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Activated Sludge 1

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• Suspended growth type• Unique – the name activated

sludge was originated in referringto the return sludge (biomass),since these masses ofmicroorganisms were observed tobe very “active” in removingsoluble organic matter fromsolution

Introduction

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Introduction

INFLUENT FROM

PRIMARYCLARIFIE

R

TREATEDEFFLUEN

T

WASTEDSLUDGE

SLUDGERETURN

 Aerationtank/reactor 

Sedimentationtank/secondary

settling tank/fnalclarifer 

%icroorganisms(activatedsludge&biomass) are

settled in the 'nal

learsupernatant

from the'nal clari'er

is the planteuent*+cess activated sludgeis wasted from thesystem to maintain the

properfood&microorganism

Two major

components

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-rganic matter decompose in aerobic condition

.ir (o+ygen) is supplied through/Mechanica aerator or di!u"er, -r both

.ir supply also gives #i$in% in wastewater ombination of suspended solids and

microorganisms is called &#i$ed i'uor"u" ended "oid" )MLSS*+

eration tan, 

-rganic %atter 0 -1   -1 0 21-0 new cells

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Aeration tank

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Diffusers

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Mechanical Surface Aerator 

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Solid separation unit inwhich the cells(biomass) from the

reactor are separated(settled) and returned tothe reactor

Secondary settling tank

/sedimentation tank

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.dvantages

• 2igh quality of euent (345 6-7removal)

• *uent quality is controlled by

sludge return

7isadvantages

• 8eed high s"ill labour

• 2igh capital, operation andmaintenance costs

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%odi'cation

•Step aeration

• 9apered aeration

• -+idation:ditch

• *+tended aeration (*.)

• Sequencing 6atch ;eactor

(S6;)

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Ste( Aeration-

<n$uent addition at intermediatepoints provides more uniform6-7 removal throughout tan"

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Ta(ered Aeration-.ir is added in proportion to 6-7e+erted

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-+idation 7itch/ plan view

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;eactor 7esign

• 9ype of reactor: plug:$ow system

: complete:mi+ system

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REACTOR DESIGN

and the conce(to. #a"" baance

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Ma"" /aance•

 9he mass balance concept is based on thefundamental physical principle that mattercan neither be created nor destroyed

• . balance on a continuous processes at

steady state may be written as/

<nput 0 =eneration > -utput 0 onsumption

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.eration tan" inal lari'er

INFLUENT EFFLUENT

WASTEDSLUDGE

SLUDGERETURN

6iomass <8?U*89 0 6iomass =;-!92 > 6iomass *?U*89 0 6iomass!.S9*7 S?U7=*ood <8?U*89 : ood -8SU%*7 > ood *?U*89 0 ood !.S9*7

S?U7=*

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Schematic f C mplete-Mix

eact r

INFLUENT0 RAW

SEWAGE

TREATEDEFFLUEN

T

WASTEDSLUDGE

SLUDGERETURN

 Aerationtank 

FinalClarifer

@o, So, Ao

∀, S, A@o 0 @r

A, S @o – @w

Ae, S

@u, Au@r, Au

@w, Au

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%ass balance for biomass

<nput 0 =eneration > -utput 0 onsumption

6iomass <n 0 6iomass =rowth > 6iomass -ut/io#a"" In 1 /io#a"" Gro2th 3 /io#a"" E4uent 1 /io#a"" Wa"te "ud%e

INFLUENT0 RAW

SEWAGE

TREATEDEFFLUEN

T

WASTEDSLUDGE

SLUDGERETURN

 Aerationtank 

FinalClarifer

@o

, So

, Ao

∀, S, A@o 0 @r

A, S @o – @w

Ae, S

@u, Au@r, Au

@w, Au

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*q BC

/io#a"" In

/io#a""

Gro2th

/io#a""E4uent

/io#a""Wa"ted

"ud%e

%ass balance for biomass – de'ned boundary (C)

INFLUENT0 RAW

SEWAGE

TREATEDEFFLUEN

T

WASTEDSLUDGE

SLUDGERETURN

 Aerationtank 

FinalClarifer

@o, So, Ao

∀, S, A@o 0 @r

A, S @o – @w

Ae, S

@u, Au@r, Au

@w, Au

uwewod 

 s

o

oo  X Q X QQ X k 

S  K 

 XS k  X Q   +−=−+

∀+   )()(

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*q BC

/io#a"" In

/io#a""

Gro2th

/io#a""E4uent

/io#a""Wa"ted"ud%e

!hereD@o,@w > <n$uent and waste:sludge $ow rate,mE&dAo, A, Ae, Au > biomass concentrations in in$uent,

reactor, euent and clari'erunder$ow(waste sludge), respectively,"g&mE

So, S > soluble food concentration in thein$uent and reactor, respectively, "g&mE

∀ > volume of reactor, mE

F s > half saturation constant,"g&mE

"o > ma+imum growth rate constant, d:C

> :C

%ass balance for biomass

uwewod 

 s

ooo   X Q X  )QQ(  ) X k 

S  K  XS k (  X Q   +−=−+

∀+

% b l f b (f d)

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%ass balance for substrate (food)

<nput 0 =eneration > -utput 0 onsumption

ood <n : ood onsumed > ood -utFood In 5 Food Con"u#ed 3 Food E4uent 1 Food Wa"ted Sud%e

INFLUENT0 RAW

SEWAGE

TREATEDEFFLUEN

T

WASTEDSLUDGE

SLUDGERETURN

 Aerationtank 

FinalClarifer

@o, So, Ao

∀, S, A@o 0 @r

A, S @o – @w

Ae, S

@u, Au@r, Au

@w, Au

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INFLUENT0 RAW

SEWAGE

TREATEDEFFLUEN

T

WASTEDSLUDGE

SLUDGERETURN

 Aerationtank 

FinalClarifer

@o, So, Ao

∀, S,A

@o 0 @r

A,S

@o – @w

Ae, S

@u, Au@r, Au

@w, Au

E'6768

FoodIn

FoodCon"u#e

d

FoodE4uent

FoodWa"ted"ud%e!hereD

 G > decimal fraction of food mass converted to

biomass

S QS QQS  K Y 

SX k S Q

wwo

 s

o

oo  +−=

+∀−   )(

)(

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.ll reactions occur in the reactor 9herefore, the volumeD ∀ representsthe volume of the reactor onlyH

INFLUENT0 RAW

SEWAGE

TREATEDEFFLUEN

T

WASTEDSLUDGE

SLUDGERETURN

 Aerationtank 

FinalClarifer

@o, So, Ao

 

, S, A

@o 0 @r

A,S

@o – @w

Ae, S

@u, Au@r, Au

@w, Au

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Fro# E'6 769 )#a"" baance .orbio#a""*

/io#a"" In

/io#a""

Gro2th

/io#a""E4uent

/io#a""Wa"ted"ud%e

A""u#(tion 9- 9hein$uent and euentbiomass concentrations arenegligible compared tobiomass at other points inthe system

E'6 76:

uwewod 

 s

o

oo   X Q X QQ X k S  K 

 XS k 

 X Q   +−=−+∀+   )()(

uwd 

 s

o  X Q ) X k S  K 

 XS k (    =−

+∀

uw

 s

o k  X 

 X Q

S  K 

S k +

∀=

+

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Fro# E'6 768 )#a"" baance .or.ood0"ub"trate"*

FoodIn

FoodCon"u#e

d

FoodE4uent

FoodWa"ted"ud%e

A""u#(tion 8- 9hein$uent food concentrationSo is immediately diluted to

the reactor concentration S

because of the complete:mi+ regimeA""u#(tion :- 9he foodconcentrations in wasted

sludge outlet is negligible

E'6 76;!here G > decimal fraction of foodmass converted to biomass > (mg&? biomass&mg&? food

utili#ed)

S QS QQ

S  K Y 

SX k S Q

wwo

 s

o

oo  +−=

+

∀−   )(

)(

)(   S S  X 

Y Q

S  K 

S k o

o

 s

o−

∀=

+

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#binin% E'6 )76:* and )76;* %i<e"-

E'6 76=

The h>drauic detention ti#e in the aerationtan,0reactor-

E'6 76?

d o

ouw

k  )S S (  X 

Y Q

 X 

 X Q

−−∀=∀

o

Qt   ∀=

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The #ean ce5re"idence ti#e )"ud%e a%e 0the a<era%e ti#e that ce )#icroor%ani"#"*

"(end in the reactor*

"titutin% E'6 )76?* and )767* into E'6 )76=*-

E'6 767

E'6 76@

uw

c

 X Q

 X t 

  ∀=

o

ck tX 

S S Y 

t −

−=

  )(1

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The <ou#etric oadin% rate L i" the #a""o. /OD in the inBuent di<ided b> the <ou#eo. the reactor

The concentration o. bio#a"" in the reactor)MLSS* i" .ound b> "o<in% E'6 )76@*-

E'6 76

E'6 769

)1(

)(

cd 

oc

t k t 

S S Y t  X 

+

−=

∀=∀  oo

 L

S Q

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The .ood5to5#icroor%ani"#" ratio i" u"ed toe$(re"" /OD oadin%" 2ith re%ard to the

bio#a"" in the reactor-

he recircuation ratio i"-

E'6 7699

E'6 7698

 X 

S Q

 M 

 F    oo

∀=

Q

Q R   r =

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*+ample – .ctivated Sludge

An acti!ated sludge system is to "e used for secondarytreatment of #$%$$$ m&/d of municipal wastewater'

After primary clarification% the ()D is #*$ mg/+% and it

is desired to ha!e not more than * mg/+ ()D in the

effluent' A completely mixed reactor is to "e used% and

pilot-plant analysis has esta"lished the following

kinetic !alues, . $'* kg/kg% kd . $'$*d-#' Assuming an

M+SS concentration of &$$$ mg/+ and an underflow

concentration of #$%$$$ mg/+ from the secondary

clarifier% determine,

a the !olume of the reactor " the mass and !olume of solids that must "e wasted

each day

c the recycle ratio

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Proce"" Bo2 chart

 Aerationtank 

FinalClarifer

Qo = 10000 m3/d

So  = 0.15 kg

BOD/m3

X (!SS" = 3.0kg/m3

0

 X# = 10.0 kg/m3

Se = $0.005 kgBOD /m3

. $'* kg/kg% kd . $'$*d-#

 Q%X#0r u

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Soution-C Select tc > CI day, solve *q(BJ) with t > ∀&@

∀ = 1611 m3

o

c

k  X 

S S QY 

t −

−=

  )(1

05.00.3

)005.015.0)(5.0(000,101.0   −∀

−=

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1 .t equilibrium conditions, *q(BB) applies

%ass ⇒ 

Kolume ⇒ 

uw

c

 X Q

 X t   ∀=

c

uW 

 X  X Q  ∀=

daykg  X QuW 

  /3.48310

)0.3(1611 ==

daymmkg d kg Q

W   /3.48

/10/3.483   3

3==

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E . mass balance around the secondary clari'ercan be written as follows/

.ssuming that the solids in the euent arenegligible compared to the in$uents and

under$ow ( Xe LL X )/

uwr ewor o  X QQ X QQ X QQ   )()()(   ++−=+

uwr r o  X QQ X QQ   )(0)(   ++=+

 X  X 

 X QQX Q

u

uw

−=

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∴ 9he recirculation ratio is

d mmkg mkg 

d kg mkg d mQr 

/4217/3/10

/3.483))/0.3(/000,10(

3

33

33

=

−=

42.0000,10

4217===

Q

Q R   r 

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*+ample – *+tended .eration

 An &'tended Aeration S) treat* 1500 m3 o+*e%age dail, %it- a BOD onentration o+300 mg/!. Cal#late t-e ol#me re#ired +ort-e aeration omartment. 2ene -ek t-e2,dra#li 4etention ime (24" and t-e

 ol#metri loading. Determine t-e #antit, o+*l#dge %a*ted er da,. A**#me a !SSonentration o+ 3500 mg/! and a Solid*4etention ime (S4" o+ 5 da,*.

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Proce"" Bo2 chart

 Aerationtank 

FinalClarifer

Qo = 1500 m3/d

So  = 0.3 kg

BOD/m3

X (!SS" = 3.5kg/m3t (S4" = 5

d0

 Q%X#0

240

0

 L∀

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Solution

1. Find volume

C-oo*e F/ ratio a*6

0.1 kg BOD/kg !SS.da,

 X 

QS 

 M 

 F    o

∀=

5.3

3.015001.0

×∀

×=

31286m=∀∴

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= 20.6 hoursOK (18 < HRT < 24)

2. Find HRT

!15001"#  $R%    ×=

oQ )t (  $R%   ∀

=

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3. Find volumetric loading

= 0.35 kgBO/m3 

(OK ! 0.16 < < 0.40) 

∀=∀   oo

 L S Q

1286

3.01500×=

 L∀

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4. Find uantity o! sludge "asted

∴ "u#$%&%' o su*g+ #s%+*, Qw X u  =

= 180 kg/*#'

uw X Q

 X SR% 

=

25

5.31286×

%ass balance for biomass – de'ned boundary (1)

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/io#a"" In

/io#a""

/io#a""E4uent

/io#a""Wa"ted

%ass balance for biomass de'ned boundary (1)

.eration tan" inal lari'er

INFLUENT EFFLUENT

WASTEDSLUDGE

SLUDGERETURN

uwewo   X Q X  )QQ( QX   +−=


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