What is the primary clarifier?

• A. A settling tank that follows preliminary treatment such as screening and grit removal
• B. A chemical mixing chamber
• C. A disinfection unit
• D. A secondary biological reactor

Answer: (C)

A primary clarifier is a gravity settling tank placed after preliminary treatment (such as screening and grit removal). Its main job is to allow solids that settle by gravity to drop to the bottom, forming sludge, while lighter materials or fats rise to the surface and are skimmed off. This step produces clearer liquid for the next stage and reduces the burden on the biological treatment that follows. It’s not a chemical mixing chamber, which would introduce reagents; it’s not a disinfection unit, which treats the water to kill pathogens after biological treatment; and it’s not a secondary biological reactor, which uses microorganisms to biologically degrade organics.