Operator terms · Water and wastewater
What is a clarifier?
A clarifier is a tank where water slows down so heavier solids settle to the bottom and clearer water flows off the top. Primary clarifiers remove settleable solids before biological treatment; secondary clarifiers separate the biomass after it. Settling is simple physics: give solids time and they sink.
Last reviewed 2026-08-03
Primary and secondary
A primary clarifier settles out raw solids before biological treatment. A secondary clarifier settles the biomass afterward, sending most of it back as return activated sludge.
How it works
Slowing the flow lets gravity pull particles down. The settled sludge is scraped from the bottom, and the clarified water flows over weirs at the top.
Common questions
- What does a clarifier do?
- Lets solids settle out of water by slowing it down.
- What is the difference between primary and secondary?
- Primary settles raw solids; secondary settles the biomass after biological treatment.
- Is a clarifier the same as a settling tank?
- Yes, the terms are used interchangeably.