The sludge from a primary clarifier typically contains what percent solids?

The correct answer indicates that the sludge from a primary clarifier typically contains 4% to 6% solids. This range is representative of the concentration of solids that accumulates during the primary treatment phase of wastewater treatment, where heavy solids settle out of the liquid due to gravity.

In a primary clarifier, wastewater is allowed to stand to enable solids to settle, thereby separating out a significant amount of the suspended material. The settled solids, referred to as primary sludge, usually have a solid content that falls within this specific percentage range. This concentration is critical for subsequent treatment processes, as it impacts the design and operation of the treatment plant.

The values provided for the other ranges either underestimate or overestimate the typical solid concentration found in primary sludge. This specificity is essential for operators to understand, as it helps in calculating treatment requirements and managing the overall efficiency of the wastewater treatment system.