What settles in the clarifier and results from the trickling filter process?

The process of treating wastewater in a trickling filter involves the use of a fixed film process where microorganisms grow on the surface of media as the wastewater flows over it. As the wastewater is oxygenated and the organic matter is metabolized by the microorganisms, the resulting end product consists of partially degraded material and microorganisms, which can settle in the clarifier.

Humus sludge, formed from the breakdown of organic matter by these microorganisms in the trickling filter, is what settles in the clarifier. This sludge is rich in organic matter and is generally referred to as humus due to its decomposition process. It plays a crucial role in the overall treatment process because it helps improve the quality of the effluent that is eventually discharged. The settling of humus sludge also facilitates the separation of treated water from the solid matter, allowing for effective further treatment or discharge.

In contrast, biosolids are typically a more refined product resulting from the stabilization of sewage sludge, activated sludge refers specifically to the biomass produced in an activated sludge system, and primary sludge is the settled solids from primary treatment processes, which occur before the trickling filter step. These distinctions highlight why humus sludge is the correct answer in this context.