVI if the SSV30 is > 200 mL/L. It is

considered more representative of full-scale sludge compaction, but may not define

solids-liquid separation rates.

(3) SVI2.0 (unstirred) and SSVI2.0 (stirred). These tests are conducted for 30 minutes

at a MLSS of 2000 mg/L (2.0 g/L) and represent an effort to standardize the

procedure for comparison. The uSVI2.0 has the same limitations as the uSVI30 tests,

and the SSVI2.0 is an attempt to eliminate the effects of varying MLSS on both the

settleability rate as well as the solids-liquid separation rate. The SSVI2.0 procedure

has an advantage over the uSVI and SVI2.0 tests for plant-to-plant comparisons.

(4) SSVI3.5 (stirred). This test is similar to the SSVI2.0 procedure except that it is

conducted at 3500 mg/L (3.5 g/L) MLSS. This procedure is best suited for the range

of MLSS concentrations typically employed in full-scale advanced wastewater

treatment plants. As the SVI increases, the SSVI3.5 test results will be increasingly

higher than those of the SSVI2.0.

1-23

(5) DSVI30 (unstirred, diluted). This procedure involves dilution of the MLSS until the

resulting 30-minute settled volume is about 200 mL/L. A tolerance of ±40 mL/L

does not have a large effect on the resulting DSVI value. In comparison to the uSVI

test, this procedure will provide a more representative value of the solids-liquid

separation rate and a better projection of the potential concentration characteristics

of the settled solids due to minimization of bridging and MLSS concentration

effects. This procedure is excellent for plant process control but would not be as

effective for plant-to-plant comparison as SSVI2.0 or SSVI3.5.

Many treatment facilities operate with MLSS of 2500 to 4000 mg/L, a range in which the

uSVI test is meaningless when the SSVI is > 80 mL/g. Thus, it is necessary to employ either the

SSVI or the DSVI procedure in order to accurately define solids-liquid separation characteristics.

The studies of Koopman and Cadee (1983) indicate that the DSVI and SSVI procedures produce

reasonable, comparable results. Lee et al., (1983) correlated total extended filament length with

uSVI, uSVI1.5, uSVI2.5, DSVI and SSVI2.5 and judged that DSVI was the most quantifiable

procedure for use in judging clarifier settling and compaction characteristics. This conclusion

was also supported by the work of Pitman (1984) and Yamamoto and Matsui (1988).

Test vessels employed for the SVI procedures have been one or two liter cylinders and

one or two liter settlometers These vessels will produce different values for SVI with the same

sludge. Keinath and Wahlberg (1990) conducted tests in 1 L cylinder and 2 L settlometers using

the SVI and SSVI procedures. A portion of their results is presented in Figures 1.9 and 1.10. A 2

L settlometer is the preferred settling vessel for all types of SVI tests due to reduced wall effects

and thus is more analogous to full-scale settling. However, as shown by Rachwal (Figure 1.8),

the uSVI test is invalid above SSV30 ≥ 150 to 200 mL/L (% volume). The DSVI test uses 160 to

240 mL/L SSV30 as an acceptable measure to define the DSVI.

The presence of hydrous bulking organisms is not necessarily reflected in a high SVI.

The SVI tests may hide the presence of the hydrated organisms due to the bulk settling

characteristics of the mixed liquor in test vessels. A small hydrated fraction (<10%) can be

1-24

1-25

1-26

entrapped by the more rapidly settling fraction of denser floc particles. However, in the clarifier

there is an opportunity for the slower settling, lower density, hydrated floc particles to segregate

and accumulate in the area above the more concentrated settled sludge being transported to the

return sludge hopper. These floc form fluffy sludge blankets of low total suspended solids (TSS)

concentrations, which are susceptible to being flushed into the effluent during diurnal or wet

weather flows.

The portion of the MLSS that is hydrated but causing problems could be quite small. If

the MLSS is 3000 to 3500 mg/L, 10 % of these solids, or 300 to 350 mg/L TSS, could form the

dilute blankets and easily contribute to a 30 to 150 mg/L increase in TSS, which may overflow

the clarifier during these periods.

1.6 Dilute Sludge Volume Index Procedure

The DSVI procedure involves adding a known volume of secondary effluent (diluent) to

the MLSS sample to reach 1000 mL (cylinder) or 2000 mL (settlometer). For example, if the

undiluted MLSS settled to 500 mL/L (SSV30), then a 0.5 dilution factor (DF) will likely result in

a settled volume of 180 to 220 mL/L or within the target range of 200 mL/L ± 40 mL/L (160 to

240 mL/L SSV30).

Initially, for training experience, the operators will usually set up two or three settling

vessels to target the correct dilution factor (DF) and usually within a week or two of experience

will rely on just one test vessel. The dilution factor and settled volume from the previous shift is

the best guide to determine the required DF for the next shift. For example, if in the previous

shift, the DF was 0.5 at an SSV30 of 240 mL/L, the next shift should try a 0.4 DF. The lower DF

of 0.4 would have produced (0.4/0.5)(240 mL/L) = 192 mL/L or close to the 200 mL/L target

volume had a 0.4 DF been employed on the shift reporting 240 mL/L SSV30.

Generally, the adjustments of the dilution factor from one shift to the next is not more

than 0.1 DF. The DF values and calculations for DSVI are provided below.

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Dilution Factor (DF) Volumes of MLSS and Secondary Effluent

Dilution MLSS EffluentFactor(1) mL/L(2) mL/L(2)

1.0 1000/2000 0

0.9 900/1800 100/200

0.8 800/1600 200/400

0.7 700/1400 300/600

0.6 600/1200 400/800

0.5 500/1000 500/1000

0.4 400/800 600/1200

0.3 300/600 700/1400

0.2 200/400 800/1600(1)DF = the dilution factor is a decimal fraction of MLSS in total volume(2)1000 mL cylinder / 2000 mL settlometer

Calculation Procedure:

DSVI =)/,)((

)/,( 30

LgMLSSDF

LmLSSV(1.6-1)

Example:

If MLSS was 3200 mg/L (3.2 g/L), SSV30 was 225 mL/g after diluting 600 mL of mixed liquor

with 400 mL of effluent (DF = 0.6), the DSVI is:

DSVI =)/2.3)(6.0(

/225

Lg

gmL(1.6-2)

= 117 mL/g

Wastewater treatment plant operators are strongly encouraged to employ SSVI30 or

DSVI30 procedures since they will accurately identify changes in the settling properties of the

MLSS. The uSVI procedure, as shown in Figure 1.8, is unreliable and can provide misleading

information, which can result in the employment of an improper mode of operation for SVI

control in the plant.

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1.7 Conversion of uSVI to DSVI or SSVI

The conversion of uSVI to DSVI or SSVI poses some problems due to the inaccuracies

of the SVI test. Keinath and Wahlberg (1990) presented data showing the relationship of uSVI

and SSVI in 1L and 2L cylinders (Figures 1.9 and 1.10). The SVI and DSVI sludge loadings vs.

SVI and return sludge concentration graphs of Daigger (1995) illustrate the differences between

the SVI, DSVI and SSVI3.5 values. The SVI, SSVI3.5 and DSVI secondary clarifier graphs of

Daigger are reproduced in Section 2.0

As shown in Figures 1.9 and 1.10, the relationship between SVI procedures is poor and

conversion of uSVI to SSVI has limitations when the uSVI > 300 mL/L (Figure 1.8).

An excellent reference on the influence of sludge bulking on clarifier design and

performance is the 1997 IAWQ (now IWA) publication Secondary Settling Tanks. This

document provides background information on the various measurement techniques for SVI and

support for the DSVI procedure. It was noted that it is necessary to define the MLSS

concentration at which the SSVI procedure is conducted: SSVI2.0 or SSVI3.5. Even the SSVI

procedure at different MLSS concentrations will provide different SSVI values. The SSVI

increases with higher MLSS as does the uSVI, but to a much lesser degree.

The relationship between SVI and DSVI was investigated by Merkel (IAWQ, 1992b)

employing a large data base. The resulting equation for the SSV30 range of 300 to 800 mgL/L for

the SVI test was

DSVI = SVI (300/ SSV30)0.6

In the IAWQ (1997) review and discussion of the Merkel equation, it was concluded that

the limits for SSV30 were appropriate and the equation is considered to be the best representation

of the relationship. This document summarized three sets of Western Cape (South Africa) full-

scale UCT data and developed the plot of measured DSVI vs. calculated DSVI from the Merkel

equation. This plot from IAWQ (1997) is reproduced as Figure 1.11. Although the level of

1-29

variance from actual to the measured DSVI is appreciable, DSVI is a better measurement. The

best procedure is SSV3.5 which is used widely overseas.

The IAWQ document provides additional and important background data and

observations on the bulking organisms and the development of measurement and control

methods for limiting the mixed liquor SVI. The emphasis of the control modes is natural

bioselection although it includes information generated by European researchers – some of

which, but not all, have been included in the citations of this report. The significance and

interrelationship of secondary clarifier capacity and performance have been fully developed in

the IAWQ document.

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2-1

Chapter 2.0

Benefits of Bioselection

The process benefits of a lower and stable sludge volume index (SVI) are well known to

plant operators. Control of the activated sludge process is simplified and the effluent quality is

more stable with minimal sludge blankets in the clarifier. In addition, the biological average and

peaking capacities of the biological system are increased – that is, BOD5 loading and hydraulic

flow. Sludge bulking conditions limit clarifier solids and liquid processing capacity. They also

limit the mixed liquor suspended solids (MLSS) concentration of the aeration basins due to a

lower return sludge solids concentration and a lower solids handling capacity of the clarifier.

Control of SVI is important to the reliability of the activated sludge system. Palm et al.,

(1980) documented the effect of the total extended filament length (TEFL) on the SVI. His

results of their studies (Figure 2.1) are important to understanding the value of controlling the

DSVI to the range of 60 to 100 mL/g. Historically, an SVI of ≤ 150 mL/g has been considered an

acceptable value for a non-bulking sludge. However, as shown by Palm et al., the SVI at 150

mL/g is in an area of instability and a small increase in TEFL can result in an escalating SVI and

reduced clarifier and aeration capacity. This figure, like that of Rachwal (Figure 1.8), also

reveals the lack of precision of the SVI test. The equivalent diluted SVI (DSVI) for the SVI at

150 mL/g would be 100 to 120 mL/g.

An uncontrolled increase in the SVI often results in a need to change the operating modes

of the aeration basins and clarifiers, which can upset the biological process. Since sludge bulking

has been the most common cause of reduced capacity and failures of activated sludge facilities,

the concept of bioselectors and its attendant benefits have been welcomed and widely accepted

as a standard practice.

Daigger (1995) developed a series of clarifier loading graphs as a function of undiluted

SVI (uSVI), stirred SVI (SSVI) and DSVI of the MLSS and return sludge concentration. These

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2-3

relationships and graphs, Figures 2.2, 2.3 and 2.4, and similar graphs developed by Pitman

(1984), Daigger and Roper (1985), Keinath (1990) and Mines et al., (2000) provide a basis to

define the benefits of bioselection in a wastewater facility. Using Figure 2.2, the aeration and

clarification requirements for a plant with and without bioselection and the maximum SVIs are

as set forth in Table 2.1. The bioselector reduced the aeration volume by 14%, clarification area

by 38% and RAS flow by 40%. Furthermore, at ≤ 120 mL/g SVI the process will be more stable,

produce a higher quality effluent and be easier to control. There is a greater potential for the SVI

to increase from 150 to above 250 mL/g without bioselectors, but with bioselectors the more

probable uSVI is ≤ 100 mL/g. Further, as indicated by Figure 2.1, there is no rational method of

predicting the range of SVI when it exceeds 150 mL/g. It is not known how much of the uSVI

results are impacted by the problems of poor reproducibility of this SVI procedure. Historically,

the bulking problem has required designers to oversize the aeration and clarification facilities in

order to handle higher SVIs. Employing bioselectors will reduce both the aeration and

clarification volume requirements.

Table 2.1 Benefits of Sludge Bulking Control by Bioselection

Parameter Units w/o Bioselection w/ Bioselection

Flow (L/s) mgd 438 (10.0) 438 (10.0)

MLSS mg/L 3000 3500

Max uSVI (DSVI) mL/g 250 (130) 120 (80)

Aeration Mass 103 kg (103 lb) 29.5 (65) 29.5 (65)

Max. Clarifier SLR kg/m2.d (lb/ft2.d) (18) (30)

Return Sludge Conc.(1) mg/L(1) 7200 11,600

RAS Flow L/s (mgd) 313 (7.14) 189 (4.32)

Aeration Volume 103m3 (103ft3) 9.84 (347.2) 8.43 (297.6)

Clarifier Area 103m3 (103ft3) 1.94 (20.85) 1.21 (12.97)

(1) Design set at 80% of the RAS suspended solids (RSS) concentration in Figures 2.3 and 2.4 toallow for operational factors.

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2-5

The bioselector reduced the required aeration volume and clarifier area. Without

bioselectors, the aeration volume would need to be 16.7% larger and the clarifier surface area

would increase by 60%. It is expected that the smaller plant with bioselectors would also produce

a better effluent quality.

It is not recommended that the clarifier solids loading rates (SLR) and RSS

concentrations (Xu) in Figures 2.2, 2.3 and 2.4 be employed without an allowance (or safety

factor) to account for the normal load variations occurring in a typical wastewater treatment plant

as well as the inaccuracy of SVI measurements. With a good floor slope (≥ 1:12), center sludge

withdrawal mechanism and properly sized collection equipment, the RSS concentration will

exceed 80% of the values on the graph. Rapid sludge removal equipment may achieve only 60-

65% of the RSS concentrations. The SLRs set forth and the RAS/Q ratios will be proprotionally

higher. The predicted RSS and operating RSS of 9000 and 7200 mg/L, respectively, were

generated from the Daigger’s Figures 2.3 and 2.4.

Graph OperatingFigure uSVI DSVI SLR RSS RSSNo. mL/g mL/g lb/ft2.d mg/L mg/L 2.4 250 – 18 9,000 7,200

2.3 120 80 30 14,000 11,600

The procedure for employing the Daigger (1995) graphs is described below and

illustrated in Figure 2.5 using DSVI input data.

1. Locate the line of the maximum operating DSVI: 80 mL/g

2. Draw a horizontal line from the operating maximum SLR to the maximum DSVI and

note equivalent underflow solids concentration (RSS) and draw vertical line: 14,000

mg/L

3. Multiply the RSS value by 0.80 (14000 mg/L x 0.8) to determine operating RSS value of

11,600 mg/L to use to define the RAS flow:

2-6

4.

2-7

2-8

2-9

)/,()/,/,(

)/,(sLQ

LmgMLSSLmgRSS

LmgMLSS

−

)/438()/3500600,11(

)/3500(sL

Lmg

Lmg

− = 189 L/s

The graphs are developed such that all conditions to the left of the DSVI (SSVI3.5 and

SVI) line are considered operable clarifier designs. However, the design engineer should

recognize that there are optimal SLRs as a function of the RSS concentration. That is, an

underflow based on 18,000 mg/L at 80 mL/g DSVI would require a 300% larger clarifier. Thus,

judgment is required in the final selection of a SLR consistent with the overall process design

and operation and the configuration of the clarifiers (IAWQ 1992b, IAWQ 1997, WEF 1998).

While the Daigger DSVI and SSVI3.5 curves project similar underflows at the SLRs and

RSS concentrations selected, there are differences at other conditions. The curve employed must

be the same as the settling test procedure. The DSVI or SSVI3.5 test should be employed due to

the greater inaccuracy of the uSVI test as noted earlier.

As of the year 2002, bioselectors have been employed in hundreds of U.S. and overseas

facilities and are considered to be standard practice for new and renovated activated sludge

plants. Bioselectors have allowed the upgrading and rerating of plants for higher capacity and/or

for biological nutrient removal (BNR) by operating at much higher MLSS and solids retention

times (SRTs) in existing basins and increasing sludge handling capacity of clarifiers. Some of

these facilities will be reviewed in Chapter 5: Process Experiences with Bioselection.

While bioselectors provide control of many filamentous and foaming organisms, they do

not fully control some organisms. Anoxic bioselectors can control Nocardia growth in the

laboratory studies and full-scale facilities, but anaerobic selectors have not been successful

(Jenkins et al., 1993). Microthrix parvicella has been shown to be resistant to control in anoxic

bioselectors. Kucman (1987) reported high denitrification rates in activated sludge dominated by

M. parvicella, raising the possibility that this organism could use oxygen derived from nitrates.

2-10

Most filamentous organisms are inefficient users of combined oxygen (NO_

2 and NO_

3) and thus

can be controlled in mixed or aerated bioselectors with low or no DO.

In 1993 Jenkins et al., summarized the experiences regarding the effectiveness of

bioselectors and that information is reproduced in Table 2.2. New information from the

increasing number of operating facilities and ongoing studies will continue to expand that

information as well as the content of this study.

Thus, while there are some organisms that resist control by bioselectors, the overall

experiences are a marked improvement (a lowering) in the SVI. In general, the higher the

operating SVI before the installation of bioselectors, the greater the improvement that will be

realized in the plant operating capacity and performance.

Table 2.2 Effectiveness of Bioselectors in Controlling Filamentous Organisms

Effective Not Always Effective

S. natans type 0041

type 1701 type 0675

type 021Na type 0092

Thiothrix spp.a M. parvicella

N. limicola

H. hydrossis

Type 1851

Nocardia spp.b

aNot effective when caused by nutrient deficiencybAerobic selectors not always effective; anoxic selectors effective (Cha et al., 1992).

In many cases, bioselectors can be installed within existing basins as a series of small

compartments using 6 to 10% of the total aeration volume. The loss of this oxic volume is easily

offset by the increased MLSS and SLRs resulting from the modified operation of the system at a

low SVI. In denitrification systems, internal recycle of nitrified mixed liquor to the bioselector

zones results in highly efficient denitrification with no measurable loss of operating capacity.

3-1

Chapter 3.0

General Design Arrangement of Bioselectors

Since the bulk of the soluble biochemical oxygen demand (sBOD5) – or soluble chemical

oxygen demand (sCOD) – removal from solution occurs in the first 5 to 10 minutes of contact of

wastewater and return sludge, the bioselectors must be installed in the initial (first) contact zone

(ICZ) of the influent wastewater and recycled oxidized activated sludge. In most cases, the

recycled solids will be in the return activated sludge (RAS) from the secondary clarifiers.

However, the recycled solids also can be mixed liquor suspended solids (MLSS) from the oxic

zone or the discharge from the aeration basin. Heide and Pasveer (1974) demonstrated successful

bulking sludge control by recycling oxidized MLSS from an existing oxidation ditch and

contacting the MLSS with raw sewage in a six-stage external bioselector (Figure 1.3). The

food/mass (F/M) gradient as defined by Chudoba et al., (1973a, 1973, 1974) was also noted in

the Heide and Pasveer study and it followed the earlier staged aeration of the British Ministry

(1969). All staged system (F/M cascade) modes were successful in controlling the SVI.

While there are cases where single-stage bioselectors have been effective, the multi-

staged concepts of Chudoba et al., and Heide and Pasveer are recommended to ensure that the

bioselector design will be fully effective in controlling filamentous organism growths.

3.1 F/M Cascade Design

Bioselectors will most often comprise three or four complete mix activated sludge

(CMAS) reactors (zones) in a series for the initial contact of the return sludge and the wastewater

to be treated. These small compartments will provide a F/M gradient, which has been

demonstrated to be the most important process factor in the environmental conditions favoring

the production of a non-bulking sludge. The general arrangement of bioselectors for an advanced

wastewater treatment plant is shown in Figure 3.1. In this flowsheet, there can be an option for

biological phosphorus removal (Bio-P) and the anoxic zone allows for removal of the nitrates

(denitrification) in the internal recycle (IR) stream. When nitrification (N) and denitrification

(deN) are not required, the anoxic zones after the bioselectors and internal recycle are not

3-2

3-3

employed. However, the bioselectors are equally effective in controlling bulking in conventional

secondary treatment.

When rectangular, longitudinal flow reactors are employed, the bioselectors are often

incorporated into the influent end of the aeration basins. For complete mix and orbital basins, it

is more common to construct the bioselectors (and anoxic zones, if required) externally to the

aeration basin. Some typical bioselector arrangements in U.S. installations are shown in Figure

3.2. The term ‘semi-aerobic’ (Albertson et al., 1987) has been employed for bioselectors that are

aerated at a low rate, but oxygen deficient (Albertson et al., 1991), or where there is a recycle of

nitrate-containing oxidized MLSS to the initial contact zone employing mechanical mixing. The

environment of all types of bioselectors and the terminology are now more definitive. The

method and degree of aeration, mixing and internal recycle of (or lack of) nitrate-containing

return sludge and mixed liquor are employed to define the type of bioselector.

The staging volume of the bioselector zones is similar for the U.S. and Czech designs

except that the U.S. recommendation (Albertson, 1987, 1992) is three stages of 25, 25 and 50%

of the total volume, while the Czech design (Wanner and Chudoba, 1988; based on Kroiss, 1985)

employs four equal volumes. Both concepts may or may not employ air for mixing when Bio-P

removal is required. The best performance for Bio-P removal is with mixing only and without

internal recycle. The question of whether the low DO aeration or anoxic mode results in a lower,

more consistent SVI is yet to be answered. The unaerated volume requirements for Bio-P

removal are larger than required for bulking sludge control. However, Bio-P designs are not a

focus of this report. Bioselectors designed for bulking control, with and without denitrification,

will generally reduce the effluent total phosphorus (TP) to 1.5 to 3.0 mg/L, depending on

incoming wastewater characteristics and the nitrogen inventory in the system.

The F/M gradient of the staged bioselector involves consideration of the COD, sCOD,

BOD5, and sBOD5 of the wastewater discharged to the initial contact zone of the bioselector. If

only inhibited or carbonaceous BOD5 (cBOD5) is available, the cBOD5 can significantly

understate the true strength of the wastewater, and comparable COD and sCOD should be

collected to define COD and sCOD to the cBOD5 and soluble cBOD5 ratios. When the

COD/cBOD5 ratios exceed 2.1, employ the COD and sCOD values for design. The COD and

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3-5

sCOD are the most accurate measurements of the wastewater’s available ‘food’ and these are the

favored input data for bioselector and aeration design. Since the sCOD (or sBOD5) is the

component involved in the bioselection mechanisms, these data should be employed when

available.

Municipal facilities and industrial facilities may receive significantly higher organic

loading in 8- or 16-hour periods. Because the biological growth characteristics is a function of

the load over a short period (5-15 minutes in the initial contact zone), it is necessary to consider

diurnal or peaking load (COD, BOD5) periods when they exceed 140% of design. The design

recommendation of this report will allow for typical municipal wastewater diurnal peaking

conditions, and if the peaking factors exceed 140% of the average load, then the cascade F/M

values must be adjusted (lowered) and the bioselector volumes increased.

Albertson (1987, 1992, 1994) and Jenkins et al., (1993) have suggested general design

criteria for aerated and anoxic bioselectors treating typical municipal wastewater and the

loadings are set forth in Table 3.1. However, there is neither general agreement by researchers

and practitioners on the F/M loading profile nor the number of profiles at this time. There is also

a question whether anaerobic bioselectors (SXANM), anoxic bioselector (SXAXM and SXAXAL),

and aerated (SXAH and SXAL) bioselectors should use the same F/M values. This will be

reviewed in Chapters 4 and 5. Currently, bioselector design criteria are most often different from

plant to plant and the results also differ, which may or may not be due to the bioselector design.

3-6

Table 3.1 General Design Guidelines for Bioselector Sizing(after Albertson 1987,1992,1994)

Zone F/ΣM

Sx-1 Sx-2 Sx-3

COD kg/kg.d 10-12 5-6 2.5-3

sCOD kg/kg.d 5-6 2.5-3 1.25-1.5

BOD5 kg/kg.d 5-6 2.5-3 1.25-1.5

sBOD5 kg/kg.d 2.5-3 1.25-3 0.63-0.75

Notes: (1) The sCOD or sBOD5 (f1.5μm) is the best basis for design of bioselectors.

(2) Use parameter (COD, etc) that generates the smallest bioselector volume.

(3) The COD/BOD5 and sCOD/sBOD5 range is presumed to be 1.8-2.2. Use COD and

sCOD only if outside this range. No criteria are presumed for cBOD5. Data for sBOD5

and sCOD should be collected when plant data are cBOD5.

(4) The diurnal peak load is assumed to be ≤ 1.4 average, 8-hour loading ≤ 1.3 average

and 16-hour loading ≤ 1.3 average. Reduce F/M criteria if loadings are higher.

The compartment sizing of the bioselectors to generate a F/M gradient has evolved

considerably. Still, the general mechanistic concept of bioselection using a F/M gradient in three

or four stages that was initially presented by Chudoba et al., (1973a, 1973b, 1974) and Heide and

Pasveer (1974) is still considered valid.

Wanner (1994) reported that the aerated mode was successful at the suggested initial

contact zone loadings in Table 3.1. His results are included in Table 3.2 with additional data.

3-7

Table 3.2 Prior Art Batch and Continuous Flow Experience with Bulking SludgeControl Concepts and the DO and BOD5 Mass Loading in the ICZ

Selector Feed DO F/M SVI (Test)Author Mode Mode mg/L kg/kg⋅d mL/g

Davidson, 1959 AN Continuous 0.0 1.0 34 (u)

Bhatla, 1967 AL Continuous 0.0 >2.5 <120 (u)

British Ministry, 1969 A Continuous Unkn 0.8 <75 (u)

Milbury, 1971 AL Continuous 0.0 >2.0 <100 (u)

Chudoba, 1973 AL Continuous <0.5 >2.5 <100 (u)

Heide & Pasveer, 1973 AN/Ax Continuous 0.0 >5.0 <100 (u)

Batch 0.0 infinity 50 (u)

Rensink, 1974 A Continuous Unkn 3.6 <100 (u)

Tomlinson, 1976 A Continuous Unkn >2.0 <100 (u)

Spector, 1977 AN Continuous <0.7 >3.0 <100 (S)

Chudoba & Wanner, 1988 AH Continuous ~1.0 12.0 <50 (S)

Daigger and Nicholson, 1990 AH Continuous 2.0 14.7 <75 (D)

Albertson et al., 1992 AL Continuous <0.3 5.0 <100 (D)

Albertson and Hendricks, 1992 AL Continuous <0.3 7.0 <100 (D)

Linne et al. AL Continuous Unkn >5.0 <80 (D)

A – aerated, DO level unknown

Daigger and Nicholson (1990) reported on the performance of four wastewater treatment

plants employing selectors. The selectors employed different modes and configurations. The

design and performance characteristics of the bioselectors at Upper Occoquon Sewage Authority

(UOSA), VA; Northside WWTP, Tulsa, OK; Fayetteville, AK; and Tri-City, OR are summarized

in Table 3.3.

3-8

Table 3.3 Summary of the Design and Operation of Bioselectors by Daigger andNicholson (1990)

Item Units UOSA Northside Fayetteville Tri-CiyFlow (% of Design) mL/d (%) 80 (79) 84 (80) 60 (94) 11.9 (37)

Bioselector Type ANSX Aerated ANSX AXSX

SX Stages (L/W) No. 1 (5) 2 (17) 6 (1) 1 (?)

ICZ F/M (BOD5) kg/kg.d 14.8(1) 25.6(2) 4.4 0.8

Total SX F/M kg/kg.d 4.9 3.2 0.7 0.8

SX Retention Time (Q) minutes 14 20 93 86

SX DO mg/L 2-3 1.0 0 0

NO3-N Recycle ? No No Yes(3) Yes

Aeration Design No.-Type 2-CMAS 1-CMAS 4-CMAS 1-CMAS

Aeration Operation Mode Series – Series –

Average SVI (month) mL/g 80 150 90 70

Maximum SVI (month) mL/g 150 >300 180 180

Effluent BOD5 mg/L 6.2 5.5 7.4 7.7

Effluent NH4-N mg/L 0.4 1.4 1.1 1.3

(1) Estimated by dye testing – equivalent to three stages/compartments(2) Estimated by this author to exceed eight equivalent stages(3) Not employed during evaluation phase.

The only selector that was not considered effective by the authors was at Northside in

Tulsa, OK. This was a two-stage bioselector aerated channel with a 17:1 length:width, which

would likely have the equivalent stages equal to eight or more compartments. This would

produce an F/M in the initial contact zone of about 25 kg BOD5/kg MLSS⋅d. At this loading,

there is a high potential for hydrous bulking. The bioselectors were also aerated at 1.0 mg/L DO.

This range of DO is known to encourage filamentous growth in bioselectors. In fact, if the SVI is

too low and causing excess turbidity due to rapid settling sludge, increasing the DO to 0.5 to 1.5

mg/L is recommended to encourage growth of filamentous organisms. Thus, it is possible that

bulking is a result of generating both filamentous and hydrous bulking organisms in the aerated

channel.

3-9

The F/M in the Tri-City bioselector is considered to be low and a single-stage unit is less

than optimum. Large, first-stage anoxic zones common to the early South African and American

Bardenpho process plants also had bulking problems with maximum SVIs of 150 to over 200

mL/g.

A rigorous comparison of the four bioselectors is not possible because the SVI (uSVI)

test was employed. Once the SSV30 value exceeds 250 to 300 mL/L, the SVI fails to define the

true settling characteristics of the MLSS. That is, at MLSS of 3500 to 4000 mg/L, a uSVI of 140-

150 mL/g could be a DSVI value of 80-110 mL/g and good settling would occur in the clarifier.

The MLSS data were not available to make any approximation of the relative DSVI values.

Martin and Daigger (1997) reported on bioselector performance in four wastewater

treatment plants. The results achieved by the bioselectors were inconsistent, but there was an

appreciable difference in the design criteria. The operational characteristics of the facilities are

provided in Table 3.4

Table 3.4 Summary of the Design and Operation of Bioselectors by Marten andDaigger (1997)

Beloit Green Bay Green Bay Landis Tri-CityItem Units WI North, WI South, WI NJ OR

Flow (Design) m3/d 30,100 NA NA 31,000 51,000

Bioselector Type AXSX AXSX AXSX AXSX AXSX

SX Stages No. 1 1 1 3 1

ICZ F/M (BOD5) kg/kg⋅d 0.7-1.2 1.0-1.6 1.2-2.2 0.63 0.5-1.4

Total SX F/M kg/kg⋅d 0.7-1.2 1.0-1.6 1.2-2.2 0.21 0.5-1.4

SX Retention Time (Q) minutes 162 60 72 528 48

Air Added ? No No No No Yes/No

SX DO mg/L 0 0 0 0 0

NO3-N Recycle ? Yes Yes Yes(3) Yes Yes

Aeration Design No.-Type PF(1) PF(1) PF(1) 4-CMAS 1-CMAS

3-10

Beloit Green Bay Green Bay Landis Tri-CityItem Units WI North, WI South, WI NJ OR

Aeration Operation Mode – – – Series –

Aeration SRT Days 8-12 8-11 8-11 14-33 4-11

Average SVI mL/g 100 110 125 160 100

Maximum SVI mL/g 120 250(2) 250(2) 280(2) 160

Nitrifying ? Yes Yes Yes Yes Yes

(1) No estimate of the equivalent number of stages(2) Chlorine employed to control filamentous organisms

In three of the five activated sludge systems, it was necessary to use chlorine to control

filamentous growth. That is, the filamentous growths could not be controlled by the bioselector

and also scBOD5 breakthrough occurred in the Beloit and Green Bay plants, even at the low F/M

ratios. Thus, the question arises whether the bioselectors, with the exception of the Beloit units,

failed to control bulking due to the design and operation of the bioselector or due to wastewater

characteristics. Based on the literature surveyed and recommendations of other researchers and

practitioners, possible reasons why the bioselectors were sometimes ineffective (maximum

month SVI > 150 mL/g) were:

• The single-stage bioselector did not have F/M cascade – after Chudoba (1973a, 1973b),

Albertson (1987, 1991) and Jenkins et al. (1993).

• The low F/M in the ICZ, less than 2-2.5 kg BOD5 /kgMLSS⋅d, did not provide adequate

stress (Wanner, 1994) in the initial contact zone necessary to control filamentous growth –

after Chudoba (1973a, 1973b), Albertson (1987, 1992), and Wanner (1993).

• The single-stage bioselectors are less effective in the removal of sCOD than multi-stage units

and sCOD breakthrough can cause filamentous growths.

• Biological volumes necessary for denitrification are too large for bioselectors, which need to

be designed on a higher F/M gradient in three to four stages followed by the anoxic zones.

• In the case of one facility, the F/M in the long, rectangular bioselector (Northside) would be

very high (at > 25 kg/kg.d) in the equivalent volume of the initial contact zone. This F/M

could cause hydrous bulking. However, this bioselector was aerated at 1.0 mg/L DO, a DO

that is known to produce filamentous organisms. This document discourages use of aerated

3-11

bioselectors at a DO range of 0.3 to 2.0 mg/L unless the objective is to raise the SVI by

growing filaments when the SVI is too low for good clarification.

• Lack of adequate oxygen in the oxic zones to fully regenerate the return sludge.

3.2 Contact Loading Analysis

Contact loading (CL) is the ratio of soluble organics (sCOD) mass to the mass of

biological solids in the initial contact zone and is defined by mg sCOD/g TSS. The contact

loading in the initial contact zone will not be a design consideration for most municipal

wastewaters with sCODs of 125 to 250 mg/L. At 3000 to 3500 mg/L MLSS in the initial contact

zone, the contact loading is 25 to 60 mg sCOD/g TSS when the zone is anaerobic. If the ICZ also

receives an internal recycle flow, the CL will be lower. However, it can be a definite

consideration for those municipal and industrial facilities that have an above average sCOD in

the influent to the initial contact zone. There will be a point where the soluble food/mass (FS/M)

loading criteria will result in a higher value than the recommended limiting contact loading.

Under these conditions, the contact loading criteria govern the bioselector design.

In the initial contact of return sludge (RAS and/or MLSS), the majority of the sCOD is

removed in a few minutes with minimal oxidation (Kroiss, 1985). This conversion to cell storage

has a limited capacity for healthy (non-bulking) sludge generation. Excessive levels of food will

result in exo-cellular protoplasm (ECP) accumulation (hydrous/viscous bulking) and possibly

breakthrough of soluble substrates to the oxic zone where filamentous growths can develop.

Thus, the contact loading is the ratio of the amount of soluble substrate per contacting unit of cell

weight in the ICZ without the time function of the F/M ratio.

While there has been successful bulking sludge control at contact loadings up to 200 mg

sCOD/g TSS, the recommended limiting value is 100 mg/g (10% of cell weight) for the average

daily value. It is recognized that the peak loading periods could result in CL values up to 140

mg/g. The limiting sCOD into the initial contact zone as a function of the MLSS is shown in

Figure 3.3. As discussed earlier, bioselection is favored by higher MLSS and the resulting

increased oxygen demand in the ICZ.

3-12

When the influent (Qi) sCOD is higher than indicated in Figure 3.3, then an additional

source of activated sludge solids must be added to the initial contact zone. The solids must be

fully oxidized (regenerated) MLSS from the effluent end of the aeration basin via an internal

recycle (IR). The IR (Figure 3.4) requirements are defined by:

CL ≤ 100 mg/L ≤ )/,)(/,()/,)(/,(

)/,)(/,(

LmgMLSSsLIRLmgRSSsLRAS

LmgsCODsLQ ii

+(3.2-1)

When the contact loading is greater than 100 mg/g, the above equation suggests that there

are two possible responses to reduce the contact loading.

• Increase the RAS flow of RSS (higher MLSS) within the capability of the secondary

clarifiers to process the (Q + RAS)(MLSS) solids loading.

• Turn on or increase the IR flow. The IR flow is preferred due to lower power

requirements, better control, and no adverse impact on the secondary clarifiers. If the

system is nitrifying, denitrification will occur in the bioselectors.

When treating strong municipal and industrial wastewater, it is necessary to check the

contact loading of a bioselector design to be assured there is not a problem of high contact

loading that could result in hydrous sludge (excess ECP) production.

3.3 Sludge Age

Washout of filamentous organisms is difficult because their growth rate (sludge age)

spans such a large range as shown in Figure 3.5 (Wanner, 1994). This procedure has been

employed for secondary level of treatment using 1- to 2-day solids retention time (SRT), but it is

not practical for biological nutrient removal (BNR) processes requiring a 5- to 15-day SRT.

There is probably a minimal sludge age (defined by SRT) where bioselectors lose their

effectiveness. The exact SRT resulting in washout of preferred biomass (rapid settling) is not

known and may be both site and temperature dependent. However, bioselection has consistently

proven effective at SRTs of > 3 to 4 days and as low as 1.6 days at the Deer Island pilot plant

(Bowen et al., 1992 and MWRA, 1995).

3-13

3-14

3-15

3-16

3.4 Design Features

It is necessary to eliminate uncontrolled backmixing between the bioselector zones. The

design should employ submerged walls (Figure 3.6) to prevent the entrapment of floating

material in the compartments. Floating material is unsightly, can be odorous and

counterproductively act as a seed for increased filamentous growth. The design of the submerged

baffle walls should allow for about 1.2 mm (0.5 inch) head loss per baffle at the maximum

month combined flow of wastewater, RAS and IR at each stage. An opening at the bottom of the

baffle to allow filling and drainage of the component is necessary and eliminates the need for the

baffle walls to structurally withstand the hydraulic load.

A sizing procedure for the submerged baffle walls, described by Albertson (1999), has

been successfully employed in many facilities including those at Columbus, OH (Jackson Pike

and Southerly); Phoenix, AZ (91st and 23rd Ave); Santa Fe, NM; Baltimore, MD (Back River);

Pierce Co, WA; Lakeland, FL; and Wash, DC. (Seneca).

The submerged wall height is designed to produce about a 1.2 mm (0.5-inch) water

column (WC) head loss (ΔH) as shown in Figure 3.6. The head loss equations of Fteley and

Stearns (King, 1939) with the coefficient (C) provided can be employed to generate the data

necessary to calculate the information for the wall configuration. The flow over the baffle would

include maximum influent flow and RAS as well as the internal recycle flow. A sample

calculation is provided in Table 3.5.

A slot at the end of the partition wall has been employed, but it is not as effective unless

it is designed to have sufficient headloss to prevent backmixing. With aeration on one side and

mixing on the other side, backflow at the bottom of the slot will occur if the head loss is too low.

The submerged baffle eliminates backmixing and does not trap solids. The surface of the

aeration basin is clear of foam as shown in the photograph (Figure 3.7) of the four-pass aeration

basin at the 23rd Ave Water Reclamation Facility in Phoenix, AZ.

3-17

3-18

3-19

Table 3.5 Calculation of Baffle Wall Height

Input Data

Avg/Max Month Flow – 8/11 mgd Baffle Wall Length(1) – 30 ft

Max. Month RAS – 8 mgd Basin Depth – 16 ft

IR Flow @ 4 QAMF – 32 Head Loss (ΔH = 0.5 in) – 0.0417 ft

Total Flow – 51 mgd (79.1 ft3/sec)

F&S Eq Qt = CL(ΔH)0.5(H + d/2)

= 0.2042 CL(1.5H – 0.0208) (@ ΔH = 0.0417 ft)

H = 0139.0))()(3063.0(+

LC

QT

1st Estimate H = 2.25 ft

d/H =25.2

0417.025.2 − = 0.9815

Fteley and Stearns C = 3.304

∴H = 2.62 ft from Fteley and Stearns Equation

2nd Estimate H = 2.60 ft

d/H = 0.9840

C = 3.310

∴H = 2.61 ft OK = 2.60 ft submergence

(1) Basin width is also 30 ft.

Values of Fteley and Stearns Submerged-Weir Coefficient C

d/H 0.0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

0.0 0.000 3.330 3.331 3.835 3.343 3.360 3.368 3.371 3.372 3.370

.1 3.365 3.359 3.353 3.343 3.335 3.327 3.318 3.310 3.302 3.294

.2 3.286 3.278 3.271 3.264 3.256 3.249 3.241 3.231 3.227 3.220

.3 3.214 3.207 3.201 3.194 3.188 3.182 3.176 3.170 3.168 3.159

.4 3.155 3.150 3.145 3.140 3.135 3.131 3.127 3.123 3.119 3.116

.5 3.113 3.110 3.107 3.104 3.102 3.100 3.098 3.096 3.095 3.098

.6 3.092 3.091 3.090 3.090 3.089 3.089 3.089 3.090 3.090 3.091

.7 3.092 3.093 3.095 3.097 3.099 3.102 3.105 3.109 3.113 3.117

.8 3.122 3.127 3.131 3.137 3.143 3.150 3.156 3.164 3.173 3.181

.9 3.190 3.200 3.209 3.221 3.233 3.247 3.262 3.280 3.300 3.325

Source: King, H.W. Handbook of Hydraulics, 2d ed. p. 98 McGraw Hill Book Company, Inc., New York, 1939.

4-1

Chapter 4.0

Types or Designs of Bioselectors

Nomenclature employed to define the type of bioselectors (SX) for bulking sludge control

and the mode of mixing are anaerobic (AN), anoxic (AX), and aerated, where mixing is provided

by low air rate (AL) with DO < 0.3 mg/L, high air rate (AH) with DO > 2.0 mg/L or mechanical

(M) means.

• Aerated, High DO (SXAH): The SX zones are heavily aerated to maintain a

dissolved oxygen (DO) > 2.0 mg/L for bulking sludge control. This process is

employed for both secondary and/or nitrification modes of operation with or

without limited denitrification. That is, denitrification is not a design requirement.

• Aerated, Low DO (SXAL): The SX zones are aerated at a rate whereby the DO is

< 0.3 mg/L during off-peak loadings and zero DO during average to peak

loadings for secondary and/or nitrification processes with or without limited

denitrification.

• Anoxic with Mechanical Mixing (SXAXM): An anoxic environment at zero DO

is maintained by mechanical mixing. Nitrates are recycled from the oxic zones for

enhanced levels of denitrification in the bioselectors and subsequent anoxic

zones.

• Anoxic, Aerated, Low DO (SXAXAL): An anoxic environment at near zero DO

with aerated zones as set -forth in SXAL. Nitrates are recycled for enhanced levels

of denitrification.

• Anaerobic with Mechanical Mixing (SXANM): An anaerobic environment is

maintained by employing mechanical mixing and the design limits the average

concentration of nitrates to 0.5 mg/L in the total flow entering the initial contact

zone (ICZ). That is, internal recycle is not discharged into the bioselectors. This

mode of operation is preferred for maximum levels of Bio-P removal.

4-2

-

While a bioselector may have two to four compartments (zones), the terminology ‘initial

contact zone’ (ICZ) refers only to the first compartment of the bioselector where influent and

return sludge are mixed. All bioselectors should be staged (three or four compartments) and will

thus create a food/mass (F/M) gradient (cascade). This does not mean that a single-stage

bioselector will not work, but it is less effective and thus not reliable. For those reasons and the

history of successful bioselectors, only three- or four-stage designs are recommended for this

study.

The recommendations in this section will not reflect the full range of successful operating

conditions for bioselectors. These are site-specific conditions that are not fully understood at this

time. Thus, the recommendations reflect what is considered to be the most reliable design criteria

and process approach with the available data. There can exist numerous successful systems that

are not within the suggested guidelines. In fact, there are, for unknown reasons, complete mix

activated sludge (CMAS) systems that do not bulk (Tomlinson, 1976), while many others,

seemingly with similar operational conditions, that do bulk. Tomlinson’s survey of 65 English

plants clearly revealed the benefits of multistage aeration basins for producing lower SVIs.

However, there were CMAS plants with SVIs <100 mL/g as well as plants at >500 mL/g.

It is recognized that the ‘anaerobic’ bioselector is not actually anaerobic (methane

producing) as facultative organisms are the dominant group present. However, it is a convenient

definition and provides differentiation as described in a Bio-P and bioselector patent (Spector,

1977). This patent is the source of distinguishing an anaerobic zone (< 0.5 mg/L nitrate level in

the total flows entering the initial contact zone) vs. an anoxic zone, where anoxic is defined as

>0.5 mg/L NO3-N. The term ‘anaerobic’ was also used earlier by Davidson (1957) to describe

the first contact zone of his bioselection process which was also patented. Albertson (1987,

1992,1994) differentiated the type of bioselectors in quantitative terms as shown in Table 4.1.

This terminology has been employed throughout this document.

4-3

Table 4.1 Characteristics of Initial-- Contact Zone of Bioselectors

Environment Criteria

Aerated, High DO (SXAH) - air or oxygen added, DO > 2.0 mg/L, without nitrates present

Aerated, Low DO (SXAL) - air added, DO = 0.0 to 0.3 mg/L,NO3-N < 0.5 mg/L in mixed liquor formed

Anoxic (SXAXM) - no added air or oxygen, DO = 0.0 mg/L,NO3-N > 0.5 mg/L in mixed liquor formed

Anoxic, Aerated, - air added, DO < 0.0-0.3 mg/LLow DO (SXAXAL) NO3-N > 0.5 mg/L in mixed liquor formed

Anoxic, AeratedHigh DO (SXAXAH) - not recommended at this time

Anaerobic (SXANM) - no added oxygen or air, DO = 0.0 mg/L, mechanically mixed,NO3-N < 0.5 mg/L in mixed liquor formed

F/M Cascade - 2.5 to 3 kg sBOD5/kg MLSS⋅d in ICZ with 3-4 stages

The environmental conditions set forth above are somewhat arbitrary and are not

exclusive. Prediction of bulking conditions in a system is problematic and sometimes conditions

that should control filamentous growths have not done so, and the opposite situation can be true.

The objective must be to develop an environment where there is the maximum opportunity to

control growth of undesirable species of bacteria. In the author’s 20-plus years of experiences

with multistage bioselectors (1980-2005), the process has been 90-95% successful in the control

of filamentous organisms. Some failures are due to inappropriate designs, lack of understanding

of the control mechanisms, and/or changes in or inadequate definition of the influent

characteristics.

The following sections present alternative bioselector designs with and without aeration

and mechanical mixing for aerated, anoxic and anaerobic bioselectors. The recommendations

may agree or conflict with other cited references. This is not unusual for a developing

technology that does not have a sufficient quantity of readily available and reducible full-scale

data.

4-4

Because bioselector zones probably involve only the soluble (defined by filtration at 1.2-

1.5 μm) fraction of the wastewater organics, it is more correct to use the soluble chemical

oxygen demand (sCOD), or soluble biological oxygen demand (sBOD5), fraction to define the

soluble food/sum of the mass (FS/ΣM) gradient. It is also necessary to protect against

overloading of the biomass in plants with higher than normal peaking loads to prevent periodic

overloading and subsequent hydrous sludge production. In small wastewater plants, the higher

peaking factors dictate use of a lower (FS/M) gradient. As an example, the recommended FS/M

criteria in a three-stage anoxic (SXAXM or SXAXAL) bioselector are set forth in Table 4.2 for 1.4

and 2.0 organic load peaking factors. When the peaking factors are higher than 1.4, the F/M in

the initial contact zone should be reduced, as shown in this table.

If the COD/BOD5 and sCOD/ BOD5 ratios are not about 2.0 ± 0.1, sCOD values should

be employed for the design or the sBOD5 FS/M adjusted to account for the difference. The sCOD

is a better measure of the loading than sBOD5 and should be employed when data are available.

It is recognized that the influent sCOD is reduced at each stage. However, it is convenient to

express the design criteria for each stage as the influent soluble value (food) divided by the sum

of the accumulated mass (Fs/ΣM) in the zones. It is a bookkeeping procedure for design and is

not representative of the conditions within the bioselection zones.

Table 4.2 Design FS/M Criteria for Anoxic (SXAXAL or SXAXM) BioselectorsTo Account for Daily Peaking Factors

sBOD5 sCOD

Peaking Load Factor (8 hr)(1) 2.0 1.4(2) 2.0 1.4(2)

Zone Average Loading

Sx-1 FS/ΣM – kg/kg⋅d 2.2 3.0 4.4 6.0

Sx-2 FS/ΣM – kg/kg⋅d 1.1 1.5 2.2 3.0

Sx-3 FS/ΣM – kg/kg⋅d 0.55 0.75 1.1 1.5(1)Interpolate FS/M loadings for intermediate or higher peaking factors.(2)Typical peaking factor for domestic wastewater treatment plants.

4-5

A typical set of calculations for bioselectors for a hypothetical wastewater is defined in

Table 4.3. These wastewater characteristics will be employed for all bioselector designs in this

section, but the peaking load factor must be considered on a site-specific basis. Calculations that

use similar design methodology will refer to the first set of calculations developed for bioselector

designs. Because the calculations are soluble (f1.5µm) substrate quantities, the origin (raw or

settled) of the wastewater is not of consequence. However, if soluble BOD5 and COD data are

available, it is strongly recommended that these data be collected prior to finalizing the design of

the bioselectors.

Table 4.3 Wastewater Characteristics Used in an Equation to EstablishCriteria for the Bioselector Design

Parameter Units Maximum Month Load 8-hr Peak

Flow L/sec 1,000 1,400

COD kg/d 43,200 69,100

sCOD kg/d 17,000 27,200

BOD5(1) kg/d 19,400 31,000

sBOD5 kg/d 7,200 11,700

TKN kg/d 3,600 --

TP kg/d 650 --

Temperature °C 15/25 --(1)cBOD5 data are not acceptable input values since they can be 0.70 to 0.90 of the BOD5.

A review of Table 4.3 reveals a sCOD/sBOD5 ratio of 2.3, or higher than 2.1; thus the

sCOD value should be employed for the design of the bioselectors. The 8-hour peaking factor is

1.6 (> 1.4) and factoring the F/M according to Table 4.2 instructions, a FS/ΣM gradient of 5.5,

2.75 and 1.38 kg sCOD/kg MLSS⋅d will be employed with the average maximum month sCOD

loading.

4-6

The calculations for the bioselector design would be as set forth below.

Zone SX-1 FS/ΣM = 5.5 kg sCOD/kg⋅d

1−XSM =dkglb

dkg

⋅/5.5

/000,17

= 3091 kg

1−XSV =3/5.3

/091,3

mkg

dkg

= 883 m3 @ 3500 mg/L MLSS

Zone SX-2 FS/ΣM = 2.75 kg sCOD/kg⋅d

2−XSM = kgdkglb

dkg3091

/75.2

/000,17−

⋅

= 3091 kg

2−XSV =3/5.3

/091,3

mkg

dkg

= 883 m3

Zone SX-3 FS/ΣM = 1.38 kg sCOD/kg⋅d sCOD/kg●d

3−XSM = kgkgdkglb

dkg30913091

/38.1

/000,17−−

⋅

= 6182 kg

3−XSV =3/5.3

182,6

mkg

kg

= 1766 m3

The aeration design for the system produced a basin average actual oxygen transfer rate

(AOTR) of 40 mg/L⋅hr for the oxic volume. Thus, the maximum aeration AOTR design for the

bioselectors would be 32 to 40 mg/L⋅hr (80 to 100%). However, the normal diffuser turndown

will generally provide 40 to 100% of the overall basin average AOTR to account for startup

conditions and lower than design loadings. The conversion to standard oxygen transfer rate

4-7

(SOTR) would be made using standard practice equations. Most often, coarse bubble diffusers

have been employed in bioselectors, but membrane diffusers have provided satisfactory service.

It is recommended that an alpha value of 0.7 be used for coarse bubble and 0.35 for membrane

diffusers in the bioselector zones. The type of diffuser employed will be dependent on the ability

to remove a basin from service while maintaining adequate facilities in operation. When there is

a mixture of coarse bubble diffusers and fine bubble diffusers, there should be a separate air

service with a control valve and flow meter to all for a constant air flow to the selector zones.

The total volume (3532 m3) of the bioselectors sized in the previous example would be

allocated based on the number of aeration basins. Generally each aeration basin has a

bioselector, but one bioselector can serve two or more basins. When one bioselector serves two

or more basins, the aeration design should allow the bioselector to remain in service at all times.

4.1 Aerated, High DO Bioselectors (SXAH)

The first published reference to a cascade of heavily aerated zones was Chudoba (1973a,

1973b, 1974), wherein the initial stage of the 2-, 4-, 8- and 16-stage reactors was targeted at a

DO level of 2 mg/L. This target level proved to be difficult to maintain in the initial contact zone

of the 8- and 16-stage reactors. The ICZ oxygen uptake rate (OUR) would be very high in both

the 8- and 16-stage systems.

Casey et al., (1975) were issued a U.S. patent (No. 3,654,147) on a heavily aerated or

oxic bioselector that set forth specific criteria for the control of filamentous bacteria. The design

criteria had a F/M component wherein the mass employed was the active mass fraction. This

fraction could be from <30 to >70% of the mixed liquor volatile suspended solids (MLVSS). The

critical parameters for the initial contact zone in the Casey patent were:

1.55 BOD5/active mass ≥ 4 kg/kg⋅d

DO ≥ 2 mg/L

OUR ≥ 100 mg O2/L⋅hr

4-8

For the most part, the patent and the above criteria were established for pure oxygen

systems. The initial contact of the influent wastewater was controlled to a specific BOD5:active

mass ratio by compartmentalization. Pure oxygen systems were designed to provide sufficient

driving force to maintain the first compartment DO at 2-8 mg/L, even with high oxygen uptake

rates. However, in practice, the DO in the first stage of pure oxygen plants was often below 2

mg/L when the process was at design load.

In full-scale pure oxygen operations, control of filamentous growth and the resulting

sludge volume index (SVI) were not always attained. In many cases, there was severe bulking

and it was necessary to operate the pure oxygen aeration basins at low (1000 to 1500 mg/L)

mixed liquor suspended solids (MLSS) in order to prevent overloading the clarifiers. Often the

installed oxygenation capacity in the first stage was not adequate to meet the oxygen demand

necessary to produce the required DO at the design MLSS. Conversely, some systems have

operated at high MLSS and zero or near zero DO successfully in the first stage with a low SVI.

While the expired patent suggests maintaining a DO greater than 2.0 mg/L is necessary, it

is probable that a higher DO was necessary to fully maintain the entire floc aerobic. That is, in

the high-rate regime common to pure oxygen activated sludge, a liquid DO of 4-8 mg/L may be

required to provide the gradient necessary for oxygen to fully penetrate the floc. It has been

reported from tests at San Francisco, CA, that a low DO in the initial stage of the pure oxygen

reactor was more effective than higher DO in the high-rate oxygen reactors. However, the

Boston, MA, pure oxygen plant with a four-stage bioselector has determined that 10 mg/L DO in

the initial contact zone is the best operating mode (Tyler, 2002).

The reported use of a heavily aerated (SXAH) ICZ was not actively pursued until 1985

(Kroiss) when an aerated four-stage bioselector was employed to treat sugar mill wastewater.

The Czechoslovakian bioselector design employed a SOTR up to 167 mg/L⋅hr in the initial

contact zone (Grau, 1991) in order to maintain >1 mg/L, preferably ≥ 2.0 mg/L DO in the initial

contact zone. The bioselector at the sugar mill was designed on the criteria set forth in Table 4.4.

4-9

Table 4.4 Technical Parameters and Results of the Bioselector (SXAH) Activated SludgeProcess in the Leopoldsdorf Sugar Mill Obtained During the Campaign in1984 (Kroiss)

Parameter Dimension Value

Flow m3/d 45,000

Selector Volume m3 400

Total Volume m3 16,400

Selector Detention Time h 0.21

Total Detention Time h 8.71

COD Loading - System kg/m3⋅d 1.23

BOD5 Loading - System kg/m3⋅d 0.93

MLSS mg/L 3,300

SRT d 8

SVI mL/g 50

Energy Consumption kWh/kg BOD5 0.7

COD Removed in Selector % of CODtot Removed 72

Oxygen Consumed in Selector % of CODtot 8.5

Influent sCOD g/m 450

Effluent sCOD g/m 45

Influent sBOD5 g/m 340

Effluent sBOD5 g/m 10

COD Removal % 90

BOD5 Removal % 97

The four-stage bioselector F/ΣM loadings were 12, 6, 4 and 3 kg BOD5/kg MLSS⋅d,

respectively. Since the COD/ BOD5 ratio is 1.32 kg/kg, the equivalent COD loading is 15.9, 7.9,

5.3 and 4.0 kg/kg⋅d. The ‘equivalent’ BOD5 at COD/ BOD5 = 2.0 kg/kg reduces the BOD5

loading effect by 34% to 7.9, 4.0, 2.6 and 2.0 kg/kg⋅d. Due to the low COD/ BOD5 ratio for the

sugar wastewater, the design based on BOD5 can be misleading. However, the COD and BOD5

in the sugar mill wastewater is mostly soluble and the equivalent soluble loadings are higher than

4-10

recommended in this document. The oxygenation (SOTR) capacity was 4 kg/m3⋅d (167 mg/L⋅hr)

for the bioselectors and 2 kg/m3⋅d (83 mg/L⋅hr) for the overall system, and the reported actual

oxygen transfer rate (AOTR) would be about 109 mg/L⋅hr based on CODR.

The Upper Occoquan Sewage Authority (UOSA) plant in Virginia employs an aerated,

high DO (SXAH) three-stage bioselector that can maintain the DO at ≥ 2 mg/L. The overall F/M

is 4.9 kg BOD5/kg MLSS⋅d and a loading of 14.7 kg BOD5/kg MLSS⋅d in the initial contact

zone. The bioselector has proven capable of reducing the SVI to an average of 74 mL/g from

SVIs exceeding 600 mL/g prior to the bioselector installation (Daigger and Nicholson, 1990).

The UOSA plant has operated at equivalent COD F/Ms exceeding those set forth by Kroiss

(1985) and about 250% higher than recommended by Jenkins et al., (1993). There has not been

any reported problem (Sellman, 2001) of exo-cellular protoplasm (ECP) accumulation after 10

years of operation. Process data from the UOSA wastewater facility are included in Chapter 5.

Jenkins et al., (1993) and Czechoslovakian (now Czech Republic) researchers Chudoba

and Wanner’s (1988) recommendations for F/M loadings of an aerated, high DO (SXAH)

bioselector are:

Jenkins et al. Chudoba and Wanner

Zone SX-1 12 kg COD/kg MLSS⋅d 12 kg BOD5/kg MLSS⋅d

Zone SX-2 6 kg COD/kg MLSS⋅d 6 kg BOD5/kg MLSS⋅d

Zone SX-3 3 kg COD/kg MLSS⋅d 4 kg BOD5/kg MLSS⋅d

Zone SX-4 – 3 kg BOD5/kg MLSS⋅d

The Chudoba and Wanner organic loadings are similar to those in the UOSA and

Hamilton, OH, (Wheeler et al., 1984) bioselectors, which have also operated successfully but

about 100% higher than the more recent Jenkins et al., reference.

The oxygen supply in the initial contact zone may need to meet up to 30 to 35 mg O2/g

MLSS⋅hr in warm wastewaters at 3500 to 4000 mg/L MLSS. Thus, SOTR values of 140 to 160

mg/L⋅hr are necessary and may not meet the target DO values of 2 mg/L. An SOTR of 160

4-11

mg/L⋅hr is about the practical transfer limit of diffused aeration employing air feed. Thus,

maintaining 1 to 2 mg/L DO in the initial contact zone of a high-rate nitrification (SRT 4-6 days)

system may be at the limit of air-supplied SOTR capabilities. Further, the proportion of sCOD in

the influent will be important in defining the maximum value of AOTR and SOTR.

The BOD5 loading in the initial contact zone of the reported operating plants is 12 to 14.7

kg BOD5/kg MLSS⋅d. Experience is limited and the reduced loadings suggested by Jenkins et

al., (1993) form the basis for the aerated, high DO (SXAH) bioselector design criteria. The

primary reason for employing loadings lower than the cited references is concern for production

of hydrous sludges from floc organic overload. Further, there is evidence that the lower loadings

will still provide good bulking sludge control. The SXAH bioselector design recommendations

are set forth in Table 4.5.

Table 4.5 Design Recommendations for Aerated, High DO (SXAH) Bioselectors

Relative Design LoadingsBioselector AOTR Volumes COD sCOD(1) BOD5 sBOD 5

(1)

Zone mg/g⋅hr % kg/kg⋅d kg/kg⋅d kg/kg⋅d kg/kg⋅d

SX-1 30-35 25 16 8 8 4

SX-2 25-30 25 8 4 4 2

SX-3 25-30 50 4 2 2 1

(1) Preferred wastewater characteristic for design.

The three-stage design shown in Table 4.2 can also be used for the four-stage Czech

design by dividing the third zone (SX-3) of Table 4.5 into two compartments creating a

bioselector with four equal zones.

If the operating SXAH bioselector was producing a higher than target SVI (stirred SVI,

diluted SVI < 90 mL/g) and the DO was 0.5 to 1.0 mg/L at a maximum air rate, the air rate

should be reduced to cause the DO to be 0.0 mg/L and retested (i.e., convert aerated to low DO

bioselector).

4-12

4.2 Aerated, Low DO Bioselectors (SXAL)

In the 1960s, some wastewater researchers and practitioners recognized that the DO, or

lack of it, in the initial length of long, rectangular basins had an impact on the treatment

effectiveness. Their observations (Vacker et al., 1967; Wells, 1969; Scalf et al., 1969; Milbury et

al., 1971, Garber et al., 1972) related not only to the biological removal of phosphorus (luxury

uptake) but also to the control of SVI. Due to the high BOD5 loading and oxygen demand,

coupled with diffuser fouling, low or no DO was present in the initial portions of the long,

rectangular basins. That is, the low DO in the initial 20-30% of the basin length encouraged

biological phosphorus removal, later called Bio-P removal, as well as producing reduced SVIs.

Tomlinson (1976) found that English wastewater treatment plants with long, rectangular basins

generally had SVIs less than 100 mL/g. Additional findings in the 1970s by Chudoba et al.,

(1973a, 1973b and 1974) and Heide and Pasveer (1974) recognized the significance of the high

organic loading in the initial contact of return sludge and influent wastewater as it related to the

control of bulking organisms. The results of Chudoba’s studies in 1973 are shown in Table 4.6.

Table 4.6 Results of 1973a Chudoba Laboratory Staged Aeration Studies

Reactor No. I II III IV

Volume – L 4 4 4 4

Retention Time – hr 8 8 8 8

Compartments – No. 1 4 8 16

COD – mg/L 650 650 650 650

BOD5 – mg/L 230 230 230 230

MLSS - mg/L 1900 3280 2940 3100

SRT – days 3.0 4.8 4.7 4.5

COD F/M – kg/kg TSS⋅d 1.03 0.595 0.663 0.629

BOD5 F/M – kg/kg TSS⋅d 0.44 1.02 2.29 4.36

Settled Effluent

TSS – mg/L 53 8.9 13.1 15.0

COD – mg/L 107.8 30.6 43.0 50.7

BOD5 – mg/L 40.8 11.0 10.2 10.8

4-13

Reactor No. I II III IV

NH4+ – mg/L 2.9 0.4 3.4 3.6

NO_

2 – mg/L 29.0 7.9 17.9 22.0

NO_

3 – mg/L 16.3 33.5 23.1 25.9

Org N – mg/L 4.9 4.0 3.9 4.9

PO4–3 – mg/L 3.1 0.9 0.2 2.0

SVI – mL/g 517 300 91 51

ICZ COD F/M – kg/kg TSS⋅d 1.03 2.38 5.30 10.06

ICZ BOD5 F/M – kg/kg TSS⋅d 0.44 1.02 2.29 4.36

Chudoba et al., experiments with 1, 2, 4, 8 and 16-stage and 1, 4, 8 and 16-stage reactors

furthered the British Water Pollution Control Laboratories’ work (1969) with 1, 2 and 4-stage

reactors and correlated the SVI to the F/M gradient in the reactors. The 8 and 16-compartment

aerated reactors with the highest F/M in the initial contact stage (ICZ) produced the lowest SVI

(Figure 4.1). While the target DO in these studies was 2.0 mg/L, the operating DOs in the 8 and

16-stage ICZs were less than 0.5 mg/L (Chudoba, 1985b) due to the high oxygen demand in the

small compartments. Thus, the work of the Czech investigators was consistent with the U.S. and

British investigators (Tomlinson, 1976; Tomlinson and Chambers, 1978b) who had earlier found

SVI control in activated sludge systems with long aeration basins that provided an F/ΣM

gradient. This gradient was better maintained when the basin was baffled into compartments as

was first noted by Donaldson (1932) and later by the British Water Pollution Laboratory (1969).

Chudoba et al., (1973a, 1973b) studies confirmed these earlier observations.

In 1976, Tomlinson reported on an extensive survey of 65 English activated sludge

facilities. The focus of the study was the bulking sludge history of each facility. It was noted that

the larger plants tended to have less bulking problems. These same plants would normally have

long, rectangular tanks with more of a plug-flow mode of operation. The mathematically

4-14

4-15

calculated dispersion index to define the number of compartments was translated into the

theoretical F/M loading in the first compartment (ICZ or SX-1).

The results of this analysis, Figure 4-2, supports the position that the F/M loading must

be sufficiently high to ensure biological stress, and this F/M is >2 and most likely above 3 kg

BOD5/kg mass⋅day in the initial contact zone. However, some plants, even in a complete mix

(CMAS), will not have bulking problems. As shown, at F/Ms of 0.1 to 0.5 kg/kg⋅d, the SVI

ranged from about 70 mL/g to 680 mL/g, but only four of 25 plants with an F/M < 0.5 kg/kg⋅d in

the initial contact zone had SVIs < 150 mL/g and the average value exceeded 300 mL/g SVI.

With an F/M > 1.8 kg/kg⋅d in the initial contact zone, none of the plants had an SVI > 150 mL/g.

The DO in the initial portion of the long basins was not reported. However, typically fine

bubble diffusers in the initial portion of a long basin blinded more easily and alpha values were

lower. As the result, the DO was low and the low SVIs were due to operating in a high F/M,

aerated, low DO mode (SXAL) of bioselection in the initial contact zone of the long basin.

In the aerated, low DO bioselectors, the AOTR will be much less than the oxygen uptake

rate and the AOTR needed for the MLSS in the oxic zones. For example, the 20°C OUR of a

3500 mg/L MLSS can be 30 to 40 mg O2/g⋅hr, but the design AOTR would usually not be more

than 30 to 35% of this rate. This will result in a zero DO in the initial contact zone at design

loadings. The recommended AOTR for the bioselector zones is about 80 to 100% of the average

AOTR for the aeration basin when Bio-P reactions are not required. As a consequence, the DO

will be at or near zero except during low BOD5 loading periods when biological growth is

minimal.

Earlier bioselector criteria (Albertson, 1987) employed a three-stage design with 5.6, 2.8

and 1.4 kg BOD5/kg MLSS⋅d F/ΣM gradient for the primary effluent at the Southerly plant in

Columbus, OH. Later this was modified for a COD design at a 12, 6 and 3 kg/kg⋅d F/M gradient

for primary effluent. However, it is the soluble fraction of the wastewater that is involved in the

bioselector mechanisms and this should be employed for design. The soluble fraction of raw

4-16

4-17

domestic wastewater is 30-40% and primary effluent normally has 45-55% sBOD5/BOD5

(sCOD/COD). Industrial content can modify the values; thus, a design based on soluble fraction,

preferably sCOD, would be a more reliable approach. This approach is consistent with the

contact loading (CL) criteria of mg sCOD/g TSS set forth earlier in Chapter 3, Section 3.2.

The aerated, low DO bioselectors may or may not be anoxic, but both modes will be

equally effective in controlling the growth of filamentous organisms. A bioselector in secondary

treatment mode may not be as effective at less than three days solids retention time (SRT).

However, the Deer Island, Boston, MA, bioselectors (Bowen et al., 1992) produced low SVIs at

1.6- to 2.6-day SRT. Because nitrification can occur at SRTs greater than 2-3 days, a contact-

stabilization (C-S) mode can be employed to provide a 3- to 5-day SRT, minimize nitrification,

and maintain an adequate SRT for the bioselection process and control nitrification if it occurs in

the stabilization zone. The nitrates produced would be reduced to nitrogen gas (N2) in the

bioselectors prior to the contact zone. The C-S mode is employed at Gig Harbor, WA, and the

flowsheet with operating results are provided in Chapter 5.

The aerated, low DO bioselectors operating in an anoxic mode have been successfully

applied in many facilities including large facilities at Southerly and Jackson Pike in Columbus,

OH (Albertson et al., 1992), Santa Fe, NM, the 23rd and 91st Ave plants in Phoenix, AZ

(Albertson and Hendricks, 1992 and Albertson and Stensel, 1994) and Baltimore Back River

wastewater treatment plants (WWTPs). Operating data are provided for several facilities in a

later section. When denitrification is desired, the air rate is more limited in the bioselectors and

about 30-35% of the internal recycle (IR) is discharged to the initial contact zone (Figure 3.1)

through Valve V1. If partial (65-80%) phosphorus removal is an objective, then the internal

recycle is discharged either to the third bioselector stage or to the anoxic zone after the

bioselector. The denitrification rates did not show any adverse effects of aeration at 40-50

mg/L⋅hr AOTR at Phoenix 23rd and 91st plants, which have warm wastewaters (20-33°C)

(Albertson and Stensel, 1994).

Albertson et al., (1992, 1994) reported on successful SVI control with aerated

bioselectors (SXAL) at the Jackson Pike and Southerly WWTPs in Columbus, OH, and 23rd Ave

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and 91st Ave plants in Phoenix, AZ. The bioselector ICZs were operated with a F/M of 5 to 6 kg

BOD5/kg MLSS⋅d (12-15 kg COD/kg MLSS⋅d). The air rate was low and DO was essentially

zero. Selector effluent sCOD approached the concentration (25 to 30 mg/L) present in the final

effluent. At 23rd Ave WWTP, the combination of a 25% increase in BOD5 input and an

improperly placed ICZ baffle resulted in a COD F/M of 16-19 kg/kg.MLSS⋅d (BOD5 of 7.5-9

kg/kg⋅d) in the initial contact zone, and hydrous bulking occurred until the baffle was moved to a

new position that reduced the COD F/M to 10-12 kg/kg⋅d (BOD5 at 4.8.-5.7 kg/kg⋅d).

The design criteria for the aerated, low DO bioselector are the same for processes without

denitrification (SXAL) and anoxic (SXAXAL) processes. At this time, the solids retention time

(SRT) of the aeration basin is assumed to be > 3 days. The 8-hour peaking load factor is assumed

to be < 1.4 average loading. Adjust the design F/M as noted earlier for higher peak load factors

(see Table 4.5) and if the COD/BOD5 ratios vary from 1.9-2.1. The sCOD data will provide the

most reliable design criteria. The design criteria are summarized in Table 4.7.

Table 4.7 Design Recommendations for Aerated SXAL and SXAXAL Bioselectors

Relative Design Loadings(1)

Bioselector AOTR Volumes COD sCOD(1) BOD5 sBOD5

Zone mg/g.hr % kg/kg.d kg/kg.d kg/kg.d kg/kg.d

SX-1 < 20 25 12 6 6 3

SX-2 < 20 25 6 3 3 1.4

SX-3 < 20 50 3 1.5 1.5 0.75(1) Adjust loadings if either the 8-hour load peaking factor is > 1.4 or the sCOD/sBOD5 ratio

differs from the range of 1.9 to 2.1 in the case where BOD5 data are employed.

4.3 Anoxic Bioselectors (SXAXM and SXAXAL)

The anoxic bioselector design often employs mechanical mixers rather than air mixing to

maintain the MLSS in suspension. About 30-40% of the internal recycle (IR) is discharged to the

initial contact zone and the balance to the third stage of the bioselector. A significant portion of

the nitrates formed in the aerated or oxic zones will be removed in the bioselectors and the

balance reduced in the one or more anoxic stages following the bioselectors.

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The design of the anoxic bioselectors with mixers employs a three- or four-stage F/M

gradient – the same employed for the aerated, low DO mode of operation. Thus, the bioselector

volumes in each stage will be determined in the same manner as established in the example for

the aerated, low DO process. The concerns for the potential to overload the biomass, causing

viscous bulking, exist and the same criteria for the 8-hour peaking/average loading and

sCOD/sBOD5 ratio need to be considered for site-specific designs.

At this time, it is not possible to determine whether there are conditions that favor the

mechanically mixed anoxic bioselector over the aerated, low DO mode of operation. No

advantage of curtailing the air flow was found at Phoenix 91st Ave (Albertson and Stensel,

1994), which has a warm (20-33°C) and relatively strong primary effluent (320-450 mg/L COD,

200-220 mg/L sCOD) wastewater. It is possible that aeration in the bioselectors would be more

detrimental for cold and relatively weak wastewater and with low MLSS when there would be

reduced oxygen uptake and, hence, less stress. The level of biological stress reflected by the F/M

would also be exhibited by the oxygen uptake rate of the MLSS. Under conditions of reduced

oxygen demand (mg/L⋅hr), the effectiveness of bioselection could be diminished and the rate of

denitrification reduced. Comparative studies under these conditions have not been reported in the

literature.

Marten and Daigger (1997) studied five facilities with anoxic bioselectors: Beloit, WI;

North and South WWTP in Green Bay, WI; Landis, NJ; and Tri-City, OR. Their conclusion from

the study was that the F/ΣM should be in the range of 0.7 kg cBOD5/kg MLSS⋅d in cold

wastewaters and up to 1.2 kg/kg⋅d in warm wastewaters to produce improved SVI control. The

equivalent BOD5 and COD values to the cBOD5 values were not reported. Also, the ICZ

loadings were not provided and ICZ loadings are probably the most significant criteria.

However, there were still excursions to higher SVIs as indicated by the reported results in the

earlier Table 3.4.

The level of aeration in the Columbus and Phoenix facilities (see data in Chapter 5)

would result in only a small fraction of the sCOD removal by oxidation, probably 5-15%, in the

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bioselectors. Thus, aeration would seem to be an unlikely reason for the higher operable F/M in

these facilities. The range of SVI was significantly lower than reported for unaerated anoxic and

anaerobic zones of plants operating at a lower F/M in the initial contact zone. Additional full-

scale data analysis of the anoxic bioselector F/M operating range is needed.

Jenkins et al., (1993) suggested that oversized anoxic bioselectors will not produce

unacceptable results. However, South African and U.S. orbital systems with single-stage lower

F/M anoxic zones have had bulking (SVI >200 mL/g) problems. There is the distinct probability

that large differences in bioselector design can cloud the comparison of data from several plants.

The staging of the bioselectors can be critical and currently the overall professional opinion and

results support a three- to four-stage bioselection zone. Chudoba et al., (1973a, 1973b), the

British Water Pollution Laboratory (1969), Tomlinson (1976), Heide and Pasveer (1974),

Chambers and Tomlinson (1978a, 1978b), and Albertson (1987, 1992) premise that a high F/M

cascade is critical to achieve a low SVI biomass is valid. Designs by this author using these

references have produced low SVIs in many installations.

In the Jenkins et al., publication, the overall F/M in the bioselector is provided, not the

F/M in the initial contact zone. At Beloit, WI, the F/M is 2.1 to 3.6 kg/kg⋅d in the initial contact

zone and the biological train with the higher F/M produces the lowest range of SVI. The three

anoxic stages at Landis WWTP in Vineland, NJ, have only an F/M of 0.45 to 0.9 kg/kg⋅d and

may not exert sufficient selective pressure on the system. The single-stage bioselectors at Green

Bay, WI, do not have a cascading F/M, which may be the cause of periodic higher SVI

excursions.

At this time, the information on the F/M design for mechanically mixed anoxic

bioselectors is conflicting. Jenkins et al., (1993) recommended a COD F/M profile for anoxic

bioselectors. No design criteria for a sCOD nor BOD5 F/M gradient were provided.

Zone SX-1 6 kg COD/kg MLSS⋅d

Zone SX-2 3 kg COD/kg MLSS⋅d

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Zone SX-3 1.5 kg COD/kg MLSS⋅d

During the plant startup or lower BOD5 (COD) loading periods, the F/M in the first

selector zone (SX-1) may be too low to produce the level of biological stress that Wanner (1994)

considered necessary to promote (select) the growth of rapid settling floc formers. A heavily

aerated SX-1 has proven operable at BOD5 F/M loadings up to 14 kg/kg⋅d in the initial contact

zone (Jenkins et al., 1993; Kross, H., 1985, Grau and Wanner, 1988). Loading recommendations

for both aerated, low DO and anaerobic bioselectors have been 5.5 to 6.0 kg BOD5/ kg⋅d in the

initial contact zone. The addition of oxygen, DO in the influent, DO in the recycle, DO by the

aeration, or the recycle of nitrates to the initial contact zone may reduce the biological stress

necessary to select the preferred biomass in the biological reactor. However, this aspect has not

been researched and both lowly and highly aerated bioselectors have been effective.

By introducing oxygen into the initial contact zone, the loss of selectivity is apparent

from the reduction in the level of phosphorus removal in aerated (high or low) and anoxic

bioselectors. At the same time, the bioselectors with oxygen added in any form have worked well

with substantially higher F/M ratios than the criteria set forth by Jenkins et al., (1993) and noted

earlier. The recommended anoxic bioselector loadings of this document are presented in Table 4-

8. The preferred and most reliable criteria will be defined by sBOD5 and/or sCOD data.

Table 4.8 Design Recommendations for Mechanically Mixed Anoxic (SXAXM)Bioselectors(1)

Relative sCOD sBOD5

Zone Volumes Loading Loading COD BOD% kg/Σkg mass⋅d kg/Σkg mass⋅d kg/kg⋅d kg/kg⋅d

SX-1 25 5.0 - 6.0 2.5 - 3.0 10 -12 5 - 6

SX-2 25 2.5 - 3.0 1.3 - 1.5 5 - 6 2.5 - 3

SX-3 50 1.3 - 1.5 0.63 - 0.75 1.5 - 3 0.75 - 1.5

(1) Adjust loadings if either the 8-hour load peaking factor is > 1.4 or the sCOD/sBOD5 ratiodiffers from the range of 1.9 to 2.1 in the case where BOD5 data are employed.

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Mixing in the anoxic bioselectors can be provided by either vertical-mounted turbine

mixers or slide rail-mounted axial flow mixer pumps. The general power requirement guidelines

for the two types of mixing systems are:

Turbine Mixers: 8 to 13 w/m3 (0.3 to 0.5 hp/1000 ft3)

Axial Flow Mixers: 8 to 13 w/m3 (0.3 to 0.5 hp/1000 ft3)

The mixing power requirements can be reduced by using lower shaft speeds (larger

impellers) at a higher capital cost. The installed cost of the slide rail-axial flow mixer is less than

the bridge-mounted, vertically submerged turbine mixer. As noted in the section discussing

aerated, low DO bioselectors, coarse bubble aeration has been employed successfully in many

nitrification denitrification (NdeN) systems, but could interfere with Bio-P removal.

4.4 Anaerobic Bioselectors (SXANM)

In order to optimize biological removal of phosphorus, the introduction of free and

combined sources of oxygen into the bioselector zone must be minimized. This maximizes the

availability of sBOD5 (sCOD) for the Bio-P reactions discussed in Chapter 1 on process theory.

Thus, it is desirable to have minimal DO in the influent to the initial contact zone and low

nitrates in the return activated sludge (RAS). There is no internal recycle of nitrates to the

bioselector zones whenever Bio-P is to be optimized.

There are not sufficient data to define whether the anaerobic bioselector should have the

same F/M gradient as employed for the aerated, low DO and aerated anoxic bioselectors.

Bulking sludges have been reported as a problem in the A2O process and South African WWTPs

that have low F/M single-stage anaerobic ICZs. Since the single-stage processes typically had 2-

to 4-hour retention time in the first anoxic zones, the F/M loadings were low – less than 1 to 1.5

kg BOD5/kg⋅d – and provided inadequate biological stress for bioselection.

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Further, it is not known whether the best process design for bioselection and bulking

control will permit optimization of phosphorus removal. The emphasis of this document is the

control of bulking sludges; thus F/M gradient recommendations set forth for aerated, low and

high DO bioselectors would not necessarily be valid for Bio-P removal. If the Bio-P removal

requirements result in a longer period than necessary for bioselection, then the volume of the

anaerobic zone could be expanded by adding a fourth zone, where the required volume and the

recycle of nitrate-containing mixed liquor from the oxic zone would be discharged downstream

of the anaerobic zone.

Wanner et al., (1987) concluded that SVI control would be achieved if nearly all of the

soluble organics were removed in the initial contact zones. They also concluded that a cascading

F/M gradient was a more effective design but did not provide numerical criteria. Results of

Rensink and Donker et al., (1985) suggested that in completely mixed anaerobic, anoxic and

oxic zones, filamentous organisms were suppressed only if there was P release (polyphosphate

depolymerization) in the anaerobic zone. However, good bulking control was achieved at Tree

Top (1993), Cranston Print Works (1992), and other studies on industrial waste where excess

phosphorus was not available. An alternative pathway of glucose storage resulted in the removal

of sCOD and, hence, bulking control.

A two-stage anaerobic bioselector was piloted for the Deer Island project (Bowen et al.,

1992) and compared to a pure oxygen process without a bioselector. This control study

demonstrated the benefits of bioselection to control SVI. The initial or zone settling velocity

(ZSV5) increased more than 100% and the concentration of the return sludge solids (RSS) more

than doubled. The significant findings are set forth in Table 4.9.

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Table 4.9 Deer Island, Boston, MA, Pilot Study ResultsICZ F/M

Period Mode SRT ZSV5 RSS SVI F/M TotalDays m/hr mg/L mL/g kg/kg⋅d kg/kg⋅d kg/kg⋅d

1 SXAN-COAS 1.0 5.5 14,222 62 5.2 0.58

2 COAS 2.1 1.8 6,312 96 2.8 0.71

3 COAS 2.6 2.3 5,685 190 2.5 0.62

4a SXAN-COAS 3.3 3.4 13,124 64 3.1 0.34

4b SXAN-COAS 1.6 5.1 10,046 50 4.0 0.50

SXAN-COAS – Anaerobic bioselector; COAS – conventional oxygen activated sludge

Note: Two-stage anaerobic bioselector at 568 L and four-stage aerobic reactor at 1987 L totalvolumes, respectively. The ICZ is 284 L in Periods 1, 4a and 4b and 497L in four-stage aerobicreactor.

The anaerobic bioselector in the pure oxygen mode was effective in controlling the

growth of filamentous or bulking sludge as evidenced by the higher zone settling velocity

(ZSV5), lower SVI and higher return sludge solids concentration (RSS). The first stage of a pure

oxygen reactor has been considered an aerobic selector (Tracy, 1975) but was not as effective.

The significant difference could be the lower F/M in the initial contact zone of the COAS

reactor.

The net sludge solids for the bioselector arrangement (activated sludge) was 14% higher

than for that of the conventional activated sludge system (without bioselector zones). An

increase of net yield has been noted in other studies with bioselectors.

The increase in the net biomass yield would, in part, be due to accumulation of excess

phosphorus (Bio-P) in the bioselector arrangement. While the anaerobic mode provides the

highest level of removals, Bio-P removals in anoxic and low DO bioselectors are typically 65-

80%. For each additional kg of TP removed, about 4 to 4.5 kg of waste sludge will be produced

and will increase total waste sludge proportionally.

Since the bioselector process involves conversion–storage–oxidation of soluble organics,

it is possible that a portion of the stored product is not oxidized and thus it increased the net

4-25

sludge yield. Net yields, when hydrous sludge is being produced, increase due to the difficulty of

oxidizing this fraction of exo-cellular protoplasm (ECP) storage. Reported poor SVI control with

bioselection could be in part due to excess ECP content.

Further Deer Island, WWTP studies (Phase 3) of anaerobic vs. aerobic bioselectors were

undertaken in 1994-95. The test operation was designed to simulate design capacity operation in

the pilot-scale reactors and clarifiers. The four-stage aerobic and anaerobic bioselectors have a

volume of 17.85 m3/stage (630 ft3/stage) and the four-basin pure oxygen activated sludge

bioselectors have a volume of 58.02 m3/basin (2048 ft3/basin). Total system volume is 303.46

m3/train (10,712 ft3/train). The results of these tests are summarized in Table 4.10 (MWRA,

1995).

In these studies, the SVI was controlled by the anaerobic and aerobic bioselector at the

lowest F/M loadings. The F/M loadings in the initial contact zone of the four-stage bioselector

were very high, 50 to 100% higher than recommended in this document. The SVIs were

generally good with the anaerobic bioselector producing lower SVIs. It appeared that the

anaerobic bioselector did result in a higher net yield although the data were not consistent. At the

higher loadings and flow, the aerobic bioselector effluent quality was better than that from the

anaerobic bioselector system.

The anaerobic bioselector did not release a significant amount of phosphorus as would be

expected. The evaluation attributed this fact to the low SRT and colder temperatures. This

supports the position that the bioselector(s) was not operating in a true bioselection mode, and

with the low SRT, the sludge may not have been adequately regenerated.

The full-scale bioselectors at the Deer Island WWTP have operated better in an aerobic,

high DO mode (Tyler, 2002) than in a mixed anaerobic mode. In recent years, the aerated

(SXAH) mode has operated at about 10 mg/L DO from pure oxygen aeration at a MLSS of 1000

to 1500 mg/L (Tyler, 2002). The SRT is 1.2 to 1.4 days and the SVI is 120 to 160 mL/g. Tyler

reported that converting to a two-stage bioselector by removing walls in the model-scale unit

4-26

appeared to improve operation. The F/M would be 50% of the full-scale design and of the data

reported in Table 4.10.

Table 4.10 Phase 3 Secondary Treatment Performance at Deer Island WWTP

Aeration(1) F/M Loading(2) Net Yield Sec. EffluentPeriod Flow MLSS SRT SVI BOD5 COD BOD5R BOD5 TSSTrain m3⋅d mg/L days mL/g kg/kg⋅d kg/kg⋅d kg/kg mg/L mg/L

12/24/93-12/23/94

Train 1 (AH) 2273 1244 2.3 152 7.1/0.42 21.3/1.25 0.92 5 9

Train 2 (AN) 2064 1643 2.2 152 6.0/0.35 18.5/1.09 1.31 5 8

12/24/94-1/3/95

Train 1 (AH) 2651 1075 2.1 92 10.5/0.62 23.8/1.40 0.76 8 10

Train 2 (AN) 2693 1045 2.2 95 10.5/0.35 18.5/1.09 0.75 5 8

1/9-25/95

Train 1 (AH) 3788 1766 2.0 112 10.9/0.64 25.5/1.50 0.83 9 9

Train 2 (AN) 3841 1944 2.0 90 9.7/0.57 23.8/1.40 1.10 17 23

1/29-2/14/95

Train 1 (AH) 4546 2143 1.7 121 10.9/0.64 25.5/1.50 0.83 9 9

Train 2 (AN) 4523 2177 1.2 80 11.4/0.67 34.0/2.00 1.75 22 43

2/15-3/3/95(3)

Train 1 (AH) 4546 2272 1.5 78 12.2/0.72 32.3/1.90 1.27 9 8

Train 2 (AN) 4496 2208 1.9 61 12.4/0.73 32.3/1.90 1.30 14 16

3/4 -13/94

Train 1 (AH) 4546 2127 2,8 71 11.9/0.70 28.9/1.70 1.48 29 40

Train 2 (AN) 4364 1590 1.7 63 12.6/0.74 30.6/1.80 1.49 35 53

3/14-31/95(3)

Train 1 (AH) 4167 2655 2.6 114 9.9/0.58 25.5/1.50 0.96 11 15

Train 2 (AN) 4201 2685 1.9 72 8.7/0.51 25.5/1.50 1.22 17 28

All Data Averages

Train 1 (AH) 3689 1957 2.1 117 10.7/0.63 27.2/1.60 1.05 12 15

Train 2 (AN) 3580 1942 1.9 94 8.5/0.50 25.5/1.50 1.26 16 22

4-27

(1) Aerobic SRT included solids in the bioselector volume (Train 1) and only the aeration volume

solids in the anaerobic (Train 2) bioselector system.(2) Bioselector ICZ F/M /Overall system F/M.(3) Polymers employed at the same dosage in both systems for clarification.

The pilot-scale tests were conducted at the lower range of the recommended F/M in the

initial contact zone and were successful in limiting the SVI to <100 mL/g when the F/M in the

initial contact zone was >2.8 kg/kg⋅d. Since the tests were using cBOD5 at a cBOD5/COD ratio

of 0.4, the equivalent BOD5 F/M would have been >3.5 kg BOD5/kg MLSS⋅d. The model-scale

tests were conducted at cBOD5 loadings of 6 to 12.6 kg/kg⋅d and COD loadings of 18.5 to 30.6

kg/kg⋅d in the initial contact zone. These higher F/M loadings in the initial contact zone at

Phoenix 23rd Ave Water Reclamation Facility (WRF) caused hydrous bulking and the baffle was

moved to decrease the loading. The hydrous bulking resulted in poorer clarification although the

DSVI was only marginally affected, increasing from 60-90 mL/g to 110-120 mL/g.

The Fayetteville, AR, WWTP (Jenkins et al.,1993) employs a six-stage anaerobic

bioselector prior to a four-stage oxic zone. The system is arranged so that it can operate in an

AN/OX (A/O) or an AN/AX/OX (A2O) mode. The SVI prior to the bioselector installation could

only be maintained below 150 mL/g with nearly continuous use of chlorine to the RAS. Since

the bioselector was installed, the SVI has averaged 86 mL/g and has not exceeded 185 mL/g.

The highest SVIs occur during wet weather with dilute sewage and influent DOs of 8-10 mg/L.

In this mode, it would be difficult to achieve a truly anaerobic state in the bioselector. The

operating characteristics of the Fayetteville WWTP are:

Bioselector Activated Sludge SystemICZ Total SX Total Volume

F/M – kg/kg⋅d 1.6 0.27 0.045

Retention – hrs 0.25 1.5 9.0

Jenkins et al., (1993) do not provide specific design criteria for anaerobic selectors, but

noted that the required liquid retention time is 0.75 to 2.0 hours – much longer than the typical

15-30 minutes employed for aerated and anoxic bioselectors. They do advise that the anaerobic

4-28

zone may be divided into zones with the same gradient as they set forth for anoxic bioselectors.

Because the retention time for the anaerobic bioselectors is longer than required for bioselection,

there would then be three initial stages of 6, 3 and 1.5 kg COD/kg MLSS⋅d zones followed by a

fourth anaerobic zone to produce the total required retention time. The anaerobic zone volume is

often defined by criteria for Bio-P removal, which requires a longer retention time as opposed to

bulking sludge control. However, lower SVIs are associated with higher phosphorus content in

the MLSS (Bio-P) as shown in Figure 4.3.

Wanner (1994) noted that the aerated ICZ F/M of 1.0 kg BOD5/kg MLSS⋅d for a brewery

wastewater was not high enough to produce selective stress to control filamentous growth.

However, the removal of the air supply controlled SVI to the 100-120 mL/g range; this

modification converted the bioselector from aerobic to an anaerobic mode. Based on experiences

with soluble wastewaters at other sites, a higher ICZ F/M would likely have further depressed

the SVI to the range of 40 to 70 mL/g.

Historically, the Newark, OH, wastewater treatment plant had experienced a problem of

foaming and bulking problems (Albertson, 1987; IAWQ 1992). A three-stage anaerobic

bioselector was installed and operated with a F/ΣM of 1.59, 0.79 and 0.59 kg BOD5/kg MLSS⋅d,

respectively. The SVI decreased from an operating range of 100 to 400 mL/g to 80 to 200 mL/g

and averaged 124 mL/g. While this was a significant improvement, the higher SVIs would still

limit plant clarification capacity. Based on data presented in this document and Wanner’s (1994)

experience, the F/M in the initial contact zone was too low to produce the stress necessary for

full bioselection – that is, eliminate most filamentous bacteria.

The reported F/M in the anaerobic ICZ designs are much lower than employed for

aerated (SXAH and SXAL) and anoxic (SXAX) bioselectors. The SVIs of the SXAH, SXAL and

SXAX aerated or mechanically mixed systems at higher ICZ F/M ratios appear to be more stable

and averaged a lower SVI. There are only minor excursions in SVIs for these systems with F/Ms

≥ 3-6 kg BOD5/kg MLSS⋅d in the initial contact zone. Thus, it is possible that in the cited

system,

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4-30

ICZ F/Ms were too low to provide the minimum selective stress to fully control the filamentous

growth. This appears to be the case for some anoxic (SXAX) systems. At the Fibra WWTP (Okey,

1997), the anaerobic (SXANM) bioselector produced very low SVIs treating a highly soluble

wastewater, but the operating ICZ loading was not available (see Chapter 5).

From a technical standpoint, all bioselectors are operating on the same basis – providing

an environment where soluble substrates can be removed and stored in floc formers prior to

aerobic zones, where filamentous organisms would otherwise use the same soluble substrate to

proliferate. Therefore, there should be similar F/M gradients for anaerobic (SXANM)

bioselectors. Based on this analogy, the criteria set forth in Table 4.11 are proposed for SXANM

bioselectors:

Table 4.11 Design Recommendation for Three-Stage Anaerobic (SXANM) Bioselectors

AN Zone Fs/ΣM Zone (sCOD) Fs/ΣM (sBOD5)kg/kg⋅d kg/kg⋅d

SX-1 6 ≥3

SX-2 3 1.5

SX-3 1.5 0.75

Contact Loading <100 mg sCOD/s MLSS

If additional anaerobic volume is required for Bio-P release and subsequent removal, it

should be added as a fourth zone to maintain the F/M cascade necessary for bioselection.

Due to the minimal and conflicting amount of available data, it is recommended that the

design engineer keep current with operating experiences of anaerobic bioselectors and adjust the

loadings as required to achieve best results.

Mixing in the anaerobic bioselectors can be provided by either vertical-mounted turbine

mixers or slide rail-mounted axial flow mixer pumps. The general power requirement guidelines

for the two types of mixing systems are:

4-31

Turbine Mixers: 8 to 13 w/m3 (0.3 to 0.5 hp/1000 ft3)

Axial Flow Mixers: 8 to 13 w/m3 (0.3 to 0.5 hp/1000 ft3)

5-1

Chapter 5.0

Process Experiences with Bioselectors

The following data presentations provide background information on the use of bio-

selectors in U.S. facilities. The mode of operation in the initial contact zone (ICZ) of the

bioselector is noted.

5.1 Davenport, IA – Aerated, Low DO and Anaerobic (SXAL and SXANM)

The Davenport wastewater treatment plant (WWTP) plant has two aeration trains of four

complete mix activated sludge (CMAS) zones in each train. The zones can be arranged to

operate as three and up to eight zones in a series (staged aeration). Prior to the time of modifying

the operation to bioselector-staged aeration (1987), the SVI ranged from 100 to over 400 mL/g

without any basis to define causative factor(s). A submerged turbine aerated each zone and the

air flow was adjusted to maintain the desired DO in each zone. Normal practice during sludge

bulking occurrences was a target DO of 1 to 2 mg/L in the ICZ.

The plant was designed to process an average flow of 1140 L/s (26 mgd), a peak flow of

1754 L/s (40 mgd) and process 3420 L/s (78 mgd) through primary clarification. In the initial

nine years of operation, the presence of bulking conditions limited the plant’s secondary

treatment capacity to 887-1096 L/s (20-25 mgd) due to rising sludge blankets in the secondary

clarifiers.

In 1987, the plant modified the operation (Davenport 1987) of the first zone (of six zones

in operation) to be in a low DO mode by restricting the air flow to the submerged turbine. The

impact of the 0.0-0.3 mg/L DO in the first zone on the SVI is displayed in Figure 5.1; the cartoon

in the figure reflects the plant operating staff’s reaction to the bioselector’s reduction of the SVI.

The reduction in the SVI allow for secondary treatment of flows up to 1750 L/s (40 mgd)

without incurring excessive depths of sludge blankets in the secondary clarifiers.

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In subsequent periods of time, the plant staff returned to the normal practice of 1-2 mg/L

DO in the first stage and the SVI increased beyond acceptable levels in 1987 and 1988. In 1989,

a process control procedure was instituted where the air rate was increased (higher DO) when the

SVI was < 70 mL/g and decreased (lower DO) when the SVI > 90 mL/g. This action produced a

stable operating SVI as shown in Figure 5.2.

By 2001, the BOD5 loadings to the Davenport WWTP had increased significantly and the

effectiveness of the single-stage bioselector had diminished. In 1987, the first stage F/M was 1.2

to 3.0 kg/kg⋅d, averaging about 1.8 kg BOD5/kg MLSS⋅d. The loading increases were offset by

operating eight zones, but SVIs had reached as high as 150 to 200 mL/g while the annual

average SVI was <100 mL/g. The waste characterization had changed due to increased soluble

organic wastewaters and in 2002 the bioselectors operated in an anaerobic mode (SXM). A three-

stage bioselector (SXAH) with 3.0 kg sBOD5 kg MLSS⋅d in the initial contact zone (ICZ or SX-1)

was under design in 2002 with plans to modify the staged aeration to contact-stabilization to

increase the BOD5 capacity.

Davenport is an example of the range of responses to bioselectors. In this case, a single-

stage zone with a low F/M was operable for a number of years. Whether the addition of a new

industrial waste input was the cause of the increased SVI cannot be determined at this time, since

similarly designed facilities may or may not have bulking problems without bioselectors.

5.2 Columbus Southerly, OH – Aerated, Low DO (SXAL)

The Southerly WWTP had a historical problem of uncontrolled bulking of activated

sludge prior to mid-1988 when the plant was expanded to 4990 L/s (114 mgd) and the aeration

basins were modified to include a three-stage bioselector. Prior to modification to a 10-stage

reactor, two-pass basins, which were 274.4 ml x 7.92 mw x 4.45 mwd (900 ft x 26 ft x 14.6 ft),

had an SVI varying from 150 to 500 mL/g in a plug-flow mode. In the step-feed mode of

operation, bulking problems increased. Due to the strong, highly soluble influent, the initial

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aeration stages had a low DO. The wastewater was a mixed domestic and industrial wastewater

with about 40% of the influent loading from a large brewery.

The three-stage bioselectors (Figure 5.3) were designed (Albertson et al., 1992) with an

F/ΣM gradient of 5.6, 2.8 and 1.4 kg BOD5/kg MLSS⋅d. The COD gradient was about 13, 6.5

and 3.3 kg/kg⋅d. The bioselectors and first anoxic zone were equipped with jet aerators, which

could either provide mixing, aeration-mixing or low DO (AL) aeration only. The aerated

bioselectors were designed to operate at 0.0-0.3 mg/L DO. An internal recycle of 1.25 Q to the

anoxic selectors was provided but not always employed. This produced an aerated (or unaerated)

anoxic environment with denitrification in the bioselectors and in the first anoxic/oxic zone

(Ax/Ox-1). A total of six 61 m∅ x 4.57 m (200 ft∅ x 15 ft) SWD mechanical flocculating

secondary clarifiers with central sludge withdrawal using spiral scrapers replaced the existing

rectangular basins.

Figure 5.4 displays the average monthly SVI profile for the first 14 months of operation

of the upgraded wastewater treatment plant. The low and stable SVI and daily SVI range reflect

the effectiveness of the anoxic bioselectors. The bioselectors operated with aeration (jets off)

only to minimize power usage. Maximum and minimum daily SVIs were generally ≤ +15 mL/g

of the monthly average value. The average MLSS was 3250 mg/L with an average SVI of 78

mL/g. The settled secondary effluent quality averaged 2 mg/L cBOD5, 6 mg/L TSS, 1.5 mg/L

TKN, 0.13 mg/L NH4-N and 1.1 mg/L TP. The plant treated monthly flows up to 5739 L/sec

(131 MGD) for the first 2.5 years of operation as shown in Table 5.1.

For flexibility in operating a SXAH, SXAL, SXANM and anoxic modes (AX), the

bioselectors were equipped with jet aerators. The plant staff has operated the system with jets

only (anaerobic), air only (low DO) and air plus jets (low DO) without internal recycle. The

treatment performance, bulking sludge control and effluent quality were unchanged. There was

5-10 mg/L of NO3-N in the return sludge at 0.4 RAS/Q or 1.4 to 2.8 mg/L NO3-N in the total

flow to the initial contact zone (1a, Figure 5.3). However, no NO3-N was present in the initial

contact zone if the internal recycle pumps were not operating. Still, the system has operated

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efficiently at MLSS of 4000-5000 mg/L due to the low SVIs. Based on the criteria set forth in

the BNR patents, the initial contact zone would still be considered anoxic (>0.5 mg/L NO3-N in

the total influent flow) with the 5-10 mg/L NO3-N in the RAS flow.

The Southerly plant has continued to produce low SVIs and similar effluent quality with

increasing loads. The aeration basins and final clarifiers have since been expanded to process

additional flow and loadings. The flexibility offered by the more costly and power intensive jet

aeration in the bioselector zones is no longer considered necessary.

5.3 Columbus Jackson Pike, OH – Aerated, Low DO (SXAL)

The Jackson Pike WWTP historically processed about 4380 L/s (100 mgd) in secondary

treatment with periodic bulking incidents that limited treatment capacity. The construction of the

12 aeration basins was similar to that of the Southerly WWTP – each 274.4 ml x 7.92 mw x 4.45

md (900 ftl x 26 ftw x 14.6 ftd). The 12 basins (Figure 5.5) were being operated in a plug-flow

mode on a mixed domestic and industrial wastewater.

The sBOD5 content of the wastewater was low at about 25% of the influent BOD5. Based

on a requirement to minimize investment, a two-stage bioselector with a 5.5/1.4 kg/kg⋅d F/ΣM

gradient was employed. Mixing/aeration of the bioselectors and the oxic zones was modified

from coarse bubble to tubular membrane diffusers. No provision for an internal recycle for

denitrification was provided for this plant, which is slated for retirement in the future.

The treatment facility was renovated to nitrify a flow of 2630 L/s (60 mgd) to an effluent

NH4-N of less than 1.0 mg/L. The SVI (Figure 5.6) in the first 11 months of operation (1988-

1989) ranged from 52 to 100 mL/g, averaging about 75 mL/g. The average effluent quality over

this period was 3.1 mg/L cBOD5, 7.3 mg/L TSS, 1.5 mg/L TKN and 0.09 mg/L NH4-N, while

treating slightly more than design capacity. The treatment capacity was later further increased

25% above the original design flow without loss of treatment efficiency. The summary data for

the first 2.5 years of operation are shown in Table 5.2.

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The two-stage bioselector did not produce as stable an SVI as did the Southerly three-stage unit.

However, in itself, this is not a reason to establish that the three-stage design would have

produced a more stable SVI. The Jackson Pike wastewater had a low soluble BOD5 content,

while the brewery wastewater resulted in Southerly’s wastewater having a higher than normal

sBOD5 content. As noted earlier, the bioselection process is most effective on soluble

wastewaters. Further, as Wanner and Grau (1988) suggested, there could be secondary bulking

organism growth in the oxic zones due to the high BOD5 particulate content at Jackson Pike.

With a maximum monthly SVI of 108 mL/g, the bioselector was effective regardless of the

daily-monthly fluctuations.

5.4 Santa Fe, NM – Aerated, Low DO (SXAL)

The 263 L/s (6 mgd) Santa Fe WWTP consists of two 1932 m3 (0.51 mg) Barrier

Oxidation ditches. The available internal recycle flow rate was up to 300% and primary clarifiers

were not employed. The four rectangular final clarifiers were 3.72 m (12.2 ft) deep with a total

area of 2007 m2 (21,595 ft2) and employed traveling sludge siphons to remove the settled sludge.

Due to mechanical failure of the ditch aeration system, it was necessary to replace the aeration

system. This necessitated removing 50% of existing treatment capacity for several months. The

remaining oxidation ditch could not meet more than 40% of the total oxygen demand.

While the interim construction permit relaxed the TN from 10 mg/L to 25 mg/L, these

levels still required nitrification and 40-45% TN removal until the first ditch was modified and

put into operation. The TN limit was then set at 17 mg/L until the second ditch was also

modified to diffused aeration.

Because the plant had a history of bulking sludge and foaming problems, a two-stage

aerated bioselector was installed in each of the existing lead anoxic basins (Figure 5.7). All

bioselector and anoxic basins were retrofitted with a high density of 9"∅ membrane diffusers

designed to operate at 0.0-0.3 mg/L DO in the bioselector and first anoxic stage. The single

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oxidation ditch was operated to generate the minimum TN value and maximum oxygen transfer

capacity, but the ammonia oxidation capacity of the single oxidation ditch was limited.

The aerated zones were operated at 0-0.5 mg/L in the bioselectors (SXAX-1, SXAX-2, and

SXAX-3) and generally at 4-6 mg/L DO in the second zone (AX-4) with an internal recycle of

250% Q. The influent flow averaged 265 L/s (6 mgd) with a temperature range of 16-19°C in the

5.5 months of operation in 1993. The summary of the operating results is provided in Table 5.3.

The aeration capacity of the oxidation ditch was further limited by the loss of an

additional aeration pump during the conversion. While the loss further inhibited the ability to

nitrify fully, it decreased NO3-N to less than 0.5 mg/L during the last 30 days of operation.

However, the modified aeration anoxic facility was able to provide treatment to meet final

criteria with 50% of the oxic volume off-line and the balance of the oxygenation capacity

impaired. The SVIs averaged 103 mL/g and 10-day averages ranged from 79-132 mL/g. The

1993 COD, TSS and TKN loadings in the raw wastewater were 132%, 123% and 117%,

respectively, of the 1991 data used for the design of the failed aeration system. The bioselectors

and aerated anoxic basins with one aeration basin on-line met the effluent requirements at 110 to

115% of original design loadings.

5.5 Gig Harbor, WA – Aerated, Low DO (SXAL)

The secondary treatment was expanded from 26.3 to 70.2 L/s (0.6 to 1.6 MGD) in 1997.

The activated sludge process was modified from complete mix (CMAS) to contact-stabilization

(C-S) with bioselectors. The return sludge after the stabilization period was mixed with the

influent wastewater and processed through a three-stage bioselector prior to the contact zone

(Figure 5.8). In the ultimate expansion of the plant to 153 L/s (3.5 mgd), the influent flow will be

reversed and primary treatment will be provided by the two secondary clarifiers. Two new 19.8

m∅ (65 ft∅) secondary clarifiers will be constructed. With primary treatment, the C-S mode of

operation will be able to process the full flow of 153 L/s.

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At a flow of about 60-80% of design capacity, the original facility averaged 5 mg/L

cBOD5 and 13 mg/L TSS from mid 1988 through 1991. Maximum monthly effluent TSS was

approaching the effluent limits of 30 mg/L. A variable and unpredictable SVI limited the

aeration MLSS and the clarification capacity and was also the cause of higher effluent TSS.

The operation and performance of the expanded wastewater treatment plant is presented

in Table 5.4. The plant was loaded at 40-50% of its hydraulic design capacity and the first-stage

bioselector zone (Sx-1) had an average F/M of 3.4 lb cBOD5/lb MLSS⋅d. The average SVI was

71 mL/g and the monthly averages ranged from 55 to 81 mL/g.

The design criteria for the low DO (SXAL) bioselectors incorporated a gradient of 14.4,

7.2 and 3.6 kg COD/kg MLSS⋅d (6.8, 3.4, 1.7 kg BOD5/kg⋅d). The basis of design was 5.0 kg

sCOD/kg⋅d (sBOD5 = 2.4 kg/kg⋅d) in the initial contact zone.

5.6 Phoenix 23rd Ave, AZ – Anoxic, Aerated, Low DO (SXAXAL)

The Phoenix 23rd Ave Water Reclamation Facility (WRF) was designed for 1622 L/s (37

mgd) of secondary treatment capacity. Historically, the plant had operated at 658 to 1096 L/s (15

to 25 mgd) capacity due to a chronic sludge bulking problem with SVIs of 150 to > 500 mL/g.

Due to clarifier limitations from the bulking sludges, the MLSS concentration was maintained in

the range of 500 to 1000 mg/L. Flow capacity not processed at the 23rd Ave WRF was

transferred to the 6750 L/s (154 mgd) plant at 91st Ave. However, the operating philosophy was

to maximize the flow treated at the 23rd Ave plant.

The 23rd Ave plant had two-four pass aeration basins, each 378 ml x 7.62 mw x 4.63 md

(1240 ft x 25 ft x 15.2 ft). The basins were aerated with ceramic fine bubble diffusers and the

basins were operated in a plug-flow mode. It was difficult to maintain DO in the first pass as the

ceramic diffusers easily fouled and the fouling alpha (αF) value of the mixed liquor was about

0.27.

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During a severe bulking (SVI > 800 mL/g) period, steps were taken to rapidly implement

bioselection. Initially, canvas baffles were placed in the front portion of the basin to effect a

three-stage bioselector. The bioselection volumes were based on a F/ΣM of 6, 3 and 1.5 kg

BOD5/kg MLSS⋅d. While successful in reducing the SVI, the initial baffles deteriorated due to

bacterial decay and were replaced with submerged wooden baffles.

There was also a need to provide advanced treatment with an effluent of ≤ 10 mg/L total

nitrogen, low BOD5 and TSS. The arrangement of the bioselectors and the internal recycle for

denitrification is shown in Figure 5.9. The fine bubble diffusers were replaced by coarse bubble

diffusers in the three-stage bioselectors (SXAX-1, SXAX-2 and SXAX-3) and membrane diffusers

were installed in the first anoxic/oxic zone (AX/OX-1). The total internal recycle flow was

discharged in the initial contact zone and the bioselectors were operated in an aerated, low DO

anoxic mode.

The results of the first nine months of the plant operation (Albertson & Hendricks,1992)

after the startup of the bioselectors on February 15, 1990, and the preceding 13 months are

displayed in Figure 5.10. There was an increase in SVI30 for the first two weeks, which was

mostly an artifact of the SVI30 test (Rachwal, 1985) as the MLSS was increased from 550 to

1100 mg/L. That is, the increased SVI30 was due to higher MLSS, but the DSVI30 was

decreasing.

The DSVI decreased to ≤ 100 mL/g after 14 weeks, the MLSS was increased to 2200

mg/L and the flow to 1400 L/s (31.9 mgd). After permanent modifications were completed, the

MLSS was further increased to 3000-3300 mg/L and the flow rate to 1500-1750 L/s (34-40

mgd). The 20-month review (Figure 5.11) of DSVI data reveals an average monthly range of 60

to 120 mL/g with relatively low daily range. Subsequently, it was found that the combined effect

of a 25% increase in BOD5 loading and a misplaced baffle in the initial contact zone caused floc

overloading and some hydrous bulking. The baffle was moved downstream to reduce ICZ F/M to

about 6 kg BOD5/kg.d.

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The fully modified plant operating at an average rate of 1450 L/s (33 mgd) produced a

monthly average secondary effluent of 7.6 mg/L BOD5, 8.2 mg/L TSS, 1.3 mg/L NH4-N, 4.1

mg/L NO3-N and 2.9 mg/L TP. The plant clarification capacity was limited at this flow due to

the shallow sidewater depth (2.74 m, 9 ft) of the final clarifiers.

Subsequently, the 23rd Avenue WRF has been expanded to 2758 L/s (63 mgd) using four

aeration basins and the aerated anoxic (SXAXAL) bioselectors. The SVI ranges from 60-90 mL/g

and the effluent quality has further improved due to the newly expanded clarification capacity

with four 54.9 ∅ x 4.88 m SWD (180 ft ∅ x 16 ft) final clarifiers with spiral scrapers and central

sludge drawoff.

5.7 Phoenix 91st Ave, AZ – Anoxic, Aerated, Low DO (SXAXAL)

The 91st Ave WWTP provides service for seven cities in the Phoenix, AZ, area: Glendale,

Mesa, Phoenix, Scottsdale, Tempe, Tolleson, and Youngtown. The plant design capacity was

6.75 m3/s (154 mgd) when operating in a secondary treatment mode. The balance of the City of

Phoenix’s wastewater was processed by the 23rd Ave WRF, a 1.62 m3/s (37 mgd) secondary

treatment plant operating at 660–1100 L/s (20-25 mgd), which was converted to nitrification-

denitrification (NdeN) mode of operation in 1991 (Albertson and Hendricks, 1992) and was then

operating at 1400–1500 L/s (32-35 mgd) producing an effluent total nitrogen (TN) of ≤8 mg/L.

There are several other water reclamation plants in the service area that remove and treat

wastewater and discharge their waste activated sludge to the interceptors feeding the 91st Ave

WWTP.

The 91st Ave WWTP treatment plant consisted of three separate primary clarification

systems and then the flow was subdivided into five independent secondary treatment plants.

These plant facilities had been constructed separately over a 30-year period as flows increased in

the metropolitan area. The design capacity in secondary treatment (≤ 30 mg/L BOD5/30 mg/L

TSS) for each plant was Plant IA, 1320 L/s (30 mgd); Plant IB, 1320 L/s (30 mgd); Plant IIA,

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1050 L/s (24 mgd), Plant IIB, 1580 L/s (36 mgd); and Plant IIIA 1320 L/s (30 mgd). The

existing sludge handling facilities included in-situ primary sludge thickening, dissolved air

flotation of waste activated sludge (WAS), anaerobic digestion with sludge decanting, and solar

drying in lagoons.

The 91st Ave WWTP had a history of bulking sludge and the SVI generally ranged from

150 to over 400 mL/g. Chlorination of the return sludge and low MLSS (500-1,000 mg/L) had

been used to maintain clarification capacity. Wastewater was primarily of domestic origin

including the waste sludges from wastewater reclamation plants. New effluent requirements had

established 10 mg/L as a TN limit. Plant IIIA was chosen for a full-scale process demonstration

of nitrification-denitrification.

Plant IIIA, employed for the full-scale pilot demonstration of bioselectors in the NdeN

mode, had two 378 ml x 7.62 mw x 4.63 md (1250 ft x 25 ft x 15.2 ft) aeration basins with eight

57.9 ml x 12.2 mw x 3.90 md (190 ft x 40 ft x 12.8 ft) secondary clarifiers. The conversion to

NdeN at 3500 mg/L MLSS required extensive modifications to the aeration basins and final

clarifiers. The cost of both the bioselector and NdeN modifications was about $2,000,000 U.S. or

$16.20/m3 ($0.06/gal).

The design concept tested in Plant IIIA (Figure 5.12) was a high-rate (SRT of 4.8-7.5

days) aerated, low DO anoxic bioselector mode of operation. Due to the limited aeration volume,

the anoxic zones were also aerated at an actual oxygen transfer rate (AOTR) of 30 to 50 mg/L

with a DO of < 0.3 mg/L. During average to peak loading periods, the DO was zero. The full-

scale operation at 920 to 1540 L/s (20 to 35 mgd) with AOTRs of 40-50 mg/L⋅hr to the

bioselectors and anoxic zones was successful in limiting the DSVI to 57 to 81 mL/g at 3000-

3800 mg/L MLSS as shown in Figure 5.13.

The lowly aerated anoxic (SXAXAL) mode was successful in meeting the TN effluent of

10 mg/L except when 50% of the internal recycle capacity was lost due to pump failures.

Aeration of the anoxic zones at AOTRs of 40-60 mg/L⋅hr following bioselection did not impair

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the ability of the system to reduce nitrates. The oxygen uptake rate (OUR) values were 100-130

mg/L, thus the DO was still zero at these AOTR values. That is, the kinetic rates were about the

same with or without aeration. Average monthly effluent quality in the 12-month study period at

full capacity is provided in Table 5.5.

Based on the success of the full-scale study, the five plants in the 91st Ave complex were

converted to the low DO, aerated bioselectors and aerated anoxic zones for nitrification-

denitrification to achieve an effluent TN of 6-8 mg/L. A summary of the 91st Ave WWTP 2004

operating data was provided by J Coughenhour (2005) for the six plants within the complex.

Essentially all six plants had a monthly DSVI of 72 + 10 mL/g for the full year. The initial

success of bioselectors piloted in 1992 has continued.

5.8 Tri City, Clackamas County, OR – Anaerobic (SXAXM) and Anoxic

(SXAXAL and SXAXM)

The Tri-City WWTP is an advanced secondary treatment system designed to remove

nitrogen from a flow of 591 L/s (13.5 mgd) in the summer. While an effluent of 20 mg/L BOD5

and TSS was required in the summer, the process nitrified, and an internal recycle for

denitrification was incorporated to recover alkalinity needed due to the low alkalinity wastewater

(Figure 5.14). In the winter, the plant operated in an aerobic step-feed mode (Daigger and

Nicholson, 1990).

The anoxic selector was a single-stage zone with an F/M of 0.72 kg BOD5/kg MLSS⋅d

and retention time of 86 minutes. During the summer period, the anoxic bioselector was effective

in reducing the SVI to an average value of 79 mL/g for 1986, 1987 and 1988. When the system

was operated in a step-feed mode for the winter period, the SVI increased (Figure 5.15).

A test of employing aeration in the bioselector in the summer of 1987 resulted in a large

increase in the SVI. The F/M was about 15% of the recommended F/M in the initial contact zone

of an aerated, low DO (SX AL) bioselection. The anoxic bioselector had occasionally reduced the

SVI to 20 to 30 mL/g, which causes a turbid effluent. Similar to the experiences at Davenport,

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IA, addition of air to the initial contact zone with a relatively low F/M increased the SVI because

there was inadequate stress for full bioselection.

5.9 Upper Occoquan Sewage Authority, VA – Aerated, High DO (SXAH)

The UOSA Regional Water Reclamation Plant (Daigger and Nicholson, 1990) is a 1183

L/s (27 mgd) facility employing primary clarification, single sludge nitrification system and

secondary clarification. The secondary effluent was chemically treated prior to discharge to a

drinking water reservoir.

The existing biological facility was upgraded because the existing CMAS system

historically had a severe bulking problem with SVIs as high as 600 mL/g. Due to the high SVIs,

the mean cell residence time (MCRT) was reduced by lowering the MLSS to satisfy the limited

clarifier capacity. The dominant problem organism was the filament M. parvicella, especially in

colder weather. Following successful bench-scale tests, full-scale bioselectors were incorporated

into the expanded biological system.

The modified plant (Figure 5.16) produced a low, stable SVI averaging 74 mL/g. Four

years of SVI data, before, during construction and after construction showed significant

improvement in sludge settleability (Figure 5.17). The high DO bioselectors operated above 2

mg/L DO and removed 60% of the sBOD5 and 40% of the sCOD with a residence time of 11

minutes based on influent flow.

Evaluation of the aerated bioselector oxygen transfer characteristics and the oxygen

uptake rate (OUR) indicated that about 0.1 mg O2/mg sBOD5 removal occurred. Because the

synthesis value would normally be 0.5 to 0.6 mg O2/mg sBOD5 removed, most (70-80%) of the

sBOD5 removed was converted to storage products. These results are similar to those reported by

Kroiss (1985).

The F/ΣM in the total volume of the aerated, high DO bioselector (SXAH) was 4.9 kg

BOD5/kg MLSS⋅d and the F/M in the initial contact zone was 14.8 kg/kg⋅d. While this value of

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F/M was higher than recommended by Chudoba and Wanner (1987) as well as in this document

for an aerated, high DO bioselector (12 kg/kg⋅d in ICZ), the SVI results were very good and are

shown in Figure 5.17. However, the experiences with the aerated, low DO (SXAL) bioselector at

the 23rd Ave WRF in Phoenix, AZ, revealed that hydrous bulking occurred at an F/M of 9-10 kg

BOD5 /kg⋅d in the initial contact zone. Thus the recommended loadings for the heavily aerated

bioselector were reduced for inclusion into this document. The heavily aerated bioselectors have

continued to produce a good operating SVI (Sellman 2001). The DO in the initial contact zone is

maintained at 2.0-2.5 mg/L.

5.10 Hamilton, OH – Anoxic (SXAXAL and SXAXM)

The City of Hamilton’s activated sludge WWTP has a design capacity of 1095 L/s (25

mgd) and operated at about 80% of that capacity in 1993 (Jenkins et al). The plant treats a

mixture of domestic wastewater (about 38%) and the discharge from a fiber recover plant (paper

mill).

Modifications and expansion of the existing plant in 1987 included a new aerated MLSS

channel to the existing (T3) and two new diffused aeration (T2A and T2B) basins, which had a

hydraulic retention time of 8-12 minutes (Q + RAS). Initially, only basin T3 received MLSS

from the channel and the SVI was 80-150 mL/g. Basins T2A and T2B received primary effluent

and RAS separately in a step-feed mode and the SVI was usually over 200 mL/g. In 1982, the

system was repiped to contact primary effluent and RAS in a gently aerated channel for 7

minutes prior to entering the three basins. The SVI profile of the three basins in the facility from

1982 through 1989 is shown in Figure 5.18.

During 1985-1986, it was necessary to remove Basin T3 from service. As a result the

bioselector retention time was reduced to 5 minutes and the SVI became unstable. It was

presumed that the bioselector retention time was too short and overloaded to a point where

sBOD5 was passing through the bioselector and into the aeration basins. It was not established

whether the increased SVI was caused by filamentous organisms or hydrous bulking from excess

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exo-cellular protoplasm (ECP) storage. Once T3 was returned to service, the SVI decreased to

the earlier values.

The air compressor to the inlet channel failed in May 1985 and was not replaced. In this

period, T3 was also out-of-service. However, the SVI from 1986-1989 was generally low

without air but operating in an anoxic condition from the NO3-N recycle. The anoxic bioselector

mode performed as good or better than the aerated anoxic mode. No analyses of the biology

during the SVI excursions of 1984-85 (aerated) and 1988 are available. In both cases, only one

or two of the three trains had elevated SVIs.

5.11 Middletown, OH – Anaerobic (SXANM)

The existing Middletown WWTP had a history (Jenkins et al., 1993) of chronic bulking

problems. Extensive pilot studies were carried out using the plant’s fully aerated configuration in

an anoxic-oxic (AX-OX) flowsheet for NdeN. This mode produced a lower SVI with an aerated

bioselector design. Further pilot studies were then carried out using the four-stage, aerated, high

F/M bioselector (SXAH) concept advocated by Chudoba and Wanner (1988) and based upon

Kroiss (1985).

The four-stage, high to low F/M (12, 6, 4 and 3 kg BOD5/kg MLSS⋅d, respectively) with

a medium DO ( ≤ 1.0 mg/L) selector was able to reduce the SVI to less than 100 mL/g within

three weeks after startup and averaged 65 mL/g SVI at 3000-4000 mg/L MLSS. In the following

month, the SVI averaged 47 mL/g. However, there were SVI excursions to 160 to 190 mL/g,

which can disrupt plant operations. This bioselector operated at least part-time in the range of 0.3

to 2.0 mg/L DO, which can generate filamentous organisms. The microscopic analysis showed

that the filamentous species in the main plant were dominated by N. limicola II and type 1851

and, to a much lesser extent, types 0675 and 0041. The pilot plant had similar species, but lesser

quantities, and much larger and better settling flocs.

5-39

5.12 Star Valley Cheese Coop, Thayne, WY – Anaerobic (SXANM)

The cheese production facility employs a sequencing batch reactor (SBR) to process the

strong wastewater (sBOD5 of 600-2000 mg/L) in an anaerobic or anoxic-oxic (ANOX or AXOX )

mode, depending on whether NH4-N is available for nitrification. While over 90% of the dairy

wastewater treatment facilities have reported bulking problems, the Star Valley SVI is generally

40-80 mL/g and as low as 25 mL/g with operation at design loadings. The SVI increased to 300

mL/g when the plant was overloaded 200-400% for three to five days and the DO was zero to 1.0

mg/L at the end of the aeration cycle. An increase in the SVI was generally observed when

organic overloading prevented the DO from reaching 4-5 mg/L at the end of the oxic cycle. That

is, the sludge was not fully regenerated (Chudoba et al, 1982) and thus unable to remove a

sufficient level of sBOD5 in the unaerated period after feeding – prior to the oxic cycle. The

sBOD5 passing into the oxic period promoted filamentous as well as hydrous growth conditions.

Bioselection was promoted in SBRs when the units are batch fed (Heide and Pasveer,

1974). Batch feeding with mixing or low DO mode of operation produces the high to low F/M

condition necessary for bioselection.

The effluent BOD5 and TSS were 6-8 mg/L and 10-15 mg/L, respectively. Phosphorus

accumulation in the MLSS was noted and would be available for nutrient shortage periods.

Phosphorus and nitrogen content of MLVSS, not effluent concentrations, are the best measures

of nutrient sufficiency since there is Bio-P removal and NdeN occurs if NO3-N is present.

Nocardia sp. was a continuing problem but foaming had been reduced significantly by a change

in the plant’s cleaning detergent.

5.13 Tree Top, Selah, WA – Anaerobic (SXANM) and Aerated, High DO

(SXAH)

The apple and other fruit juice wastes produced by the plant have been treated in a 12-

day flow-through CMAS aerated lagoon for several years. The resulting MLSS of 500-1200

mg/L did not settle and the SVI was always greater than 1000 mL/g. Dominant filaments were

types 1701, 021N and 0041. Nutrient supplemented pilot studies were conducted using two

5-40

anaerobic-oxic (ANOX) batch-fed reactors with one and two hours anaerobic and one semi-

continuously fed AHOX reactor. All units were able to produce an SVI of 40-70 ml with the SX

ANOX unit producing the best clarified effluent. There was 90% removal of the sBOD5 in the

unaerated initial contact zone without phosphorus release during the anaerobic period.

Phosphorus was a limiting nutrient in the wastewater. For reasons unknown, some dispersed floc

was present in all supernatants.

Initial operation of the modified, full-scale CMAS aerated lagoon with four-hour

unaerated mixing zone and a 5:1 recycle (MLSS:Q) reduced the SVI to less than 150 mL/g. This

resulted in two phases of sludge: one of which settled very poorly, and the other that settled very

rapidly. The F/M ratio was 1.8 kg/kg⋅d in the initial contact zone and nitrification did occur. No

reasons for the dispersed solids were found. This level of treatment eliminated problems with the

effluent discharge to a municipal wastewater treatment plant.

5.14 Fibra, America, Brazil – Anaerobic (SXANM)

This WWTP treats a soluble high sugar wastewater from a Nestle plant processing

wastewater from cocoa production with a two-stage CMAS process. Each basin was fully

aerated to ≥ 2 mg/L (Okey, 1997) using mechanical aerators. In this mode, the SVI ranged

between 500 and 1000 mL/g. When one, then two, of four first-stage aerators were shut down

and the DO was reduced, the SVI decreased to the range of 250-400 mL/g (Figure 5.19). A

single-stage bioselector, which is mechanically mixed and maintained in an anaerobic mode, was

constructed and precedes the aeration basin.

The bioselector was designed on the basis of zero-order assimilation of a monosaccharide

based on 60% removal in the bioselector. Depending on the flow, the retention time in the

bioselector is one to two hours. The bioselector was started up on Day 19 (Figure 5.19) and there

was an immediate decrease in the SVI; thereafter the SVI decreased to less than 100 mL/g within

6 days. In some periods, the SVI decreased to less than 50 mL/g and the effluent turbidity

increased. The process control mode to stabilize the SVI was to bypass more or less flow around

5-41

the bioselector into the first oxic zone. This flow resulted in a controllable growth of filamentous

organisms, which stabilized the floc structure and improved effluent clarity.

5-42

6-1

Chapter 6.0

Troubleshooting Bioselectors

The actual performance of a bioselector may be less than optimal for various reasons. If

the bioselectors are constructed per design and the operation is per the plan, then the analysis

must consider a number of possible reasons for bulking conditions. The causes can be:

• Low food/mass (F/M) in the initial contact zone (ICZ) – filamentous

• High F/M ICZ – hydrous

• Air rate (DO = 0.3 to 1.5 mg/L) to bioselector – filamentous

• Limited oxygenation (regeneration) capacity in oxic zone – hydrous / filamentous

• Secondary (oxic zone) bulking – filamentous

• Very low sludge volume index (SVI) (diluted SVI) – high turbidity

• Toxic/inhibitory compounds – hydrous

• Limited nitrogen or phosphorus supply – hydrous/filamentous

• Soluble organic breakthrough – hydrous/filamentous

• Single bioselector stage

The current state of the art of bioselection is mostly results of laboratory and field

experience than a fundamental understanding of the mechanisms involved. There is a general

understanding of how filamentous and non-filamentous growths can be controlled. Further,

research and full-scale experiences have shown that soluble wastewaters will be more amenable

to bioselection than wastewaters with a high degree of particulate and colloidal solids. Lastly, it

is understood that the bioselector should remove the bulk (>80%) of the available soluble

chemical oxygen demand (sCOD) prior to leaving the bioselector and entering the oxic zone.

Many bioselectors will reduce the sCOD to the range of the final effluent in 12 to 20 minutes of

the retention time in the bioselectors based on raw or settled effluent flow.

6-2

However, this understanding does not extend to the point where we can define why one

facility will operate at 50 to 80 mL/g SVI and another with a similar bioselector design and

similar wastewater will operate at 90 to 120 mL/g. On the other hand, the six separate plants at

Phoenix 91st Ave WWTP all had SVIs about 60-80 mL/g in 2004.

It is often difficult to clearly identify hydrous bulking problems (Figure 6.1). The SVI

may or may not indicate a serious bulking condition. When hydrous bulking occurred due to floc

overload at the 23rd Ave WRF in Phoeniz, AZ, the DSVI only increased from the 60-90 mL/g

range to 100-120 mL/g range. However, the 5-minute zone settling velocity (ZSV5) was lower,

and large, fluffy floc (sometimes called straggler floc) appeared in the upper layers of the

clarifier. A two-phase sludge blanket developed in the clarifier: the lower one dense and the

upper one light, of low density and nearly buoyant. During higher flows these large, low density

flocs (hydrous) can be flushed into the overflow.

The hydrous biomass may not be visible in the bulk settling tests since the majority of the

floc can settle well, pulling down those hydrous flocs and producing a uniform settled mass.

However, this test procedure does not duplicate the dynamic conditions in the continuous

clarifier. The initial (5 min) ZSV will generally be significantly lower than the values predicted

by the Daigger and Roper (1985) or Daigger (1995) equations.

The good news is that a good bioselector design with three or four zones will maintain

the SVI at ≤ 120 mL/g (DSVI ≤ 95 mL/g) at all times. Most of the lowly aerated and anoxic

three- or four-stage bioselectors operate at < 100 mL/g SVI (DSVI < 80 mL/s). If the settling

characteristics are poorer (higher SVIs) than these values, then there can be problems with the

bioselector design, its operation, or constituents in the wastewater as will be discussed below.

6.1 Low F/M in the ICZ

The F/M gradient in a bioselector could be lower than the recommended design loadings

due to lower operating flow rate and/or lower influent BOD5 or COD concentrations or

combinations of these factors. Some of the early bioselectors were single stage and too large;

6-3

6-4

hence there was no gradient and the F/M in the initial contact zone was too low to effect good

bioselection. With the lower F/M, the mixed liquor suspended solids (MLSS) would be expected

to show a higher level of filamentous organisms. Generally, the bioselector should operate

efficiently over the range of 40 to 120% of the recommended loadings. In conditions less than

40% of these loadings, removal of an aeration train from service should be considered to

maintain a more economical operation. However, most bioselectors will be effective over the

range of 40 to 120% of the recommended design F/M gradient. Some units will be effective at

20-30% of the suggested loadings, although this situation is not predictable.

Lowering the MLSS to achieve a higher F/M may not have the desired effect. There is

some indication that the higher oxygen demand (mg/L⋅hr) produced by the higher MLSS is a

significant factor in promoting non-filamentous growth. This would also be true for weaker

wastewater in colder periods when the influent wastewater could have a higher DO. It is

suggested as the initial step of corrective action to increase the MLSS and minimize air flow (if

employed and ensure that the bioselector zones are at or near zero DO if aerated (SXAL) and > 2

mg/L if heavily aerated (SXAH).

If low F/M in the ICZ is a normal condition, add intermediate baffles to increase ICZ

F/M. If problems persist, it will be important to identify the causative organisms or other factors

that will assist in the identification of the problem.

6.2 High F/M in the ICZ

If the F/M is excessive in the initial contact zone, the biomass will accumulate and store

the soluble organics as exo-cellular protoplasm (ECP) slimes and the result is hydrous bulking.

The hydrous bulking condition can cause lowered return sludge solids (RSS) concentrations and

a fluffy, unstable sludge blanket which may overlay a more concentrated sludge layer of non-

hydrous sludge. Corrective actions include increasing the air rate to bioselector zones and

maximizing the mixed liquor concentration (lower F/M). Increasing the return activated sludge

(RAS) and internal recycle (IR) rates to the initial contact zone may not change the F/M ratio,

6-5

but it does decrease the contact loading, which is one possible cause of the accumulation of ECP.

The increased quantity of nitrates and TSS with a higher internal recycle will also help to reduce

bioselector overload due to the denitrification comsumption of soluble organics and lower

contact loading (mg sCOD/g MLSS).

If the overloading (high F/M) is a permanent situation, it is then necessary to enlarge the

volume in the initial contact zone by moving the baffle wall or removing the baffle to double the

ICZ volume. Baffle supports that are bolted to the wall and baffles that are easily moved to a

new position represent good design practice. The three- (or four-) zone bioselector design should

be retained. However, the volume in the third bioselector zone (SX-3) has less significance and

can be reduced if necessary.

6.3 Air Rate to the Bioselectors

The air rate to the bioselectors can influence the SVI. In general, higher air flow in the

aerated, low DO (SXAL) mode will increase SVI and vice versa. To minimize SVI, the air rate

should be limited to keep the DO at 0.0 mg/L during the diurnal peak BOD5 loading (and

bacterial growth) periods. If the air rate is set to maintain at ≤ 0.3 mg/L DO during the early

morning at low flows, the DO during the higher loading periods throughout the day will be zero.

In some applications, the SVI will be too low and the rapidly settling sludge leaves

turbidity (non-flocculated TSS) in the effluent liquid. In those cases, the growth of filaments can

be encouraged by increasing the air rate and DO. The alternative is to bypass the bioselector

(Okey, 1997) with a portion of the influent flow into the oxic zone(s).

In the aerated, high DO (SX AH) mode of operation, increased SVI can be due to the

inability to maintain > 2 mg/L in the initial contact zone during peak loading periods. If the air

rate cannot be increased to achieve 2 mg/L DO, then an aerated, low DO (SX AL) mode (DO =

0.0) could be evaluated in the initial contact zone of the bioselector. The balance of the

bioselector zones would be maintained at > 2 mg/L DO. If the F/M is too high (> 4 kg

6-6

sBOD5/kg⋅d), a low DO in the initial contact zone could result in the accumulation of ECP

leading to hydrous bulking.

6.4 Limited Oxygenation Capacity in the Oxic Zones

If the aeration capacity is limited – volume or actual oxygen transfer rate (AOTR) – in

the oxic zones (overloaded), then the stored organics from the bioselectors may not be oxidized,

i.e., sludge is not regenerated. The return of these regenerated solids can reduce the normal

sorption-storage capacity of soluble substrates in the bioselectors. However, the biomass will

then use an alternative means of storage producing ECP slimes and hydrous bulking will result.

The oxygen demand of the first oxic zone in a multistage reactor can be 80 to 130

mg/L⋅hr at 3.5 g/L MLSS. It is imperative to be able to maintain > 2 mg/L DO in this zone.

Lower DOs can result in excessive ECP production and hydrous bulking.

The corrective action is to reduce the loading (add aeration volume on-line, if available)

and/or increase the oxic zone oxygenation capacity if DO is limited. Short-term actions would

include maximizing internal recycle to the initial contact zone and an increased RAS flow may

be beneficial. Increasing MLSS (and the SRTOX) can also be beneficial as the diffused aeration

αF value of the mixed liquor may often increase the AOTR when the higher solids retention time

(SRT) is increased. The net effect is reduced air requirements (kg O2 supplied/kg BOD5).

6.5 Secondary Bulking (Oxic Zone)

Secondary bulking can result from the hydrolysis of particulate BOD5 to soluble

compounds. These compounds can support the growth of filaments, especially in a soluble

organic limited (low F/M) process. It is probable that low DO in the first oxic zone may

contribute to bulking situations – both filamentous and hydrous sludge production.

Compartmentalization of the initial portion of the oxic zone is beneficial. The DO should be

maintained above 2.0 mg/L, or if not possible, reduced to 0.0-0.3 in the initial oxic zone.

6-7

6.6 A Very Low SVI – High Turbidity

Bioselectors can sometimes produce very low SVIs (20 to 40 mL/g) and there can be

excessive turbidity in the overflow due to high zone settling velocity reaching 4 to 6 m/hr. This

can be due to excessive floc shear, high settling rates disrupting reflocculation, fragile flocs or

presence of toxic/inhibitory substances. The problem is more likely associated with highly

soluble industrial wastewaters where SVIs of 20-40 mL/g have been produced and the very high

rate of floc settling limits opportunity for floc growth.

The corrective action steps are:

• Evaluate shearing forces in the aeration basin to the final clarifier feedwell.

Conduct lab vs. field settling results to define the benefits of controlled settling.

Corrective actions would include checking the nutrient supplied, effluent nutrients

and the nitrogen (N) and phosphorus (P) content of the biomass. The N and P

content should be ≥ 8% N/VSS and ≥ 1.5% P/VSS in the MLSS.

• Maximize the DO in the bioselectors if the units are aerated. The aerated, low DO

(SXAL) mode should produce ≥ 1.0 mg/L DO at maximum air flow. This mode

will help generate filaments to strengthen floc.

• Provide a bypass of influent around the bioselector to the oxic zone.

• If toxic wastes are suspected, find and remove the culprit component(s).

6.7 Toxic/Inhibitory Compounds

Specific organic and inorganic compounds act similar to chlorine – that is, they suppress

the growth of filaments and reduce SVI. However, it is also possible that these compounds will

slow nutrient transport into the cells and slow normal growth. As a consequence, ECP

production can occur with hydrous bulking the result.

6-8

The corrective action is to find and remove the culprit compound. Maximizing the MLSS

and sludge inventory may have some short-term benefits. The culprit could be metals,

chlorinated hydrocarbons, complex phosphates, and a host of other organic compounds that

inhibit biological growth.

6.8 Limited Nitrogen and Phosphorus Supply/Availability

If nutrients are limited, it is possible that both filamentous and hydrous bulking

conditions could co-exist. In warm climates at high oxygen uptake rates (30 to 40 mg O2/g

MLSS⋅hr), it is possible that the cells are unable to sorb/process sufficient nutrients to match the

growth needs. Thus, the cells could bypass nutrient needs by ECP production if normal substrate

storage capacity is limited. The limiting oxygen uptake rate (OUR) value has not been defined

through research, but it may be in the area of > 35-40 mg/L⋅hr⋅g MLSS. The N and P content of

the MLSS should be in the range set forth in Section 6.6.

If the nitrogen and/or phosphorus content of the MLSS is low, then the possible causes

will need to be evaluated. They are:

• Inadequate influent nutrients. The N and P must be available and the mixed liquor should

contain at least 5 mg/L NH4-N and 1 to 2 mg/L ortho PO4-P prior to initiation of nitrification

(if occurring) and Bio-P removal. It is possible to have limited Bio-P removal with low

organic phosphorus/MLSS levels.

• Presence of toxic compounds. Generally nitrification will be adversely affected, but this

may not be obvious in high SRT systems. Conduct batch nitrification to determine if there is

a significant lag in the initiation of nitrification. Also, check the rate of nitrification (mg/mg

MLSS⋅hr) to determine if the rate is adversely affected.

6.9 Soluble Organics Breakthrough

It is possible that the bioselectors are not removing the readily bio-degradable or soluble

organics prior to the oxic zone for one or more of the reasons set forth earlier. If breakthrough

occurs, there is the opportunity for filamentous growths as well as the potential to produce

6-9

hydrous sludges in the bioselectors since the return sludge may not be fully regenerated.

Multistage bioselectors are more efficient in sCOD removal and should be employed with the

recommended F/M cascade design. If ICZ F/M is low, add intermediate baffles.

Generally, the sCOD (f1.5COD) in the effluent from the bioselector should not be more

than 125-175% of the secondary effluent sCOD for an aerated, low DO, high DO or anoxic

bioselectors. Depending on the design, the sCOD may be higher in the effluent of anaerobic

bioselectors employed to achieve Bio-P removal. The Bio-P design of a biological selector may

have anaerobic retention times of 1 to 2 hours vs. 12 to 20 minutes typical for a bioselector for

only bulking control.

A laboratory sequencing batch reactor (SBR) study is effective in defining the SVI that

can be achieved by optimizing bioselection. In this type of study, the reactor is operated on a

full-settle-draw basis, normally fed twice daily and wasting once per day. The reactor is batch

fed, mixed for a limited time if denitrification is required and then aerated at > 2 mg/L DO.

Heide and Pasveer (1974) and Albertson et al., (1992a) reported good agreement between

laboratory SBR and full-scale facilities with multistage bioselectors. This lab procedure has been

employed successfully in other facilities – Tree Top Apple Juice, WA; 23rd Ave WRF in

Phoenix, AZ; Mt. Pleasant Rifle Range Road WWTP, SC; Star Valley Cheese Co., WY; and

Southerly and Jackson Pike in Columbus, OH – to define the potential of bioselectors.

A bioselector design with staggered baffles extending partially (75-85%) across the basin

can encourage backmixing and disrupt the bioselection operation. Overflow walls with about 10-

12 mm head loss per stage is recommended.

6.10 Summary Comments

The causative organisms should be identified by microscopic examination as to whether

they are filamentous or non-filamentous (hydrous) by standard procedures. The specific

organism can be a guide to the source of the problem. An experienced, well-trained analyst is

often required to ensure accurate definition of the organisms and possible causes. The nitrogen

6-10

and phosphorus content of the MLSS should be checked to ensure that adequate quantities of N

and P are not only available but also being utilized by the organisms.

India ink staining and the total glucose content of the MLSS (Jenkins et al., 1993)

provide important data to evaluate the characteristics of the MLSS relating to settling

compaction and clarification of the mixed liquor. Hydrous sludge production can result in

significantly lower 5-minute zone settling rates (ZSV5) predicted for SVI and MLSS

concentrations using the Daigger and Roper equation (1985).

In the analysis of problem bioselectors, all tools should be considered, specifically:

• Define whether poor settling is the result of filamentous or hydrous bulking.

• Check the level of N and P in the mixed liquor volatile suspended solids (MLVSS).

• Determine whether the soluble F/M in the initial contact zone is in the proper range.

• Check the contact loading to ensure that the contact loading value is in the range

recommended.

• Evaluate the potential for toxicity.

• Determine the level of total glucose in the MLVSS.

• Determine the 5-minute ZSV5 at two or three concentrations.

• Evaluate the DO and oxygenation capacity of the oxic zone(s).

• Check the removal of sCOD by the bioselector.

• Evaluate whether the return sludge is fully regenerated and has up to 15% of the cell weight

as sCOD sorption capacity.

• Conduct a laboratory SBR study to determine the achievable SVI if full-scale SVI is high.

As the possible number of independent variables increases, the amount of data required

to define the impact on the dependent variable SVI increases geometrically. Thus, there is an

advantage to employing bioselector designs that have proved to be most effective. Based on

Czech and US experiences, bioselectors comprising three or four zones and effecting an F/M

cascade where 75-85% of the sCOD will be removed from solution have proven to be the most

successful.

7-1

Chapter 7.0

Research Needs

The technical effort required to document the rational design approach to bioselectors

and determine whether there is a common ground to designing aerated (SXAL and SXAH), anoxic

(SXAX and SXAXAL) and anaerobic (SXANM) is daunting. The fact that plants, of similar design

and receiving typical municipal wastewaters, may or may not bulk (Tomlinson, 1976) is

evidence of the magnitude of the problem faced in developing a comprehensive understanding of

the design parameters for bioselectors. The fact that an activated sludge plant may only

occasionally have bulking problems is a further measure of the difficulty faced by the

researchers.

This study revealed that there is no common ground for bioselector design whether

aerated, low DO (SXAL), aerated, high DO (SXAH), anoxic (SXAxM and SXAxAL) or anaerobic

(SXANM) mode. The initial contact zone (ICZ), often considered the most critical zone, may

have had an organic loading varying from:

BOD5: <1.0 to >14 kg/kg MLSS⋅d

COD: <2 to >25 kg/kg MLSS⋅d

and still be successful in reducing a high SVI to a more acceptable range. The question “How

good is the optimum performance?” cannot be answered by the data available in this document.

Also, bioselectors have been constructed from as few as one to up to six compartments in a

series. How many zones are required to provide optimum reduction in the SVI? This study has

provided some insight and perhaps some preliminary guidelines to direct research efforts as well

as some general comments for introducing the research needs.

7-2

7.1 General Introductory Comments

The following comments cannot be considered site-specific because there are probably too

many factors, known and unknown, involved in any adoption/acceptance of these comments.

That is, there is and may continue to be many contradictions until the biological mechanisms are

better understood. A rejection or acceptance of one or more of the following comments by

careful lab and field studies can be considered a step toward expanding our understanding of

bioselectors.

• Filamentous and hydrous bulking are on opposite ends of the floc loading and F/M

spectrum. There is a middle ground where the adverse effects of each type of bacteria

growth are minimized (Figure 1.7).

• The initial contact zone (ICZ) of the bioselector and its food/mass (F/M) and or its

contact loading will be the key to the definition of the optimum design of each type of

bioselector.

• The aerated, low DO (SXAL), anoxic (SXAXM and SXAXAL) and anaerobic (SXANM)

bioselectors should provide a F/M cascade using three or four compartments. Fewer

zones (two) would be acceptable if demonstrated effective by pilot studies. The design of

bioselectors should be based on soluble input (f1.5µm) of preferably soluble chemical

oxygen demand (sCOD), and alternately soluble biochemical oxygen demand (sBOD5), if

COD data are unavailable. The initial contact zone of the bioselector should have a

sCOD F/M >3 and <6 kg/kg MLSS⋅d (sBOD5 >1.5 and <3 kg /kg.d). The bioselector

volumes can be 25, 25, 50% or 25, 25, 25 and 25% of the total volume. The aerated, high

DO (SX AH) bioselector loading range is > 3 to < 9 kg sCOD/kg MLSS⋅d (>1.5 kg to 4.5

kg sBOD5/kg⋅d).

• The bioselector design and optimization should be based on the soluble BOD5 (or sCOD)

entering the initial contact zone of the bioselector. The success or failure of bioselection

may be wholly dependent on the ability of the bioselector to remove the majority (75-

7-3

85%) of the incoming removable soluble sCOD (sBOD5) prior to the anoxic and/or oxic

stages.

• Limited ability to remove a large fraction of the sCOD5 in the initial contact zone will be

a potential cause of variable and poor results. The oxic zone must be fully evaluated to

determine if the ‘sorptive’ capacity of the return sludge was restored (regenerated).

• Aerated (SX AL and SX AH) and anoxic (SX AX) bioselectors may prove to be more

flexible for controlling SVI than anaerobic designs and should be employed when Bio-P

is not required. When denitrification is required, the bioselector design is not modified

and additional anoxic volume as required for completion of denitrification is added as a

fourth, and if necessary, a fifth zone. The aerated bioselector may handle a higher F/M in

the initial contact zone and thus minimize the bioselector volume. This same approach –

additional zones after the bioselectors – should also be employed for anaerobic

bioselectors when Bio-P volume requirements are larger than necessary for bioselection.

• The performance of bioselectors should be compared when the design (ICZ F/M),

number of stages and operation are similar. That is, what are the performance

characteristics of a three-stage aerated, low DO anoxic (SXAXAL) bioselector loaded in

the range of 3.0 to 6.0 kg sCOD/kg MLSS⋅d (1.5 to 3.0 kg sBOD5/kg⋅d) to the initial

contact zone? What are the average and maximum DSVI (or SSVI) values? Standard

deviations? (e.g., Columbus Southerly: 80 + 15 mL/g; Jackson Pike: 69 + 17 mL/g; Gig

Harbor: 70 + 8 mL/g; Phoenix 91st Ave: 81 + 6 mL/g – based on monthly results.)

• There is an increasing body of evidence that supports staging in the bioselector, anoxic,

or oxic zones in order to achieve the lowest SVI.

• The U.S. wastewater industry should adopt as the standard the DSVI or SSVI procedure

as well as a standard size of test vessel. The traditional uSVI procedure is not always

responsive to changes in the settling properties when SSV is >250 to 300 mL. It can be

misleading and cause improper and upsetting operational steps. Further, plant-to-plant

comparisons cannot be made with any reliability.

• Bioselectors should not be designed on COD, c BOD5, or BOD5 data.

7-4

7.2 Research Target

Research can progress toward a better understanding of the basic mechanisms of

bacterial growth that cause filamentous and non-filamentous bulking and also undertake a

detailed analysis of operating installations. It is suggested that the target criteria for the DSVI

be:

Average Maximum

Month Month

DSVI – mL/g 60 < 80 80 < 100

These DSVI values will allow for the effective and economical utilization of aeration and

clarification capacity and thus were chosen for targets. While a bioselector DSVI range of 80 to

140 mL/g would be an improvement over a DSVI of 120-180 mL/g, the clarification capacity at

140 mL/g is still significantly less than <100 mL/g DSVI. The effectiveness of bioselectors must

be evaluated on the maximum DSVI of the MLSS, not the average value for a week or a month

of operation because clarification sizing is dependent on the highest expected short-term DSVI.

7.3 Recommended Areas of Research

This study revealed that there are a number of process and design factors that may or may

not affect the efficiency of bioselection. These factors include:

• The number of stages in the bioselector. Based on prior studies, a three- or four-stage

bioselector would appear to be the best design to ensure that the maximum SVI is limited

to the target range. An initial field search should determine whether the three- or four-

stage bioselector should be adopted as a basic design (Both may use same bioselector

volume).

• Define the minimum ICZ loading that could cause filamentous growths (insufficient

stress) and the maximum loading that could lead to excessive ECP (hydrous sludge)

production.

• Establish whether future bioselector design criteria should be based on sCOD (or sBOD5)

rather than total COD (or BOD5). Bioselector designs based on sCOD are the same for

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raw and primary effluent; if based on COD (or BOD5), they would be 30 to 45% larger

for raw (unsettled) wastewater of domestic origin. This document recommends use of

soluble (f1.5µm) values for design since soluble removal defines the effectiveness of the

bioselector.

• Determine the maximum design ICZ loading and operating parameters for aerated

secondary (SX AL, SX AH), anoxic (SX AX) and anaerobic (SX AN) bioselectors. Establish

the role aeration (SX AL, SX AH) plays in terms of improving the range/operability of

these bioselectors. Since there are now hundreds of bioselectors in operation, an

extensive survey of the field units and their performance would provide useful

information. Plants where the SVI is good, but doesn’t meet the above targets, should be

considered for minor modifications to enhance performance. For example, add/delete

baffles to modify the ICZ F/M ratio.

• While there is an extensive body of laboratory research on bioselectors of many designs,

these units don’t undergo the rigors of the full-scale operating plants. However, a format

to summarize both lab and field results is needed in order to compare similar

configurations and operating conditions to determine if there are guiding design criteria

common to both sets of data. Multiple regression analysis could be a useful tool for

analyzing similar designs and comparing different design concepts. Because SVI is the

dependent variable, the problem will often be the use of undiluted SVI (uSVI) instead of

DSVI or SSVI for the U.S. data. All of the necessary data defining independent variables

may not be available without further field study.

• The research should be directed to define the significance of independent variables on

DSVI. Independent variables to be considered would be:

1. Bioselector ICZ and overall F/M

2. ICZ contact loading

3. Removal (%) of sCOD (sBOD5) by bioselector zones

4. Number of Sx zones

5. Soluble/total COD (sBOD5/BOD5) ratio of wastewater

6. Aeration (AL, AH) level and DO in bioselector (SX) zones

7. Wastewater strength and variations

8. Effect of wet weather, dilution and DO

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9. Effect of mode of operation: SXAL, SXAH, SXAxM, SXAxAL, SXANM

10. Effect of system sludge age, SRT

11. Definition of the bacterial species accompanying an excursion in the DSVI in

a bioselector system

12. Oxygen demand in the ICZ as an indicator of bioselector stress

13. Process temperature

As the number of variables increases, the mass of data required is larger. Thus, there is an

advantage of pre-screening data for major variables such as: (1) number of stages; (2)

F/M in the initial contact zone; and (3) type of bioselector. Based on Czech and US

experiences, bioselectors with three- or four-zones have been most successful,

particularly in plants with long or staged oxic zones.