The sludge wasting system is used to remove what type of solids from the process to keep a healthy population in the aeration tank?

• A. Scarce
• B. New
• C. Settled
• D. Excess

Answer: (D)

The main idea here is managing the amount of biomass in the aeration tank to keep the microbial population healthy and active. The sludge wasting system removes excess solids, preventing the mixed-liquor suspended solids (MLSS) from getting too high. When too many solids build up, oxygen transfer becomes less efficient, mixing can suffer, and the population can skew toward older or less active biomass, which can destabilize treatment. By regularly removing this excess sludge, you maintain the right biomass concentration and sludge age, supporting steady organic matter degradation and good overall performance in the aeration tank. This isn't about removing scarce, newly formed, or settled solids—the goal is to prevent buildup of excess solids that can impair the process.