To what concentration does sludge in the primary clarifier settle?

• A. 4-8%
• B. 1-2%
• C. 10-12%
• D. 20-25%

Answer: (A)

In a primary clarifier, heavier solids settle by gravity and collect as sludge at the bottom. That settled sludge is a concentrated stream, typically around 4–8 percent solids by weight. This range reflects the amount of solids that thickens out of the wastewater while most of the water remains with the clarified liquid.

Why this range fits: it represents a balance where enough solids settle to reduce load on downstream processes, but the sludge isn’t so thick and viscous that pumping and handling become impractical. If the sludge were only about 1–2 percent, it would be too dilute to be effectively thickened and pumped. If it were much higher, like 10–12 percent or 20–25 percent, it would indicate a much more concentrated sludge than a typical primary clarifier outputs, often requiring additional thickening or dewatering steps.

So, the commonly expected sludge concentration from a primary clarifier is about 4–8 percent.