The air requirement for a conventional activated sludge process aeration tank will be approximately ___ to ___ cubic feet of air for each pound of bod removed

• A. 500 - 700
• B. 1000 - 1200
• C. 1500 - 1700
• D. 2000 - 2500

Answer: (B)

Oxygen delivery to the mixed liquor is the limiting factor in aerobic digestion, so sizing the aeration system uses a rule of thumb for how much air is needed per unit of BOD removed. In a conventional activated sludge tank, about 1,000 to 1,200 cubic feet of air are typically required to remove one pound of BOD. This takes into account that only a portion of the oxygen in the injected air actually dissolves and is usable, plus the oxygen demand of the biomass under typical operating conditions. That’s why the air volume is much larger than the pure oxygen needed by the microbes.

Other ranges would imply either under- or over-sizing for standard plants. Therefore, 1,000–1,200 ft^3 of air per pound of BOD removed is the best fit.