Encyclopedia of Environmental Science and Engineering, Volume I and II

BIOLOGICAL TREATMENT OF WASTEWATER 149

Kinetic Rate: Depending upon the design and operating

conditions, one or more of the kinetic rate Eqs. 10, 12, 13

and 14 for BOD removal can be applied to different types of

the activated sludge processes.

Oxygen Requirement: Oxygen is used to provide energy

for synthesis of biological cells and for endogenous respira-

tion of the biological mass. The total oxygen requirement,

∆ O 2 , can be expressed with the following equation;

∆ O 2 = a ∆ S + b X T (26)

where a  is the fraction of BOD removed that is oxidized for

energy and b is the oxygen used for endogenous respira-

tion of the biological mass, per day. In conventional aera-

tion basins, an hourly oxygen demand of 50 to 80 mg/L per

1000 mg/L of VSS is exerted near the beginning of the tank

and is reduced to 20 mg/L per 1000 mg/L of VSS in the

course of 4 to 6 hours.^14

Excess Sludge Yield: By applying material balance for

volatile suspended solids in activated sludge system, and

using the concept shown in Figure 3:

Excess solids in activated sludge system = Non-

biodegradable suspended solids in influent + Biomass

Synthesized during BOD removal – Biomass broken down

by endo genous respiration

or

 
XfX aSbX^0 T (27)

where:

∆ X = Net accumulation of volatile suspended solids, g/day

f = Fraction of volatile suspended solids present in the

influent which are non-degradable

X 0 = Influent volatile suspended solids, g/day

Temperature Effect: According to Eckenfelder and

O’Connor,^13 the value of temperature coefficient in Eq. 12

varies between 1.0 for low loading rates to 1.04 for high

loading rates. Friedman and Schroeder^38 have studied in

detail the effect of temperature on growth and the maximum

cell yield occurred at 20°C.

Elements of a conventional activated sludge system are

shown in Figure 14. In this system, the settled waste is mixed

with the return sludge at the inlet end of the aeration tank.

The microorganisms receive the full impact of any shock

load and respond accordingly with sudden increase in oxygen

demand during growth. By the time microorganisms leave the

aeration tank, the organic matter has been stabilized and the

microorganism population starts dying off. Thus, the micro-

bial population undergoes a continual shifting and never

reaches a relatively constant equilibrium.^7

A mass of activated sludge of three to four times the

mass of the daily BOD load must be kept in the system in

order to consume all the new food and also acquire good

settling properties. These types of plants have been used

for treating domestic wastewaters of low biochemical

oxygen demands. In conventional activated sludge plants

BOD ADSORBED AND SYNTHESIZED

BOD OF SETTLED EFFLUENT

SECONDARY

SETTLING

TANK

EFFLUENT

AIR DIFFUSERS

AERATION BASIN

RETURN SLUDGE

EXCESS SLUDGE SLUDGE

PRIMARY

SETTLING

TANK

INFLOW

SLUDGE DISPOSAL

BOD OF SETTLED MIXED LIQUORTIME

BOD OXIDIZED

FIGURE 14 Conventional activated sludge.

C002_001_r03.indd 149C002_001_r03.indd 149 11/18/2005 10:15:53 AM11/18/2005 10:15:53 AM