Encyclopedia of Environmental Science and Engineering, Volume I and II

it was operated at disk speeds of 13 and 17 rpm and flow

rates of 1.9 to 2.5 LPM (0.5 to 0.65 USGPM). Similarly,

Antoine^ observed that the RBCs were able to produce

acceptable effluents for boardmill, kraft and sulfite waste-

waters. For sulfite wastewater, the loading rate had to be

reduced to increase the detention time. On the other hand, the

suspended growth treatment of pulp and paper wastes has

not consistently produced effluents of an acceptable level.

B.C. Research had conducted tests on the use of the rotat-

ing biological contactor process for refinery waste contain-

ing phenols and observed it to be an effective method with

proper control on operation.^43

Anaerobic Treatment

In this process, anaerobic bacteria stabilize the organic matter

in absence of free oxygen. Anaerobic treatment has been used

widely for stabilization of sludges collected from primary and

secondary settling tanks and recently is being adopted for treat-

ment of soluble wastes in anaerobic lagoons, anaerobic filters,

etc. One of the important advantages of anaerobic processes

over aerobic processes is a high percentage conversion of

organic matter to gases and liquid and a low percentage conver-

sion to biological cells. McCarty^44 has mentioned that efficient

anaerobic treatment of soluble wastes with BOD concentration

as low as 500 mg/L is now feasible. Wastes with lower BOD

can also be treated anaerobically, although the waste treatment

efficiency will not be of the same magnitude as expected from

aerobic treatment.

Anaerobic treatment of wastewaters takes place in two

stages as shown in Figure 20. In the first stage, complex

organic materials like protein, fats, carbohydrates, are con-

verted into simple organic acids by acid forming bacteria,

but with little change in BOD or COD value. In the second

stage, these fatty acids are converted to carbon dioxide and

methane, thereby stabilizing the BOD or COD.

In a conventional anaerobic treatment process, the sub-

strate is fed into the digester continuously or intermittently. In

most of the existing digesters, the contents are mixed, mechan-

ically or with compressed gas collected from digesters. There

is no recirculation of digested sludge and the system is a typi-

cal flow through system. The hydraulic detention time, t- in

BOD OF

SETTLED MIXED LIQUOR

SLUDGE

BOD

BOD

TIME

AERATION BASIN

INFLOW

RETURN SLUDGE - 100%

SLUDGE WASTED

PERIODICALLY

SETTLING

TANK

EFFLUENT

FIGURE 19 Extended aeration activated sludge.

154 BIOLOGICAL TREATMENT OF WASTEWATER

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