Operational Problems in Activated Sludge

7/29/2019 Operational Problems in Activated Sludge

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Operational ProblemsThe most common problems in the operation of an activated

sludge plant are bulking sludge, rising sludge and nocardiafoam.

Bulking Sludge

Bulking sludge has

• Poor settling characteristics and

• Poor compactability.



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Two principle types of sludge bulking problems have been

identified.

Caused by the growth of filamentous organisms

Or organisms that can grow in a filamentous form under

adverse conditions.This is the predominant form of bulking

Caused by bound water

Bacterial floc swell and their density is reduced so they will

not settle.

Microthrix Parvicella A Filamentous Bacteria Growing with Floc



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The causes of sludge bulking are related to

1. The pysical and chemical characteristics of the WW

2. Treatment plant design limitations

3. Plant operation

1. WW characteristics that can affect sludge bulking include

fluctuations in flow & strength, pH, temperature, nutrient

content

2. Design limitations include air supply capacity, clarifier design,

return sludge pumping capacity, short circuting or poor mixing.



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Bulking is often a problem in center feed circular tanks

where sludge is removed from the tank directly under

the point where mixed liquir enters the tank. In some

cases a large part of the sludge is actually retained in

the tank for many hours rather than the desired 30 min.

If this is the case then the design is fault and change the

withdrawal equipment.



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3. Operational causes of filamentous bulking include

low dissolved oxygen in the aeration tank

insufficient nutrients: Especially quantity of nitrogen and

phosphorus important, also absence of trace element causebulking.

widely varying organic waste loading

low F/M ratio: The F/M ratio should be check to make it iswithin normal range.

Low F/M ratio encourage the growth of filamentous

organisms

High F/M may result in the presence of small disperse flocs.

insufficient soluble BOD5 gradient:



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Limited dissolved oxygen has been noted more frequently than

any other cause. If the problem is due to limited D.O. Aerating

equipment should operate at full capacity. At least 2 mg/L of

dissolved oxygen in the aeration tank (under normal loading

conditions) should be maintained.

One of the kinetic features of filamentous organisms that

relates to this condition is that they are very competitive at low

substrate concentrations whether it be organic substrates, DOor nutrients.



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Filamentous bacteria such as Beggiatoa & Thiothrix

grow well on hydrogen sulfide & reduced substrates

respectively that would be found in septic WW.

When the influent WW contains fermentation products

such as volitile fatty acids & reduced sulfur compounds

(sulfides and thiosulfate) Thiothrix can proliferate.

Prechlorination of the WW has been done to prevent to

their growth.



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Beggiatoa

Beggiatoa, a filamentous

bacterium that oxidizes

hydrogen sulfide.



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Thiothrix



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In an emergency situation or while these factors are beinginvestigated chlorine & hydrogen peroxide may be used to

provide temporary help.

Chlorination of return sludge has been practiced as a means

of controlling bulking sludge caused by filamentous

organisms.

It is ineffective when bulking is due to light floc containing

bound water.

Chlorination of return sludge in the range of 2-3 mg/L (in

severe cases 8-10 mg/L) of Cl2 per 1000 mg/L MLVSS is

suggested.



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Occasionally sludge that has a good settling

characteristics will be observed to rise or float to the

surface.

The cause of this phenomenon is denitrification.

As nitrogen gas is formed in the sludge layer, much of

it is trapped in the sludge mass & sludge rises or floats.

Rising sludge can be differentiated from bulking sludgeby noting the presence of small gas bubbles attached

to the floating solids.

Rising Sludge



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Rising sludge problems may be overcome by

Increase return activated sludge withdrawal rate (so

reduce detention time of the sludge in the clarifier)

Increase the speed of the sludge-collecting

mechanism in the settling tank

Increase sludge wasting rate (consequently

decreasing of the SRT achieved)



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Nocardia Foam

A viscous brown foam that covers the aeration basins &

secondary clarifiers has produced many problems, including

safety hazards, odors and changes in effluent quality.

The foam is associated with a slow growing filamentous organism

usually Nocardia genus.

Nocardia, Type 1863

Nocardia-like Filamentous Bacteria in ActivatedSludge Foam (Gram stained preparation).



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Nocardia foam



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Foam on weirs

Clarifier foam



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foaming in an aeration

basin

Probable causes:

Low F/M in the aeration tank

High MLSS due to insufficient sludge wasting

Sludge reaeration



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Measures for nocardia control include

1. Reducing sludge age (most commonly used)

2. Reducing the air flowrate to lower the depth of foam

accumulation

3. Adding a selector compartment to control the growth of

filamentous organisms

4. Injecting a bacterial additive

5. Chlorinating the return sludge

6. Spraying chloride solution or sprinkling powered calcium

hypochloride directly onto the foam

7. Reducing the pH (in the mixed liquor)



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The selector concept entails the selective growth of floc-

forming organisms at the initial stage of the biological process

by providing a high F/M ratio at controlled DO levels.

A selector is a small tank (20 to 60 min contact time) or aseries of tanks in which the incoming wastewater is mixed with

return sludge under aerobic, anoxic, and anaerobic conditions.

The Selector Concept

The high substrate

concentration in the

selector favors the growth

of nonfilamentousorganisms (see Fig. 8-13).

Nonfilamentous forms

Filamentous forms

Substrate, mg/L S p e c i f i c g r o w t h r a t e h - 1



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Selector designs are based on either kinetic or metabolic

mechanisms1. Kinetics-Based Selector:

Selector designs based on biokinetic mechanisms provide for

reactor substrate concentrations that result in faster substrate

uptake by the floc forming bacteria.

While filamentous bacteria are more efficient for substrate

utilization at low substrate concentrations, the floc forming

bacteria have higher growth rates at high soluble substrate

concentrations.

The kinetics-based

selector designs are

called high F/M selectors



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2. Metabolic-Based Selector:

With biological nutrient removal processes, improved

sludge settling characteristics, and, in many cases, minimal

filamentous bacteria growth has been observed.

The anoxic or anaerobic metabolic conditions used in these

processes favor growth of the floc forming bacteria.

The filamentous bacteria can not use nitrate or nitrite for an

electron acceptor, thus yielding a significant advantage to

denitrifying floc forming bacteria.



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Similarly, the filamentous bacteria do not storepolyphosphates and thus cannot consume acetate in

the anaerobic contact zone in biological phosphorus

removal designs, giving an advantage for substrateuptake and growth to the phosphorus-storing bacteria.

In some wastewater treatment facilities an anaerobic

selector has been used before the aeration tank in low

SRT even though phosphorus removal is not required.



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ACTIVATED SLUDGE

MODIFICATIONS

1. Conventional Activated Sludge Treatment

A typical flow pattern is

As the tank geometry is long and narrow the mixing

regime approaches plug-flow. The air diffusers are generally locate evenly along one

side of the tank, therefore a spiral flow pattern is

produced.



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Operating experience soon revealed a number of problemswith this design.

For example as the biomass was recycled back to the head of

the aeration tank and there mixed with incoming WW it was

observed that the oxygen requirements at this point often

exceeded the capability of the aeration system.

It was also found that such a flow arrangement increased the

probability of process failure due to shock loads of toxic or

high-strength waste because these loads were controlled at

the enterance.

Because of these deficiencies numerous modifications have

been proposed.



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2. Tapered Aeration

This modification is identical to the conventional process.

The basic difference between the two process is in the

diffuser arrangement. In tapered aeration diffusers are spaced so that more air is

supplied at the head of the tank, where the oxygen demand is

greatest, and is then decreased.

It is more economical than conventional.

more air less air



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effluent

influent

In this modification return sludge is mixed with a portion of the

WW and enters the head of the aeration tank.

WW is also fed into the tank at different points along its

length.

Advantages are

3. Step Aeration

Aeration tank

Return sludge

a) Better equalization of waste load

b) Lower peak O2 demand

c) Smaller overall aeration tank volume



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This process is an application of the flow regime of a

continuos flow stirred tank reactor (CSTR).

The organic load on the aeration tank and the oxygen

demand are uniform throughout the tank length.

Because of the rapid blending of feed and tank contents,

this process is highly resistant to shock loads.

4. Complete Mixing



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5. Extended Aeration

Extended aeration plants are generally small (applicable to

flow less than 1 MGD) because of the large aeration tanks

volumes required. Since it operates in endogenous respiration phase of the

growth curve which requires low organic loading & long

aeration time.

Theoretically the extended air process is designed such

that all substrate removed is channeled into energy

metabolism & oxidized so that no excess biomass is

produced & sludge handling is eliminated.

Since the oxygen requirement is high associated energy

costs is also high.



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Lag Log Declining Endogenous

High Rate

Conventional

Step Aeration

Contact Stabilization

Complete Mix

Extended Aeration



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Contact stabilization uses two separate tanks for the treatment

of the WW and the stabilization of the activated sludge

6. Contact Stabilization

Contact tank(adsroption)

Complete mix

Complete mix

Stabilization

tank

The first tank provides contact between the biomass and the

WW for a short period of time

20-40 min.The insoluble organic material is first adsorbed to the m.o. floc

surface, then solubilized.



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In the second aeration tank, the organic material which

is adsorbed on the biomass surface is metabolized or

“stabilized” (retention time 4-8 hr)

In these systems aeration volume requirements are

typically 50 percent less than conventional plug flowbecause contact tank operates at a short retention time.



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The flow scheme for sludge reaeration is similar to the

contact stabilization. But in this case it is assumed that all

substrate entering the reaeration tank is removed.

Thus no substrate will be present in the recycle from thereaeration tank to the aeration tank.

7. Sludge Reaeration

Aeration tank

Complete mix

Complete mix

Reaeration

tank



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For this particular process modification, a low MLSS

concentrations are combined with high volumetric BOD

loadings.

This system is characterized by short HRT, high sludge

recycle ratio, high F/M loading.

The subtrate removal efficiency is low (typically 60-75 %)

mainly because the plant effluent generally contains a high

solids concentration. (This high solids concentration is a

result of the physiological state of the organisms in the

aeration tank.)

Therefore, the high rate process can not be used where a

high quality effluent is required.

8. High Rate Activated Sludge



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Lag Log Declining Endogenous

High Rate

Conventional

Step Aeration

Contact Stabilization

Complete Mix

Extended Aeration



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High quality oxygen is used in stead of air in the

activated sludge process. This system is based on the principle that the rate of

transfer of oxygen is higher for pure oxygen than for

air.

This results in higher availability of dissolved oxygen

leading to improved treatment and reduced production

of sludge.

9. Pure Oxygen Aeration



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The oxidation ditch consists of aring or oval channel and is

equipped with one or more

rotating rotors for WW aeration.

Screened WW enters the ditch, isaerated and circulates at about

0.25-0.35 m/s.

Typically operation an extended

aeration mode with long

detention times.

10. Oxidation Ditch



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11. Orbal Process

The orbal process is a variation

of the oxidation ditch.

And uses a concentric channels

with the same structure.

Wastewater enters the larger

outer channel and flows towardthe center through at least two

more channels before entering

an internal clarifier or a

distribution box.

Disk aerators mounted on a horizontal

shaft provide aeration.

12 Sequencing Batch Reactor



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This is a fill and draw type reactor system.

In SBR operation the process are

carried out sequencially in the

same tank.

SBR systems have five steps in

common.

5. Idle

1. Fill

2. React

3. Settle(sedimentation clarification

4. Draw /decant

12. Sequencing Batch Reactor

13 Bi l P



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13. Biolac Process

Biolac® is a process that combines longsolids retention times with submerged

aeration in earthen basins.

A major advantage of the Biolac® system

is its low installed cost.

The BioFuser® fine bubble diffusers require no

mounting to basin floors or associated anchors

and leveling. These diffusers are suspended

from the BioFlex floating aeration chains.

Note:



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DESIGN EXAMPLE

Design a complete-mix activated sludge systemGiven: Avarage design flow: 0.32 m3/s (6.3 Mgal/d)

Peak design flow: 0.8 m3/s (21.9 Mgal/d)

Raw WW BOD5 : 240 mg/L

Raw WW TSS : 280 mg/LEffluent BOD5 20 mg/L

Effluent TSS 24 mg/L

WW Temperature : 20 oC

Operational parameters & biokinetic coefficients:θc = 10 d, MLVSS=2400 mg/L (can be 3600 mg/L), VSS/TSS =0.8TSS conc. in RAS =9300 mg/L, Y = 0.5 mg VSS/mg BOD5, kd= 0.06 /d

BOD5/ultimate BODu = 0.67

Assume: 1) BOD5 and TSS removal in primary clarifiers are 33 & 67 %respectively.

2) Specific gravity of the primary sludge is 1.05 and the sludge has 4.4% of solids content

3) Oxygen consumption is 1.42 mg per mg of cell oxidized.

Note:

Prepare a hand-out for exam!!



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Secondary clarifier design

Pilot plant study for settling tank

MLSS (mg/L) 1200 1800 2400 3300 4000 5500 6800 8100

Initial settlingvelocity m/h

4.1 3.1 2.1 1.2 0.77 0.26 0.13 0.06

Plot MLSS settling curve on log-log paper.

İ

n i t i a l s e t t l i n g m / h

MLSS conc. (mg/L)

From this fiqure determine the solid fluxes.



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X

MLSS

(mg/L)

V1

initial settling

Velocity, m/h

(X*V1)

Solid flux

kg/(m2.h)

1000 4.2 4.2

1500 3.7 5.55

2000 2.8 5.6

2500 2.0 5.

3000 1.5 4.5

4000 0.76 3.04

5000 0.41 2.04

6000 0.22 1.32

7000 0.105 0.74

8000 0.062 0.5

9000 0.033 0.3

2

4

S o l i d s f l u x ( k g / m 2 . h

)

Solids conc. in return

sludge mg/L

2000 4000 6000 8000 10000

Determine limiting solids flux for an

underflow concentration of 9300 mg/L

(desired underflow)

The limiting solid flux value = 1.3 kg/m2.h

D i fl t d l ifi Q



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Design flow to secondary clarifiers, Q

Q = avarage design flow + return sludge flow – MLSS wasted

= 27563 + 12942 – 283 m3/d

= 40222 m3/d

Use two clarifiers each one w/ flow of 20200 m3/d

Area of clarifier

SF

XQA

A: area of the secondary clarifier m2

Q : influent flow of the clarifier m3/h

X: MLSS concentration kg/m3

SF: limiting solids flux kg/m2.h

For each clarifier 2

2

331942

./3.1*)/1000*1/24(

)8.0//2400(/20200 mhmkgkggdh

mgdmA

A= π.r 2 =1942 r ≈ 25 m

Determine recycle ratio required to maintain MLSS conc at 3000 mg/L(Q+Qr) 3000 = Q X + Qr Xu Q (3000-X) = Qr (Xu-3000)

Qr/Q = 3000-X/(Xu-3000) = α =recycle ratio

When Xu = 9300 mg/L and X= 92.8 mg/L

α = 0.46

Q = influent flow

Qr= recycle ratio

X= influent SS concXu underflow SS conc

Operational Problems in Activated Sludge

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