Which term refers to the initial stage of wastewater treatment where solids settle out?

• A. Primary sewage treatment
• B. Sludge
• C. Bar screens
• D. Secondary sewage treatment

Answer: (A)

In wastewater treatment, the first stage focuses on letting suspended solids settle out by gravity in a sedimentation tank, which is described by primary sewage treatment. Heavier particles drop to the bottom as sludge and are removed, while the clearer water moves on for further treatment. This step reduces the amount of solids and some organic matter entering the biological, or secondary, treatment that follows. Bar screens are used earlier to remove large debris, not the settling that characterizes this initial stage; sludge refers to the settled solids themselves, and secondary sewage treatment is the later biological stage.