In which season should MLSS be higher?

• A. Winter
• B. Spring
• C. Summer
• D. Fall

Answer: (A)

MLSS is the concentration of solids, mostly biomass, in the aeration basin. Temperature strongly affects microbial growth and the need for biomass retention. In winter, cooler temperatures slow the microbes and the rate at which they consume waste; to keep treatment performance stable, plants often operate with a longer sludge age (wasting less), which lets more biomass accumulate in the reactor. That buildup raises the MLSS in the mixed liquor. In warmer seasons, faster microbial activity means the plant can maintain treatment with less biomass in suspension, so MLSS tends to be lower. Hence MLSS is higher in winter.