The results of a 30-minute settleable solids test is 410 mL/L. What is the SVI if the MLSS concentration is 2,000 mg/L?

• A. 102.5
• B. 205
• C. 220.5
• D. 120

Answer: (B)

SVI tells you how much sludge settles in a given time relative to how concentrated the solids are. It’s calculated by taking the settleable volume after 30 minutes (in mL per liter) and dividing it by the MLSS concentration in g per liter.

Here, the settleable volume is 410 mL/L and MLSS is 2,000 mg/L, which is 2 g/L. So the SVI = 410 / 2 = 205 mL per gram of MLSS. This value reflects how well the sludge settles—larger numbers mean poorer settling. In this case, 205 mL/g indicates relatively lower settling efficiency compared to typical good-settling ranges.