Why is the sludge returned from the secondary clarifier to the aeration tank in activated sludge used for nitrification?

Why is the sludge returned from the secondary clarifier to the aeration tank in activated sludge used for nitrification?

• A. To provide microorganisms
• B. To remove excess solids
• C. To heat the tank
• D. To remove color

Answer: (A)

Returning activated sludge to the aeration tank keeps a ready supply of microorganisms in the mixed liquor. Nitrification is done by nitrifying bacteria that live in this biomass, so recycling sludge maintains enough active microbes and proper solids retention time for these slow-growing organisms to convert ammonia to nitrite and then nitrate during aeration. Without returning sludge, biomass would be washed out and nitrification would drop. The other options don’t provide the microbial population needed for nitrification.

Returning activated sludge to the aeration tank keeps a ready supply of microorganisms in the mixed liquor. Nitrification is done by nitrifying bacteria that live in this biomass, so recycling sludge maintains enough active microbes and proper solids retention time for these slow-growing organisms to convert ammonia to nitrite and then nitrate during aeration. Without returning sludge, biomass would be washed out and nitrification would drop. The other options don’t provide the microbial population needed for nitrification.