What is the solid matter that settles out of wastewater during sewage treatment?

The solid matter that settles out of wastewater during sewage treatment is referred to as sludge. During the treatment process, wastewater undergoes physical, chemical, and biological treatment stages designed to remove contaminants. As part of this process, heavier particles and organic solids, which include materials such as human waste, food particles, and other debris, naturally settle to the bottom of the treatment tanks due to gravity. This accumulation of solid waste at the bottom is what is known as sludge.

Sludge is significant because it can be further processed and treated to reduce its volume and toxicity. The remaining liquid after the sludge settles is referred to as effluent, which is the treated liquid that may go on to be discharged into water bodies or reused, depending on the treatment standards met. Scum, on the other hand, refers to the floating layer of material that can form on the surface of the wastewater and is typically made up of less dense organic materials and fats. Biomass generally refers to the mass of living biological organisms, particularly in the context of microbial populations involved in breaking down organic material during treatment, but it does not specifically denote the settled solids. Thus, sludge is the correct term for the solid matter that settles out during sewage treatment processes.