Compared
Return vs. waste activated sludge (RAS vs. WAS)
RAS and WAS are two things an operator does with the sludge that settles in a secondary clarifier. RAS (return activated sludge) is pumped back to the aeration tank to keep enough microbes working. WAS (waste activated sludge) is the extra removed so the microbe population does not grow out of balance.
Last reviewed 2026-08-04
| RAS (return) | WAS (waste) | |
|---|---|---|
| What | Sludge returned to aeration | Sludge removed from the system |
| Purpose | Keep enough active microbes | Control the microbe population |
| Where it goes | Back to the aeration tank | To sludge handling |
| If skipped | Too few microbes to treat | Too many microbes, poor settling |
The bottom line
RAS keeps the process alive by returning microbes; WAS keeps it balanced by removing the surplus.
Common questions
- What does RAS do?
- It returns settled microbes to the aeration tank to keep treatment strong.
- What does WAS do?
- It removes surplus sludge so the microbe population stays in balance.
- Do both come from the clarifier?
- Yes, both are drawn from the sludge that settles in the secondary clarifier.