Encyclopedia of Environmental Science and Engineering, Volume I and II

152 BIOLOGICAL TREATMENT OF WASTEWATER

E. In recent years, several wastewater treatment plants

have been designed to operate with pure oxygen instead

of conventional use of air in activated sludge treatment

process. The obvious advantage of pure oxygen aeration

is the higher oxygen concentration gradient maintained

within the liquid phase, and this condition permits

higher concentration of biomass in the aeration tank.

This process has been shown to be more economical

due to less energy requirements and in some cases has

produced a better quality effl uent. Signifi cant increase

in volumetric loading rate, reduction in sludge produc-

tion, elimination of foaming problems and decrease in

treatment costs are claimed to be advantages.^40

A pure oxygen activated sludge system developed by

Union Carbide Corporation is shown in Figure 18. This

process is operated at MLSS values between 3000–

10000 mg/L and the settling rate of sludge is consider-

ably improved.

F. Extended Aeration

Extended aeration plant is the one where the net

growth rate is made to approach zero, i.e., rate of

growth becomes approximately equal to rate of decay.

This is achieved by increasing the aeration time in order

to keep the sludge in the endogenous growth phase for a

considerable time. In practice, it is impossible to operate

an extended-aeration system without sludge accumula-

tion, because certain volatile solids, mainly polysaccha-

rides in nature and inert organisms in activated sludge

process, accumulate in the plant. Excess sludge is not

generally wasted continuously from an extended aera-

tion, but instead, the mixed liquor is allowed to increase

in suspended solids concentration and a large volume

of the aeration tank content or return sludge is periodi-

cally pumped to disposal. Oxidation ditch plants are

designed and operated on this principle. Layout of a

typical extended-aeration plant and variation in BOD in

aeration tank are shown in Figure 19.

G. Aerated Lagoons

These are similar to the activated sludge system but

without recirculation of sludge. Mechanical or diffused

aeration devices are used for supplying oxygen and also

providing sufficient mixing. All suspended solids may

or may not be kept in suspension, depending upon the

degree of mixing. Deposited solids may undergo anaer-

obic decomposition. Mathematically, the BOD removal

rate in aerated lagoons is given by Eq. 13 and assuming

the aerated lagoon to be a completely mixed system,

without recycle and maintaining sufficient turbulence,

SECONDARY

SETTLING

TANK

EFFLUENT

EFFLUENT

INFLUENT

AERATION BASIN

RETURN SLUDGE

EXCESS SLUDGE

PRIMARY

SETTLING

TANK

INFLOW

SLUDGE

DISPOSAL

BOD

TIME

FIGURE 17 Complete mixing activated sludge.

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