Encyclopedia of Environmental Science and Engineering, Volume I and II

150 BIOLOGICAL TREATMENT OF WASTEWATER

that have plug flow design, high BOD in influent causes

higher oxygen demand at that point in the mixed liquor

and this oxygen demand diminishes as the flow passes

down the aeration tank. Most of the plants designed these

days are provided with tapered aeration, with highest air

supply near the inlet end and lowest near the outlet end of

the aeration tank.

Modifications of the Conventional Activated Sludge

Process

A. Step Aeration Activated Sludge

Step aeration process, developed by Gould^39 at New

York City, offers more flexibility than the conventional

activated sludge process. In this process, wastewater

is introduced at four or more points along the aeration

tank in order to maintain a uniformly distributed load-

ing. In addition to evening out the oxygen demand, this

also keeps sludge reaerated in the presence of substrate.

This process remains biologically more active instead

of reaching the endogenous phase near the end of the

conventional aeration tank. Step aeration system layout

and fluctuations in BOD in aeration tank are shown in

Figure 15. This method has been successfully employed

in the treatment of domestic wastewaters and industrial

wastewaters of similar nature.

B. Short Term Aeration or High Rate or Modifi ed Activated

Sludge

These systems have very high loading rates, both in

terms of organic and volumetric loading, and low mixed

liquor volatile suspended solids, thus requiring small

aeration tank capacities and reduced air requirements.

Because of shorter aeration time and lower mass of

organisms, this process provides an intermediate degree

of treatment. Organic matter is removed largely by syn-

thesis, thus exerting a high rate of oxygen demand and

producing a relatively large volume of sludge per unit

mass of BOD removed. Since the sludge still contains

certain unstabilized organic matter, the settled sludge in

secondary settling tanks should be removed rapidly in

order to avoid its anaerobic decomposition and floata-

tion. The flow diagram is similar to the conventional

system as shown in Figure 14.

C. Contact Stabilization or Biosorption

The elements of this type of plant are shown in

Figure 16. This system is ideally suited to the treat-

ment of wastewaters in which a large portion of BOD is

SECONDARY

SETTLING

TA N K

STEP AERATION BASIN

DISTRIBUTED LOADING

RETURN SLUDGE

SLUDGE

EXCESS SLUDGE

PRIMARY

SETTLING

TANK

INFLOW

SLUDGE

DISPOSAL

BOD OF SETTLED MIXED LIQUOR

TIME

FIGURE 15 Step aeration activated sludge.

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