Of Mechanical Aerators Under Field Conditions


Oxygen Transfer Rate (kg O2/kW-hr)

Surface low-speed 0.73-1.46

Surface low-speed with draft tube 0.73-1.28

Surface high-speed 0.73-1.22

Surface downdraft turbine 0.61-1.22

Submerged turbine with sparger 0.73-1.09

Submerged impeller 0.73-1.09

Surface brush and blade 0.49-1.09

(aeration rotor)

Source: Adapted from Metcalf and Eddy, Inc., 1991, Wastewater engineering: Treatment, disposal, and reuse, 3d ed., (New York: McGraw-Hill).

Note: Field conditions are wastewater temperature, 15°C; altitude, 152 m (500 ft); oxygen-transfer correction factor, 0.85; salinity-surface-tension correction factor, 0.9; and operating DO concentration, 2 mg/l.

ganisms. Factors such as waste characteristics and composition, nutrient contents, pH, temperature, and oxygen availability can cause sludge bulking. The absence of certain components in the wastewater such as nitrogen, phosphorus, and trace elements can lead to the development of a bulking sludge. This absence is critical when industrial wastes are mixed with municipal wastewater for combined treatment.

Wide fluctuations in pH and DO are also known to cause sludge bulking. At least 2 mg/l of DO should be maintained in the aeration basin under normal operating conditions. Wastewater treatment facilities should check the F/M ratio to insure that it is within the recommended range. They should also check the additional organic loads received from internal sources such as sludge digesters and sludge dewatering operations to avoid internal overloading conditions, especially under peak flow conditions.

Chlorination of the return sludge effectively controls filamentous sludge bulking. Chlorine doses in the range of 2-3 mg/l of Cl2 per 1000 mg/l of MLVSS are suggested. However, high doses can be necessary under severe conditions (8 to 10 mg/l of Cl2 per 1000 m/l of MLVSS).

Rising sludge is usually caused by the release of gas bubbles entrapped within sludge flocs in the secondary clarifier. Nitrogen gas bubbles formed by denitrification of nitrite and nitrate under anoxic secondary clarifier conditions are known to cause sludge rising. Oversaturation of gases in the aeration tank can also cause sludge rising in the secondary clarifier, especially when aeration tank depth is significantly deeper than that of the secondary clarifier (Li 1993). Reducing the sludge retention time in the secondary clarifier is effective in controlling rising sludge. Close monitoring and control of aeration in the aeration tank can also reduce rising sludge in the secondary clarifier.

Nocardia foam is associated with a slow-growing, filamentous microorganisms of the Nocardia genus. Some fac tors causing Nocardia foaming problems are low F/M ratio, long SRT, and operating in the sludge reaeration mode (Metcalf and Eddy, Inc. 1991). Reducing SRT is the most common means of controlling Nocardia foaming problems.

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