In the secondary clarifier scenario, what is the MLSS concentration used?

• A. 1,200 mg/L
• B. 2,415 mg/L
• C. 3,000 mg/L
• D. 4,000 mg/L

Answer: (B)

MLSS stands for mixed liquor suspended solids—the concentration of solids suspended in the mixed liquor inside the aeration basin. In a secondary clarifier scenario, using a mid-range MLSS like about 2,400 mg/L provides enough microbial mass to treat organics effectively while keeping the sludge dense enough to settle well in the clarifier. This balance helps maintain stable sludge age and good clarifier performance, producing a clear effluent. Values that are too low (around 1,200 mg/L) risk insufficient biological treatment, while values much higher (near 3,000–4,000 mg/L) can lead to poorer settling and potential bulking issues. Thus, 2,415 mg/L is a representative, balanced choice for this scenario.