Tennessee Wastewater Operator Practice Exam

What is most likely to be found at the bottom of a primary clarifier?

1. Fines

2. Settleable solids

3. Floating debris

4. Biosolids

The correct answer is: Settleable solids

In a primary clarifier, the primary function is to remove settleable solids from wastewater through a process of sedimentation. The wastewater enters the clarifier, allowing heavier particles to settle to the bottom due to gravity. This settling process results in the accumulation of solids at the bottom of the clarifier, primarily composed of suspended particulates and organic materials which have a higher density than the liquid. Settleable solids refer specifically to those particles that have a propensity to settle out of the wastewater over time. These can include organic matter, inorganic particles, and other solids that are removed in the primary treatment phase before the water moves on to secondary treatment processes. This accumulation of settleable solids forms a sludge layer at the bottom of the primary clarifier, which is then typically removed for further processing or disposal. While fines, floating debris, and biosolids may be present in the wastewater, they do not primarily settle at the bottom of the primary clarifier. Fines might refer to very small particles that remain suspended in the water column, floating debris consists of materials that do not sink, and biosolids, while related to treated sludge, generally refer to processed materials from secondary treatment and not specifically to what settles in the primary clarifier.