Lecture 3

Activated sludge and lagoons

ACTIVATED SLUDGE PROCESS

Activated sludge process is used during secondary treatment of wastewater. Activated

sludge is a mixture of bacteria, fungi, protozoa and rotifers maintained in suspension by aeration

and mixing [1].

In this process, a biomass of aerobic organisms is grown in large aerated basins. These

organisms breakdown the waste and use it as their food to grow themselves.

Activated sludge processes return settled sludge to the aeration basins in order to

maintain the right amount of organisms to handle the incoming "food".

Activated sludge processes have removal efficiencies in the range (95-98%) than

trickling filters (80-85%). [2]

WORKING OF ACTIVATED SLUDGE SYSTEM [3]

A primary settler (or primary clarifier) may be introduced to remove part of the suspended solids present in the influent and this reduces the organic load to the activated

sludge system.

The biological reactor or aeration tank is filled with a mixture of activated sludge and influent, known as mixed liquor. It is necessary to maintain certain mixed liquor

suspended solid (MLSS) in the aerated tank maintain good removal efficiency.

The aeration equipment transfers the oxygen necessary for the oxidation of organic material into the reactor, while simultaneously introducing enough turbulence to keep the

sludge flocs in suspension.

The continuous introduction of new influent results in a continuous discharge of mixed liquor to the secondary settler where separation of solids and liquid takes place.

The liquid leaves the system as treated effluent, whereas some part of the sludge is recirculated to the aeration tank called as return sludge and rest of sludge is taken for

anaerobic digestion.

DESIGNING OF ACTIVATED SLUDGE SYSTEM

Suppose, Q is the flow rate of influent (m3/d), QW is the flow rate of waste sludge (m3/d),

Qr is the flow rate of return activated sludge (m3/d), V is the volume of aeration tank (m3), S0 is

the influent soluble substrate concentration (BOD g/m3), S is the effluent soluble substrate

concentration (BOD g/m3), Xo is the concentration of biomass in influent (g VSS/m3), XR is the

concentration of biomass in return line from clarifier (g VSS/m3), Xr is the concentration of

biomass

VSS/m3)

(a) E

(b) M

F

R

X

X

Q

in sludge d

[4]. VSS sta

Equations us

Mass balanc

or Xe =0

Recycle ratio

CW r

VXQ X

0C

QY S1VX

rQQX

rX Q Q r

r

QX QQX

drain (g VS

ands for vola

Figu

sed for desig

e around cl

=

0d

Sk

X

eW XQQ

r WQ Q X W rQ XX

QQr

SS/m3) and

atile suspend

ure 4.3.1. Ac

gn of aerati

arifier

rWe QQX

rX

Xe is the c

ded solids.

ctivated slu

ion tank

rr XQ

concentration

udge process

n of biomas

s

ss in efflueent (g

Problem 4.3.1: An activated-sludge system is to be used for secondary treatment of 15,000 m3/d

of municipal wastewater. After primary clarification, the BOD is 170 mg/L, and it is desired to

have not more than 25 mg/L of soluble BOD in the effluent. A completely mixed reactor is to be

used, and pilot-plant analysis has established the following values: hydraulic detention time

( )=10 d yield coefficient (Y)=0.5 kg/kg, kd=0.05 d-1. Assuming an MLSS concentration of

4500 mg/L and an underflow concentration of 12,000 mg/L from the secondary clarifier,

determine (1) the volume of the reactor, (2) the mass and volume of solids that must be wasted

each day, and (3) the recycle ratio.

Solution: Given that Q=10,000 m3/d, =10 d

Using

V=1611 m3

Using =10

If the concentration of solids in the underflow is 12,000 mg/L

For Xe =0

C

C

0d

C

QY S S1 kVX

3 3 31 -1

3

15,000 m /d 0.17 kg/m 0.025 kg/m0.1 0.05 d

4.5 kg/m d V

CW r

VXQ X

W rQ X =724.95 kg/d

33

724.95 kg/d 60.41 m /d12 kg/m

WQ

3 33

3 3

15,000 m / 4.5 kg/m 724.95 kg/d 8903.34 m /d12 kg/m 4.5 kg/m

W rr

r

QX Q X dQX X

rQ 8903.34Recycle ratio 0.59Q 15,000

PONDS AND LAGOONS

Other than activated sludge processes, ponds and lagoons are most common suspended-

culture biological systems used for the treatment of wastewater.

A wastewater pond, alternatively known as a stabilization pond, oxidation pond, and

sewage lagoon, consists of a large, shallow earthen basin in which wastewater is retained long

enough for natural purification processes.

Classification of lagoons is based on degree of mechanical mixing provided.

Aerobic lagoon: The reactor is called an aerobic lagoon, when sufficient energy is supplied to

keep the entire contents, including the sewage solids, mixed and aerated. To meet suspended-

solids effluent standards, solids are removed from the effluent coming from an aerobic lagoon.

Facultative lagoon: In facultative lagoon, only enough energy is supplied to mix the liquid

portion of the lagoon, solids settle to the bottom in areas of low velocity gradients and proceed to

degrade anaerobically and this process is different from facultative pond only in the method by

which oxygen is supplied. Facultative lagoons are assumed to be completely mixed reactors

without biomass recycle [5].

Aerobic lagoons with solid recycle: The aerobic lagoon with solids recycle is same as extended

aeration activated-sludge process, but an earthen (typically lined) basin is used in place of a

reinforced-concrete reactor basin. It is necessary that the aeration requirement for an aerobic

lagoon with recycle must be higher than the values for an aerobic flow-through lagoon to

maintain the solids in suspension.

DESIGN OF LAGOONS

Process design considerations for flow-through lagoons [6]

BOD removal Effluent characteristics Temperature effect Oxygen requirement Energy requirement for mixing Solids separation

AB

W

=hydrauIf

the next.

A

relating t

significan

W

coefficien

Problem

and 6600

the avera

Q

SS

SS

k

Applying ma

BODin = BOD

Where, S/S0=

ulic detentio

f several rea

A substrate

A wide rang

to both the re

ntly. k value

Where, k20 =

nt ranges fro

m 4.3.2: Was

0 m3/d durin

age tempera

0QS QS V

0S 1S 1 k V

n0S 1 S 1 k

T-20T 20k k

ss balance o

Dout + BODco

=fraction o

n time (d-1),

ctors are arr

balance wri

ge of values

eactor and w

e at any temp

reaction rat

om 1.03 to 1

tewater flow

ng the summ

ature of the w

V kS

1V Q 1 k

n1

n

0

n lagoon giv

onsumed

f soluble B

V= reactor

ranged in ser

itten across a

for k is av

wastewater af

perature can

te constant a

.12.

w from a sma

mer. The ave

warmest mo

ven in above

BOD remain

volume (m3)

ries, the efflu

a series of n

vailable in t

ffect the valu

be find out b

at 20C (ran

all communi

erage temper

onth is 30C

e figure

ning, k=rea

), and Q= flo

uent of one p

reactors resu

the literatur

ue of k, wate

by following

nges from 0.

ity averages

rature of the

C. The avera

action rate

ow rate (m3/

pond becom

ults in follow

re. Although

er temperatu

g equation:

.2 to1.0) and

3400 m3/d

e coldest mo

age BOD5 is

coefficient

/d).

mes the influe

wing equatio

h many vari

ure affects it

d =temper

during the w

onth is 10C

s 200 mg/L

(d-1),

ent to

on:

iables

most

rature

winter

C, and

with

70% being soluble. The reaction coefficient k is 0.23 d-1 at 20C, and the value of temperature

coefficient is 1.06. Prepare a preliminary design for a facultative pond treatment system for the

community to remove 90% of the soluble BOD.

a) Find volume of facultative lagoon to remove 90% of the soluble of BOD.

b) Find the dimensions of three square lagoons in series with depth 1.5 m.

Solution:

(a) Estimation of rate constants at given temperature

Summer: 30 20 -125 0.23 1.06 0.411 d k Winter: 10 20 -110k 0.23 1.06 0.128 d (b) Estimation of volume of lagoon

Summer: 0S 1 20 1 VS 200V 1 0.4111 k 6600Q V=144525.5 m3

Winter: 20 1 V200 1 0.128 3400 V=239062 m3

(c) Estimation of dimensions of three square lagoons in series

Q, S0 Q,S1 Q,S2 Q,S3

n

QnVk1

1 SS

i0

n

:Summer 3

i0.411 V200 120 3 6600

355607.13 miV

Winter: 30.128200 1

20 3 3400

iV

3iV 91980.8 m

Vi 1

Vi 2

Vi3

REFERENCES

[1] http://dnr.wi.gov/org/es/science/opcert/doc/Activated_Sludge_intro.pdf

[2] http://www.ragsdaleandassociates.com/WastewaterSystemOperatorsManual/Chapter%20

8%20-%20Activated%20Sludge.pdf

[3] http://www.wastewaterhandbook.com/documents/11_introduction.pdf

[4] http://www.lenntech.com/wwtp/wwtp-activated-sludge-process.htm

[5] Peavy, H. S., Rowe, D. R., Tchobanoglous, G. Environmental Engineering, McGraw-Hill,

1985.

[6] Tchobanoglous, G., burton, F. L., stensel, H.D. Wastewater Engineering: Treatment and

Reuse-Metcalf&Eddy, Inc., Tata McGraw-Hill, 2003.