Which stage of wastewater treatment is primarily designed to remove settleable solids before biological treatment?

• A. Primary treatment
• B. Secondary treatment
• C. Tertiary treatment
• D. Quaternary treatment

Answer: (A)

Removing settleable solids before the biological step is the job of primary treatment. In this stage, wastewater flows into primary clarifiers where gravity lets heavier solids settle to the bottom and scum is skimmed from the top. This physical separation reduces the amount of solids and some organic matter entering the biological treatment that follows, making the downstream processes more efficient and less prone to clogging or overloading. The biological stage then focuses on breaking down dissolved organics and remaining particles. The later stages—secondary, tertiary, and quaternary—handle biological degradation and polishing after the solids have already been mostly removed.