TSS removal performance in a primary clarifier or sedimentation basin has been declining over the course of several days. The effluent is getting increasingly dark, almost black, and foul-smelling over the same period. What is the MOST likely cause of these issues?

• A. Insufficient aeration
• B. Excessive sludge removal
• C. Excessive aeration
• D. Insufficient sludge removal

Answer: (D)

When a primary clarifier’s ability to remove solids declines over days and the effluent becomes darker and more foul-smelling, the most likely issue is not removing enough sludge from the basin. If sludge isn’t withdrawn regularly, solids build up and form a thick sludge blanket. This makes settling less effective, so more solids pass into the effluent. The accumulated organic matter then degrades under low-oxygen conditions, leading to anaerobic activity that produces darkened water and a strong, foul odor.

In contrast, issues like not enough aeration or too much aeration pertain to the aeration basin rather than the primary clarifier, and removing too much sludge would typically reduce solids in the effluent rather than cause dark, odorous effluent.