In which treatment stage is scum typically removed?

Scum is typically removed during the primary treatment stage of wastewater treatment. This stage is designed to remove large solids and floatable materials from the wastewater. In primary treatment, the wastewater is held in a large settling tank, allowing the heavier solids to settle at the bottom while lighter materials, such as scum (grease and oils), float to the top. The scum is then skimmed off and removed to prevent it from interfering with subsequent treatment processes. This removal is crucial because scum can inhibit the efficiency of biological treatment processes by forming a barrier that prevents microorganisms from effectively breaking down organic matter. The primary treatment stage is an essential first step in the wastewater treatment process, focused on separating solids from liquids.