Operator terms · Wastewater
What is return activated sludge (RAS)?
Return activated sludge, or RAS, is the settled sludge full of living microorganisms that an operator pumps from the clarifier back into the aeration tank. Returning it keeps a healthy population of microbes working on the incoming wastewater. Waste activated sludge, WAS, is the excess removed to keep the balance.
Last reviewed 2026-08-03
Why sludge is returned
The microbes that clean wastewater live in the sludge. Returning settled sludge to the aeration tank keeps enough of them present to treat the next round of incoming flow.
RAS versus WAS
RAS is returned to keep the process going. WAS, waste activated sludge, is the surplus removed so the microbe population does not grow out of balance. Managing both is daily process control.
Common questions
- Why return sludge at all?
- To keep enough active microorganisms in the aeration tank to treat incoming wastewater.
- What is WAS?
- Waste activated sludge, the excess removed to keep the microbe population balanced.
- Where does RAS come from?
- The bottom of the secondary clarifier, where the biomass settles.