Primary treatment is described as what process?

• A. When physically treated sewage water is passed into a settling tank, where suspended solids settle out as sludge; chemically treated polymers may be added to help the suspended solids separate and settle out.
• B. When the water is aerated to remove dissolved gases
• C. When chlorine is added for disinfection
• D. When sludge is digested anaerobically

Answer: (A)

Primary treatment focuses on physically removing solids from wastewater. In a settling tank, suspended solids settle to the bottom as sludge, and these solids can be aided by coagulants or polymers to improve settling. This stage mainly reduces the amount of solids (and some organic matter) before the water moves on to biological treatment. The other options describe processes used in later stages: aeration targets dissolved organics through biological oxidation, disinfection with chlorine happens after treatment to kill remaining microbes, and anaerobic digestion treats the sludge itself rather than the incoming wastewater.