If the pounds of MLSS in the basin stay constant while pounds of TSS in the influent double, the sludge age will:

• A. Increase
• B. Stay the Same
• C. Be Unlimited
• D. Decrease

Answer: (D)

Sludge age, or mean cell residence time (MCRT), is determined by the amount of solids in the basin divided by the rate at which solids are wasted from the system. If the mass of MLSS in the basin stays the same while the solids entering per day doubles, the system must remove more solids daily to keep MLSS constant. That means the solids wasted per day increases, while the MLSS in the basin remains fixed, so the ratio (MLSS mass divided by waste rate) decreases. In other words, sludge age drops. For example, with 2000 lb of MLSS and a waste rate of 400 lb/day, MCRT is 5 days. If the inflow doubles and waste increases to 800 lb/day to keep MLSS at 2000 lb, MCRT becomes 2.5 days.