grade 3 & 4 -activated sludge.ppt

7/29/2019 Grade 3 & 4 -Activated Sludge.ppt

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Activated Sludge


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Introduction Term "activated sludge" refers to wastewater

being mixed with air or oxygen for a length of

time to develop a brown floc which consistsof billions of microorganisms and othermaterial

Process provides the environment to keep

these aerobic microorganisms undercontrolled conditions so they can removemost of the suspended or dissolved organicsfrom the wastewater

The environment is provided by four basicsystems: aeration, sedimentation, return


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Introduction: Typical Activated

Sludge Process


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Introduction: Definitions of

TermsACTIVATED SLUDGE

floc of microorganisms that form whenwastewater is aerated

MIXED LIQUOR mixture of activated sludge and

wastewater in the aeration tank

MIXED LIQUOR SUSPENDED MATTER(MLSS) measure of the amount of suspended

solids in the mixed liquor expressed in

mg/l


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Introduction: Terminology MEAN CELL RESIDENCE TIME (MCRT) inverse of the net growth rate and is equal to the average time

a microorganism spends in the treatment process. The MCRT isan important kinetic parameter that is very useful in processcontrol

FOOD TO MICROORGANISM RATIO (F/M) ratio of the amount of food expressed as pounds of COD (or

BOD) applied per day, to the amount of microorganisms,expressed as the solids inventory in pounds of volatilesuspended matter. The F/M is mathematically related to theMCRT, and is also an important process control tool

RETURN ACTIVATED SLUDGE (RAS)

settled mixed liquor collected in the clarifier underflow andreturned to the aeration basin


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Introduction: Terminology

WASTE ACTIVATED SLUDGE (WAS)

excess growth of microorganisms which mustbe removed to keep the biological system inbalance. Various control techniques have beendeveloped to estimate the amount of WAS that

must be removed from the process COMPLETE MIX ACTIVATED SLUDGE

an ideal mixing situation where the contents ofthe aeration tank are at a uniformconcentration

PLUG FLOW ACTIVATED SLUDGE

an ideal situation where the contents of theaeration tank flows along the length of the tank


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Introduction: Terminology

BACK MIXING

mixing the contents of a tank in the longitudinal or floworiented direction

TRANSVERSE MIXING (or CROSS ROLL)

mixing in a direction across the direction of flow SLUDGE REAERATION

practice of aerating the RAS before it is added to themixed liquor

PROCESS LOADING

organic loading range as measured by the F/M

CONVENTIONAL LOADING

process loading of 0.2 to 0.5 lbs BOD applied/lbMLVSS/day

HIGH RATE LOADING

process loading of two to three times the conventional

loading rate


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Introduction: Terminology EXTENDED AERATION LOADING

low rate loading that is one half to one tenth ofthe conventional loading rate

SETTLEABILITY measure of the volume occupied by the mixed

liquor after settling in a graduated cylinder for30 minutes. Settleability is generally expressed

as a percentage based on the ratio of thesludge volume to the supernatant volume

SOLIDS INVENTORY (VOLATILE SOLIDS) amount of volatile suspended solids in the

treatment system


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Solids Inventory

Suspended matter (SS) that makes up the mixedliquor consists of living and nonliving organic matte

The living organic matter is referred to as being

active and contains microorganisms responsible fortreating the wastewater

The more accurately the concentration of activemicroorganisms is known, the more consistently the

activated sludge process can be controlled Many attempts have been made to accurately

measure the "active" concentration of the SS


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Solids Inventory Common means of estimating the microorganism

concentration is the measurement of volatilesuspended matter (MLVSS)

All the organic material in the SS burns to carbondioxide and water in the MLVSS determination

Typically, 70-80 percent of the MLSS will beMLVSS

The MLVSS determination provides an

approximation of the concentration of livingbiological solids, because the MLVSS alsoincludes a nonliving fraction

Even so, the MLVSS has been found to be an

acceptable representation of "active" livingmicroorganisms in activated sludge


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Treatment Flow Return Activated Sludge (RAS) from

the clarifier underflow is combined

with the influent wastewater in theaeration tank to form the mixedliquor

The mixed liquor is usually aeratedfor a period of several hours in theaeration tank


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Treatment Flow During aeration, organic material in the

wastewater is converted into newmicroorganisms, adsorbed onto floc particlesand converted (oxidized) to various otherproducts, including carbon dioxide.

Mixed liquor flows through the aeration tank

into the clarifier, where it settles to form theRAS.

Clear liquid remaining above the settledmixed liquor is called the secondary effluent,

which is discharged from the process.


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Aeration SystemsAeration provides

dissolved oxygen and

mixing of the mixed liquor andwastewater in the aeration tank.

Factors influencing aeration

requirements: Organic loading

MLSS concentration

Temperature

Other


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Aeration SystemsAeration is provided by either diffused

or mechanical aeration systems.

Diffused air systems consist of a blowerand a pipe distribution system that isused to bubble air into the mixed liquor.

Mechanical aeration systems consist ofa pumping mechanism that disperseswater droplets through the atmosphere.


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Activated Sludge Diagram


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Diffused Air System Diffused air systems are the most

common types of aeration systems used

in activated sludge plants. Diffusers are designed to either produce

fine or coarse bubbles.


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Fine Bubble Diffusers The fine bubble diffusers are easily

clogged by biological growth and by

dirty air, resulting in high maintenancecosts.

The air supply for all fine bubblediffusers should be filtered.


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Fine Bubble Diffusers


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Coarse Bubble Diffusers Coarse bubble diffusers are usually made by

drilling holes in pipes or by loosely attaching

plates or discs to a supporting piece of pipe. Coarse bubble diffusers have lower oxygen

transfer efficiencies than the fine bubblediffusers.

A typical oxygen transfer efficiency would beabout 5 percent.


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Coarse Bubble Diffusers


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Surface Aerators There are two types of mechanical aerators in

common use today. These include the

surface and turbine aerators. Surface aerators use a rotating propeller that

pumps the mixed liquor through theatmosphere above the aeration tank.

Oxygen transfer is achieved by the aeratorpropeller spraying the mixed liquor throughthe atmosphere.


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Surface Aerators Surface aerators either float or are mounted on supports in

the aeration tank. Materials such as epoxy coated steel areused in the construction of surface aerators to reducecorrosion.

The oxygen transfer efficiency of a surface aerator increasesas the submergence of the propeller is increased. However,power costs also increase because more water is sprayed.

Oxygen transfer efficiencies for surface aerators are stated interms of pounds of oxygen transferred per horsepower perhour (lb O2/hp/hr).

Typical oxygen transfer efficiencies are about 2 lb O2/hp/hr. Surface aerators are sometimes equipped with draft tubes to

improve their mixing characteristics.


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Mechanical Aerators Floating or fixed

Brush

Other


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Sedimentation System As the mixed liquor flows out of the aeration tank, it is

transferred to a sedimentation unit which is commonlycalled a secondary clarifier.

The secondary clarifier provides a reduction in flow

velocity allowing the mixed liquor to settle by gravity tothe bottom.

The design and construction of secondary clarifiers foractivated sludge treatment incorporates several methodsfor the removal of settled sludge.

These generally include the conventional sludge collectionequipment found in rectangular and circular primarysedimentation units which collects to a central hopper,and, in recent years, suction-type collectors as shown inSlide 25.


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Wasting & Return Rates Constant MLVSS

Constant F:M Ratio

Constant Sludge Age

Return Rates Constant rate Variable rate

Intermittent rate


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Process Control A portion of the activated sludge is purposely

removed by wasting it from the process.

The wasting of sludge is necessary tomaintain the desired quantity (numbers) ofactive microorganisms in the process.

A basic idea behind successful operation of

an activated sludge system is to keep abalance of microorganisms to the amount offood in the wastewater.

Proper operation makes food the only part of

microorganisms' diet that limits their growth.


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Process Control If nutrients or oxygen limit the growth of

the microorganisms, they will not settlesatisfactorily in the clarifier.

The activated sludge process depends onsettling the mixed liquor so that it can bereturned to the aeration tank to keep inbalance with the organic material in the

incoming wastewater. This balance is generally related to

process loading as expressed by the F/Mratio.

Inability to settle the mixed liquor can


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Nitrification Cycle The first step in the process, conversion of

ammonia to nitrite and then to nitrate, iscalled nitrification (NH

3

NO2

NO3

). Theprocess is summarized in the followingequations:

NH4 + 3/2 O2 NO2- + 2H+ + H2O

NO2- + 1/2 O NO3

-

It is important to note that this processrequires and consumes oxygen. Thiscontributes to the BOD or biochemical oxygendemand of the sewage. The process is

mediated by the bacteria Nitrosomonasand


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Nitrification Cycle

Decaying

Matter

Organic - N

Industrial

Fixation

Nitrogen Gas

Oxides of Nitrogen

Fertilizer

(Ammonia,

Nitrate)

Nitrogen

Fixing

Bacteria

AssimilationAmmonia

Nitrification

Nitrate DenitrificationAmmonia, Organic - N

in WWTP Influent

Denitrification inSediments

Biological Growth

in StreamNitrification in

Stream

Ammonia, Nitrate

in WWTP Effluent


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Nitrification Cycle


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ToxicityAmmonia toxicity

Ammonium

Nitrate

Nitrite


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Process Control 30 minute settleability range 400 to 800 ml/l

Aeration basin dissolved oxygen 1.0 to 3.0

mg/l Mixed Liquor Suspended Solids (MLSS) Range

Conventional, Step aeration, Complete Mix 1000 to3000 mg/l

Contact Stabilization 1000 to 3000 mg/l Extended Aeration, Oxidation Ditch 3000 to 5000

mg/l

Mixed Liquor Volatile Suspended Solids 70%

of Mixed Liquor Suspended Solids


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Process Control Temperature 8 to 22 degrees Celsius

for domestic wastewater. Industrial

discharge will significantly increase thetemperature.

Nitrogen content Total Nitrogen 15-90 mg/L

Phosphorus Content averaging around10 mg/liter


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Side Stream Impacts Return activated sludge (RAS)

Supernatant from digester

Filtrate from solids processing


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Troubleshooting If conditions in the aeration system

deteriorate, undesirable microorganisms maythrive

Typical undesirable microorganisms includethe filamentous organisms

Filamentous microorganisms

long, thread-like organisms with increased surfacearea

more competitive for growth in conditions of lowDO or low nutrient concentrations


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Troubleshooting

Filamentous organisms hinder settling

cause excessive bridging and matting of thefloc

result is a mixed liquor which does not settlewell

Poor settleability associated with thepresence of too many filamentousorganisms is known as bulking sludge.


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Troubleshooting Bulking sludge

Settles poorly

Operation in a bulking sludge condition may resultin the loss of the mixed liquor into the effluent.

Approaches to cure bulking include: treating the return activated sludge with oxidizing

agents, such as chlorine or hydrogen peroxide

improving the treatment conditions soenvironment is less favorable to the growth offilamentous organisms.

Settling problems and solids losses can also

occur during high flow periods.


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Activated Sludge Process

Variations conventional

tapered aeration

complete mix extended aeration

step aeration

contact stabilization

high rate

Sequential batch reactors

Fixed film suspended growth

Kraus


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Activated Sludge Process

Variations Variations involve changes in loading rates or

a physical rearrangement of the process.

The various levels of process loading aredescribed by the F/M ratio and MCRT.

The term "physical arrangements" is used torefer to the structural arrangement of the

aeration tank, as well as the variousarrangements of the process streams that areused to provide flexibility.


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Process Loading Ranges The three basic ranges of process

loading are for a plant operating on a

typical domestic wastewater at atemperature of about 20 degreescentigrade.

These loading ranges will be referred toas the high rate, conventional rate, andextended aeration.


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Process Loading RangesProcess Modification Aeration, D F:M MLSS

Conventional 5 to 15 0.2 to 0.4 1500 to 3000

High Rate 5 to 10 0.4 to 1.5 100 to 1000

Extended aeration 20 to 30 0.05 to 0.15 3000 to 6000


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High Rate Loading High rate loading range takes

advantage of the settleability of

sludge when the treatment systemis loaded at a fairly high rate.

Level of treatment which results is

comparable to a typical high-ratetrickling filter plant.


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Conventional Rate For typical domestic wastewater at 20

degrees C, the conventional process operatesbetween MCRT values of 5 to 15 days andF/M ratios of 0.2 to 0.5 lbs BOD applied/lbMLVSS/day.

Most large municipal treatment plantsoperate in the conventional activated sludgezone. Plants operating in the middle of thisrange produce an excellent effluent qualityand do not (in theory) nitrify.


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Conventional RateAt the lower end of this loading range,

an even better effluent is sometimes

produced, although problemssometimes occur when the plant slipsslightly or goes completely into

nitrification.


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Conventional Rate Filamentous growth and poor sludge

settleability have been associated with theconventional process at the upper loadingranges

Dispersed growth and cloudy effluent are alsoquite common. Usually the operator can seethis sort of condition coming by plotting atrend of the organic loading in his treatmentprocess (either the F/M ratio or the actualMCRT)

Other signs of a more physical nature mayalso be used by the operator to evaluate an


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Extended Aeration Rate The lowest range of process loading where

successful operation may be accomplished isthe extended aeration tank.

Generally, plants operating in this range aresmall in size and do not receive 24 hoursupervision.

Such plants are very conservative in designand generally operate with an MCRT of 20-40days and F/M ratio of 0.05 to 0.15 lbs BODapplied/lb MLVSS/day.

(The above is based on typical domesticwastewater at a tem erature of 20 de rees


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Extended Aeration Rate Extended aeration is sometimes referred to

as the "total oxidation process"

Term is derived from the fact that theseplants are designed with such low loadingsthat theory predicts that all influent BOD willbe converted to CO2

Some manufacturers claim no wasting isnecessary

In reality, even after extremely long periodsof aeration, suspended matter remains in the

effluent


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Extended Aeration Rate Although sludge wasting need not be

conducted on a daily basis in extendedaeration plants, occasional wasting is a

necessity Often the effluent of the extended aeration

processes contains small pinpoint floc, whichmay be observed passing over the weirs ofthe secondary clarifier

When loading in an extended aeration plant isin the higher portion of the loading range, anumber of operating problems may occur


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Extended Aeration Rate Because the entire aeration range is in

the nitrification zone, denitrification and

rising sludge problems may result brown, greasy foam

Filaments

poor settleability


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Activated Sludge With Re-

aeration Re-aeration processes are variations in the

arrangement of the process streams

All sludge re-aeration processes involvestabilization by aeration of the return sludgeprior to its contact with the untreatedwastewater

Most examples require different ratiosbetween the amount of return sludge underaeration and the amount of sludge in thecontact section of the process


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Activated Sludge With Re-

aeration Contact stabilization and step aeration

(step feed) are two variations of sludgere-aeration

Both processes represent the extremes ofthe contact/stabilization ratio; however,both have established a successful recordof performance

Successful process installations withcontact/stabilization ratios over the wholerange between contact stabilization andstep feed can be found in great numbers

Most have been shown to greatly increase


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Contact Stabilization


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Step Feed


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Complete Mix


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References Kerri, Kenneth D., et al. Operation Of

Wastewater Treatment Plants. Third Edition,1990, Vol. I, California State University,

Sacramento. EPA Process Control Manual.Aerobic

Biological Wastewater Treatment Facilities.March 1977. U.S. Environmental Protection

Agency, Office of Water Operations,Washington D.C.

Water Environment Federation. Design ofMunicipal Wastewater Treatment Plants.

grade 3 & 4 -activated sludge.ppt

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