In an aeration basin, what primarily controls the total suspended solids (TSS) concentration?

• A. Amount of sludge wasted
• B. Oxygen supply
• C. Temperature of influent
• D. Mixing rate

Answer: (A)

Total suspended solids in the aeration basin are set by the balance of solids entering, forming, and leaving the basin. The key control is how much sludge is wasted from the basin. Removing sludge lowers the solids inventory, pulling TSS down toward the target level. If you waste more sludge, TSS drops; if you waste less, solids accumulate and TSS rises. Oxygen supply, influent temperature, and mixing rate influence how quickly biomass grows, breaks down, or stays in suspension, but they don’t directly fix the steady-state TSS concentration. In practice, operators adjust sludge wasting to maintain the desired TSS/MLSS level.