What is "scum" in the context of wastewater treatment?

In the context of wastewater treatment, scum refers to the layer of floating organic matter and debris that accumulates on the surface of wastewater in treatment processes. It primarily consists of oils, fats, grease, and other less dense materials that float due to their lower specific gravity compared to water. This layer forms during the primary treatment process and can hinder the treatment efficiency if not properly managed.

In contrast, solid waste that sinks to the bottom would describe the sludge, which consists of heavier solid particles that settle under gravity. Treated water pertains to effluent that has undergone the purification process and is safe for discharge or reuse but does not capture the definition of scum. Contaminated soil refers to soil that has been polluted with hazardous substances and is not related to the treatment of wastewater in terms of scum formation.