In activated sludge operations, what is the effect of wasting more sludge?

• A. Wasting more reduces MLSS, MLVSS, sludge age, MCRT, and increases F/M.
• B. Wasting more increases MLSS and sludge age.
• C. Wasting more reduces F/M ratio.
• D. Wasting more has no effect on process parameters.

Answer: (A)

Wasting more sludge shortens the time solids stay in the aeration basin, i.e., it lowers the solids retention time (MCRT). As sludge is removed, both MLSS and MLVSS decrease because there’s less suspended solid material in the mixed liquor. With fewer microorganisms in the system, the average age of the sludge drops as well. Since the same incoming organic loading is now processed by less biomass, the food-to-microorganism ratio (F/M) increases. So, wasting more reduces MLSS, MLVSS, sludge age, and MCRT, and increases F/M. The other options don’t fit because they imply the opposite changes or no change.