Which clarifier is used as the initial solids-removal unit when wastewater enters the WWTP?

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

Answer: (B)

The initial solids-removal unit is the primary clarifier, a large, quiet settling tank placed at the plant’s entry. It gives wastewater time to slow down so heavier solids settle to the bottom as sludge and lighter materials rise to the top as scum. The clarified wastewater that leaves the tank has far fewer settleable solids, which protects downstream equipment and makes the next treatment stages more effective. Sludge is periodically removed from the bottom and scum skimmed from the surface.

After this step, the process moves into biological or secondary treatment to remove dissolved organics, with a secondary clarifier used after that treatment to separate biomass from the effluent. Tertiary and quaternary treatments are additional polishing steps that come later, not at the entrance.