6.Activated Sludge Process

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6. Activated Sludge Process : Introduction

• The Activated sludge Process:

1) The mostly widely used biological process for the treatment of

municipal and industrial wastewaters.

2) Strictly aerobic except anoxic variation for denitrification.

, , ,

iv) a sludge wasting line4) activated sludge : microbial aggregates (flocs) in the aeration tank

5) In 1914, E. Arden and W.T.lockett discovered the activated sludge

process in England.- They noted aeration of sewage led to formation of flocculent suspended

particles and the time to remove organic contaminants was reduced when theseflocculent particles held in the system. They referred to the suspended particles

as being “activated” -.



6. Activated Sludge Process : Introduction

• The Activated sludge Process:

n , anc es er orpora on u a m pan .

7) Successful application of the process occurred even though an understandinof how the the process actually worked was lacking :

- Man articles debated over whether the removal obtained was h sical or biolo ical.

- By 1930, the evidence in favor of a biological process was sufficiently convincing.However, an adequate theory about factors affecting removal rates was not then avail

- By 1950s and 1960s, a theory of operation has developed and was sufficient so thatrational designs could be achieved based upon characteristics of wastewater to

treated.

- It has still problems (like sludge bulking )or uncontrollable factors (like system ecology,“ microbial population dynamics”).

- In late1980s, Membrane Bioreactor (MBR) was introduced to enhance the existing

conventional system.



6.1.1 Microbial Ecology

• Two crucial characteristics :

1)The activated sludge contains a wide variety of microorganisms.

- Prokaryotes : Bacteria

- Eukaryotes : Protozoa, Crustacea, Nematodes, Rotifers

- Virus : Bacterio ha e

2) Most of them are held to ether within flocs b naturall

produced organic polymers (EPS) and electro static forces.






• Most of the other organisms are secondary consumers that feed off of

- by- products of BOD degradation- y-pro uc s rom e ea an ys s o o er organsms

• Predators, most of which are eukaryotes feed on bacteria and

bacteriophage

• Chemolithotrophic bacteria are sometimes present and obtain theirenergy from oxidation of inorganic compounds (NH4

+, NO2-, S-2, and Fe+2 ).




•

take place over time :

energy resources available in waste mixtures.

anges o e npu s an envronmen a con ons emp, , , p ,

inhibitory chemicals, nutrient availability, fluid shear, etc.)

iv) Changes in the floc’s physical characteristics : 1) aggregation strength, 2),




• The ma orit of the bacteria enera in activated slud e are Gram-

negative.

- however, recent studies using oligonucleotide probes show that Gram-positivebacteria are significant in activated sludge, too.

• Man s ecies of rotozoa have been identified in activated slud e

- order of 50,000 cells/mL (Pike and Curds, 1971)

- .

of the ciliated protozoa indicates a stable sludge.

- they tend to be highly sensitive to toxic chemicals.Hence the healthy protozoan population is indicative of a wastewater that is

relatively free of toxic chemicals

- their resence and activit are readil observed with a low- owered microsco e.

.




• Rotifers, nematodes and other multicellular forms often are found

in activated sludge system,

- Their roles in the process are not obvious.- .

• The role of bacterial viruses or phages in the overall process is

not well documented.- Their presence can cause rapid and large shifts in dominant bacterial

.

- If one species is decimated by a phage, another can replace it rapidly so

that significant perturbations in treatment efficiency are not detected

( “Redundancy ”).




•

resources, subtle changes in the treatment process can result in

ma or chan es in the microbial com osition and the floc h sical

characteristics.

- Factors that affect the microbial ecology of activated sludge:

1) Reactor system, 2) Dissolved oxygen level, 3) nutrient availability, 4) temperature, 5) pH,

6) inhibitory materials, etc..

For example, CSTR and PFR systems foster growth of quite different microorganisms, even

when the input substrate and the SRT are identical. It is because CSTR tent to maintain

consistently low substrate concentrations, while PFR tend to create more of a “Feast and

Starve” cycle.



6.1.2 Oxygen and Nutrient Requirement

¾In most situations, the electron donor(BOD) is rate-limiting for

microorganism reproduction and growth. It means that nutrients

and e- acceptor (O2) have concentrations well above their half-

saturation concentration, or K .Æ

S / (K+S) = ~1 in Monod eq when.

• For example, Dissolved oxygen : K < 1 mg/L, If DO > 2 mg/L, then O2 is

far from rate limiting.

- The literature is not definitive about just what K is for nutrients (N, P, Fe,

S, Zn, Cu, Mo, and other trace constituents). But the value appears to be

quite low, much less than 1 mg/L.

• The ox en consum tion rate is ro ortional to the rate of donorsubstrate utilization and biomass endogenous decay.

•

synthesis rate of biomass.



6.1.3 Impacts of Solids Retention Time

• SRT (θ x ) is commonly used to control not only i) treatment

efficiency of wastewater but also ii) sludge physical and biological

characteristics.

.

]39.5[1∧

+=

b K S xθ

1)-( −bqY x

θ

.r k K )r k K X q )r k K q( 3834XUAP ut1UAP

2

ut1UAPaUAPut1UAPaUAP θ θ θ θ θ −+++++−=

.

22

X k K ) X )k q( K ( )k q( K ( 4)X a2BAP

2

a2BAPBAPa2UAPBAPθ θ θ +−+−+

.22

−




3) Altering the SRT can lead to changes in sludge physiology such as

settling characteristics, EPS production,etc.

4) For various reasons, a long SRT often is not beneficial, even if the

substrate conc. can be driven lower.

- θ x is generally limited to a range between 4 to 10 days when BOD

removal and economics are to be balanced.

- x

removals of BOD deteriorate.



a) Pin-point floc

b) Normal floc




- Operation at long SRT (> 10days) allows for the accumulation of slower

rowin or anisms that are washed from the s stem if the SRT is short.

- Man of the microor anisms that can cause o erational roblems

(bulking and foaming) are relatively slow growers, compared to thebacteria that from the desirable compact floc.

- The chemolithotrophs, particularly the nitrifying bacteria, are slow

rowers that can exist in activated slud e onl when the SRT is relativellong.



• Foaming Bacteria



6.2 Process Configurations

▣ Modifications of basic activated sludge process

: trial-and-error efforts to overcome problems in activated sludge operation

s nce w en r en oc e rs scovere .

: The designer can select combinations from the three different categories.



6.2.1 Physical Configurations




1) Plug-Flow

- lon narrow aeration tan

- kinetic theory : greatest contaminant removal within a defined

treatment time- problems : high conc. of contaminants at the head end of

the aeration tank (Fig. 6.2)

Æ complete depletion of dissolved oxygen (anoxic condition)

Æ detrimental to microor anisms or anic acid roduction and

a drop in pH)

ii) industrial wastes contain substances that are inhibitory tothe bacteriaÆ slowing down or stopping the process




1) Plug-Flow




2) Step Aeration (= Step Feeding)

- distributing the influent along the length of the reactor in steps

- the concentration of influent contaminant is diluted much moreand the oxygen-uptake rate is spread out (Fig. 6.2)

Æ overcome the two problems associated with plug-flow

- Effect of step aeration

mxe quor suspen e so s s g es a e ne en

since the full sludge recycle mixes with only part of the influent flow.

concentration in plug flow, which increases the SRT for the same




3) Completely-Mixed (CSTR with settling and recycle)

- Evolved in the 1950s when reactor modeling begins.

“The simplest system for reactor modeling for biological processes ”

- Ultimate approach for spreading the wastewater uniformly

throughout the treatment system.

- .

the substrate is biodegradable.- Contaminant concentration and oxygen demand do not vary

over the reactor length (Fig. 6.2)

-most favorable with wastewaters containing nonbiodegradable materials

, , , .

are toxic to microorganisms at the modest concentration.

-

as high as in a well operating plug-flow system




4) Contact Stabilization

- g e c ency rea men re uces s gn can y o a reac or vo ume.

i Contact tank: wastewater mixed with activated slud e

HRT = 15 ~ 60minÆmost of readily biodegradable organic contaminants are oxidized

or s ore ns e e ce s, an e par cua e ma er s a sor e o e

activated sludge flocs.

ii) Settling tank: activated sludge and the treated wastewateare separated

iii) Stabilization tank: settled and concentrated activated sludge

,

stored substrates, and biomass are oxidized.




- Advantage of Contact Stabilization : reduction in overall reactor volume

ex) If sludge production is 1,000 kg MLSS/day,, , , ,

four fold concentration at settling tank. Then θ x = 8,000/1,000 = 8 d.

2,000 kg MLSS : 25%8,000 kg MLSS in one tank : 100%

Assume 4-fold concentrat

at settling tank

6,000 kg MLSS : 75%( 4-fold concentrated sludge)

then, required reactor volume for contactIf the reactor volume is 100m３

stabilization

: 25 + 75/4 = 43.8m３

for a conventional activated

sludge system,




- Disadvantage of Contact Stabilization:

i) requires substantially more operational skill and attention

,

are necessary to compute the SRT.

iii) the small volume of the contact tank makes the effluentquality susceptible to sudden increases in loading




5) Activated Sludge with a Selector

-To solve the failure of activated sludge system: sludge bulking (=

poor settleability )

- e ec or an : o c ange e eco ogy o e ac va e s u gesystem towards organisms with good settling characteristics.

( The filaments do not form storage material, while some floc formers do.)

1) Return activated sludge is contacted with the waste stream for only 10~ 30 min where complete BOD reduction is impossible. Fermentationreactions then converts carbohydrates and some proteinaceousma eras o a y ac s, w c canno e ox ze u s ore ymicroorganisms in the form of glycogen or polybetahydroxybutyric acid(PHB).

when they enter the oligotrophic environment of the normal aerationtank.

3 Fortunatel the bacteria able to store these materials also are ood at forming compact sludge floc.



6.3 Design and Operating Criteria

6.3.1. Historical Background

- Criteria used for the design and operation of activated sludge

ran e from those totall em irical to those soundl based infundamentals.

- When the activated sludge process was first invented in 1914,there was no understanding of kinetics of biological growth andsubstrate removal.Æ designs were based on empiricism.

-

1) HRT, SS,ÅÆ BOD5 removal

rganc oa ng, , xygen suppy 5 remova

3) MLSS Æ MLVSS



6.3 F/M ratio

- -

6.3.2 Food-To- Microorganism Ratio (F/M ratio)

and 1960s and still widely used because of its simplicity.

]1.6[/O

VX

Q M F

O

=

F/M = food-to-microorganism ratio,

kg BOD or COD applied per day per kg of total suspended

solids in the aeration tank

QO = influent wastewater stream flow rate (m3 /d)

V = aeration-tank volume (m3)

=



6.3 F/M ratio

- If volatile SS are used.

]2.6[/

S Q

Mv F

OO

=F/Mv = food-to-microorganism ratio on volatile solids basis, kgBOD orCOD per day per kg of volatile suspended solids in aeration tank

v =

- For a conventional design for the activated sludge treatment of ,

The F/M ratio suggested is 0.25 ~ 0.5 kg BOD / kg MLSS•d

, = , = , ,F/Mv = 0.5 kg BOD / kg MLSS•d

- g -ra e rea men : ~ g g •

Extended-aeration : 0.12 ~0.25 kg BOD / kg MLSS•d



6.3 F/M ratio

]17.3[)(ˆ

0 S S QV X S K

S q O

a

−+

+

−=

= maximum specific rate of substrate utilization (Ms / L3 T)

K = the Monod half-maximum-rate constant (Ms/L)

q̂

]3.6[)(ˆV

S S Q X S K

S qOO

a −=+

ˆ ⎞⎛

OOO

QqS Q . ⎠⎝ +

=aa VX S K VX

o When S is small left >> ri ht termF/Ma

K + S ~ K



6.3 F/M ratio

ˆ ⎞⎛ =

OOO QqS Q

ˆ

. ⎠⎝ + aa VX S K VX

]5.6[/ K

a =

]6.6[ˆa M F qS ⋅≈

- Thus, for the usual case in which we have high treatment

efficiency and a low effluent BOD concentration,

- However, M a is almost impossible to measure, which

a

e F torelated directlyisS /

a .



6.3 F/M ratio

- S can be estimated if we know the Ma,

l mgBOD K /4 5=

.ˆ

a

q⋅

l d mg K

d kgVSS kgBODq a

/4.0ˆ

/10ˆ5

⋅=

⋅=

d kgMLSS kgBOD F

q

/5.0 5 ⋅=F

oaa

7.1

.

=

==

l mg S /2.05.04.0 =×= l mg S /67.07.14.0 =×=

- The units of F/M ( ) are almost the same as

To have a good safety factor , we must have the ratio be far less than one.

q̂d kgMLSS kgBOD ⋅/5d kgVSS kgBOD a ⋅/5

q M F q M F ˆ/,1ˆ/ <<



6.3.3 Solids Retention Time

oDefinition

]7.6[wwee x

XV =

θ

V = system volume [ L3],

Qe = effluent flow rate [ L3 T-1]

Qw= waste-slud e flow rate [L3 T-1

X, Xe, Xw = the concentrations of mixed-liquor, effluent,

and waste sludges in consistent mass units, which can beactve vo at e so s, vo at e so s , or suspen e so s.

* All the parameters that comprise eq 6.7 can be measured accurately and consistently.




- As long as active biomass is not an input, any of the three solids

measurements can be used for the X values in Eq.6.7 and give the

same correct value of θ x.

- ,

measured, to estimate θ x is a major practical advantage.

- Typical values of θ x= 4 ~ 10 d

- x en e aera on un s genera y ave muc onger

θx in the range of 15 to 30 d, and sometimes longer.

- x . . .

6 3 3 S




, x

operation of the AS process,

- because it is fundamentally related to the growth rate of theactive microorganisms,

- which in turn controls the concentration of the growth-rate-

m ng su s ra e n e reac or.

]22.3[= biomassactiveof rate production

= 1- μθ x

]9.3[-

∧

bS q

Y =μ

6 3 3 S lid R t ti Ti




- This important direct relationship between effluent SS and effluentBOD forms one basis upon which the typical designs leading to a

- θxvalues have evolved fromempirical practice over the years

x .

- Effluent BOD = BOD of effluent SS + SMP + residual substrate

- If good solids separation is not achieved, the oxygen demand

from decay of active cells can overwhelm the soluble components.

- Where high BOD removal efficiencies are desired, the effluent SS

.

- Thus, the settling abili ty of the activated sludge and the efficiency

of the final clarifier take on paramount importance.





- This important direct relationship between effluent SS and effluent

θ x of 4 to 10 d originated.

- x ,

effluent SS concentrations are fairly high

- onger x va ues : ac era ocs a so en o rea up an sperse

9

population,

which through polymer production tend to hold the individualparticles together in floc,

or to the destruction of the floc through the action of predatory

, ,





oc rea up s o en no e o egn w θ x grea er an

at temperature of 20 oC, or at some longer times with lower

temperature.

9 Thus, the θ x range of 4 ~ 10 d represents a zone where

biological flocculation and clear effluents appears to be

optimal.

9 It is the preferred range for design of well-operating and

efficient activated sludge treatment systems.





Hi her SRTs allow the rowth and accumulation of slow

osecond basis for the conventional range of 4~10 days:

growing microorganisms that are not desired.

1) Nitrifying bacteria

When the oxidation of ammonia is not a treatment

goal,having nitrifier is undesirable for three reasons.

i)ammonium oxidation creates a very large oxygen demandii)the nitrifiers release a significant amount of SMPs

iii)the nitrifiers generates a significant amount of acids

which can be a roblem in low-alkalinit waters.

]1.9[6

1

3

1

6

1

4

1

6

12224 O H H NOO NH ++=+ +−+

]2.9[

2

1

4

1

2

1322−− =+ NOO NO

6 3 3 Solids Retention Time




Hi her SRTs allow the rowth and accumulation of slow

osecond basis for the conventional range of 4~10 days:

growing microorganisms that are not desired.

2) Filamentous bacteria

oCauses bulking





. .

]7.6[wwee

x

X Q X Q +=θ

V = system volume [ L3

],Qe = effluent flow rate [ L3 T-1]

Qw= waste-sludge flow rate [L3 T-1]

X, Xe, Xw = the concentrations of mixed-liquor, effluent, andwas e s u ges n cons s en mass un s, w c can e ac vevolatile solids, volatile solids , or suspended solids.





oOught to include biomass in the settler as well as

in the aeration tank

oWhat XV to use for settler ?

settler is equal to that in the aeration basin

oXV = X (Vaer + Vset);

Vaer

= Volume of aeration basin

Vset = Volume of the settler

6.3.4 Comparison of Loading Factors



6.3.4 Comparison of Loading Factors

designed very conservatively, SRT: 25- 50 d, sometimes even larger.

x = es gn va ue or x

F/Mv ratio is inversely proportional to the SRT

6 3 5 MLSS the SVI and the Recycle Ratio



6.3.5 MLSS, the SVI and the Recycle Ratio

Choice of mixed-liquor suspended solids concentration (X)

•

i) the settling characteristics of activated sludge

ii) the rate of recycle of sludge from the settling tank back to the aeration tank

iii) the design of the settling tank

• Advantage :- lead to smaller aeration basin, which translate into

High value for X

lower construction cost

• Disadvantage:

Aeration tank

-

- increase in the cost of aeration system

- increasing X requires the recycle sludge of a faster rate

- high X leads to high effluent SS and BOD

• Clearly, an arbitrary choice for MLSS is very risky





The relationship between X and the return sludge flow rate

‘ ’

r Q

]10.6[ s seei X Q X Q X Q += ]11.6[ sr X X =

0 w sr X X ==0

)0(

r w sr

e s si X X Q X Q

<<→=

→=

• A mass balance on suspended solids around control volume ‘b’

a

r i QQQ += 0

0

Q

R r =]12.6[)(1 X X Ror R X X r

r

−=

+=

b

Figure 6.1 Total suspended solids flow within a complete-mix activated sludge system





• Because of sludge settling characteristics,

and SVIr m X

the recycled sludge has upper limit of the recycled sludge(= )r

m X

]12.6[1 R

X X r

+= ]14.6[

1 R

r

mm+

=

X of Maximum X m :

r m X = 10,000 ~ 14,000 mg/L for typical good-settling activated sludge

= 3 000 ~ 6 000 m /L for bulkin slud es

• can be approximated through simple tests :r

m X

I) the Settled Sludge Volume Test, 2) the Sludge Volume Index (SVI),3) the Zone Settling rate Test





•SVI is defined as the volume in milliliters occupied by 1g of the suspended solids after settling

]15.6[)/000,1(

)/( 30

t V MLSS

g mg V gSS ml SVI

⋅

⋅=

- V 30 : the volume of the settled sludge after 30min (unit : ml)

- V t : the total volume of cylinder (unit : l)

6.3.5 Weak points of SVI



p

6.3.5 Weak points of SVI



p




, y

•An approximation to the maximum concentration

)/(106 l g ml mg X r m

⋅⋅=

• SVI and with sludge typer

m X

SVI (ml/g) (mg/l)

r

m X g m

sludge

100 10,000

Bulking sludge >200 < 5,000

Highly compactand good-settling

sludge< 50 > 20,000




y

•The effect of the recycle ratio on for various of m X r

m X

<Good sludge>

= 10,000 mg/l and = 2,000 mg/lr

m X

1

= .

< Bulking sludge>

= 2,500 mg/l and = 2,000 mg/l

R = 4

r

m X

2 <Good sludge>

R = 0.7 and = 4,000 mg/l

= 10,000 mg/lm

<Bulking sludge>

. ..

= 5,000 mg/l and = 4,000 mg/l

R=4

r

m

6.5 Bulking and Other Sludge Settling Problems



Jenkins(1992)

: different solids

-

6.5.1 Bulking Sludge



g g

• Bulking sludge

Bulking : the formation of activated sludge flocthat settles slowly and compacts poorly

: difficult removal of the slud e fromthe settlin tank for return to the aeration basin

Sludge blanket rising- Massive loss of biomass- Decrease the SRT in uncontrolled manner

Activated sludge solids in the effluent

- Destroying effluent SS and BOD quality

o a a ure o e ac va e s u ge process




• Formation of Sludge bulking

Floc microstructure with backboneof filamentous bacteria

Too many filamentous bacteriaoutside the compact floc

macrostructure

between flocs

• Effects of extended filaments’ bridges causing sludge bulking- revent the flocs from comin close to ether or com actin

- trap water within and between the flocs

-




• Onset of sludge bulking

• Microscopic examination- Identification through steady trend of more extended filaments

b a trained technician

•

- serious bulking : SVI >200 mg/L- very bad bulking : SVI≫ 500 mg/L

• Rising sludge blanket and

a low concentration of sus ended solids in the settler underflow




• Cause of sludge bulking

- ow – u ng y amen ous ac era p aero us na ans a ave

good affinity for dissolved oxygen ( a low K for O2 ). They begin to predominate

when the DO is not enough to allow good oxygen penetration into the floc.

-Low – F/M bulking by filamentous bacteria (Microthrix parvicellar )that have a

high affinity for organic substrates (a low K ) and a low endogenous decay rate (low

b).

-Reduced–sulfur bulking by filamentous bacteria(sulfur-oxidizing species,

Thiothrix ) that gain a competitive advantage from the chemolithotrophic

electron donnor (reduced sulfur compounds).




• Low-F/M bulking

- ong suc as ex en e aera on

- Microthris parvicella, Type0041, Type0092, Type0581,

• Situation with extended aeration in low-F/M bulking

- gotrop s avng a g a nty or organc su strate

- Low endogenous decay rate




-

• Reduced-sulfur bulking

- Sulfur-oxidizing species : Thiothrix , 021N

- -to eliminate all inputs of reduced sulfur

−−

34g/ M H2O2 x 4 = 136g-

++→+ 2422

=4.25 g H2O2 is needed to oxidize one g S

- ormaton o su es n a reactor rom su ate n ee wt n t e s u ge ocdue to D.O. depletion. Increased D.O.(or NO3

-) concentration is neededto prevent sulfate reduction

6.5.2 Foaming and Scum Control



• Formation of foam or scum on the surface of aeration tanks

Problem - Excessive suspended solids in the effluent

- slippery walkways around them

-

• Cause- long SRT and high wastewater temperatures- causative organisms (Nocardia and Microthrix)

•

- reducing SRT to 6 d or less- chlorination of return activated sludge

6.5.3 Rising Sludge



• Rising Sludge in the settling tank with nitrification

• entr caton n t e s u ge an et o t e sette

- gas bubbles (N2) attach to the settled sludge particles

-- These pieces of sludge blanket can increase in effluent suspended solids

- sto nitrification in the activated slud e•Solution

( No nitrate formed by nitrification No N2 gas by denitrification)

- reduce SRT and wash out the slow- rowin nitrifiers - promote denitrification as part of the activated sludge process

6.5.5 Viscous Bulking



• Viscous Bulking

- orm o non amentous u ng

- excess of extra-cellular polymer produced by floc-forming bacteria

Moderate amounts of polymer: causin bacteria flocculation

Excess amounts of polymer: detrimental to the settlin of

for good floc formation the bacterial flocs

-by sludge flocs (jelly-like)

- Poor settling caused by the high water content of the polymeric material

6.5.6 Addition of Polymers



• Quick fix solution

- between the aeration basin and the settling tankto enhance flocculation, settling and compaction

• Advantages

- effective for relief from a rising sludge blanket,

dispersed growth, or pinpoint floc

- prevention of the loss of suspended solids

• Disadvantages• - loss of the effectiveness : biodegraded by the microbial community

,which adapts to it over time.Æ rising the required dosage over timeÆ increase of the polymer cost

- normal selection process for natural floc formers is short-circuited.

us e commun y ecomes ess an ess enrc e n e goo ocformers that are needed if polymer addition is to be stopped.

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6.Activated Sludge Process