In a well-designed and operated primary clarification or sedimentation system, the expected TSS removal efficiency is:

• A. 20-39%
• B. 40-59%
• C. 60-70%
• D. 80-99%

Answer: (C)

Primary sedimentation works by letting solids settle out by gravity as the wastewater sits in the clarifier. In a well-designed and well-operated primary clarifier, a substantial portion of the settleable solids is captured and removed with the sludge, so the effluent TSS is reduced by roughly six-tenths to seven-tenths. This range (about 60–70%) reflects the reality that many solids settle readily, but a portion remains suspended because some particles are too small, stay colloidal, or require additional treatment to settle. The clarifier is not meant to remove all TSS; dissolved or very fine particles pass through and are handled later by secondary treatment. That’s why removal around 60–70% is the most representative, while clearly higher (80–99%) would imply nearly complete removal by the primary stage alone, which isn’t typical, and lower ranges (40–59% or 20–39%) indicate underperformance or poorer influent characteristics.