Operator terms · Wastewater
What is MLSS?
MLSS, mixed liquor suspended solids, is the concentration of suspended solids, mostly microorganisms, in the aeration tank of an activated sludge plant, measured in mg/L. It tells the operator how much active biomass is in the tank, which they adjust to keep treatment healthy.
Last reviewed 2026-08-03
What it tells the operator
MLSS is a gauge of how much biomass is doing the work. Too little and treatment falls behind; too much and the clarifier struggles. Operators steer it with sludge wasting.
Mixed liquor
Mixed liquor is the mix of incoming wastewater and returned sludge in the aeration tank. MLSS is simply the suspended-solids concentration of that mixture.
Common questions
- What is mixed liquor?
- The mixture of wastewater and returned sludge in the aeration tank.
- How do operators change MLSS?
- By wasting more or less sludge (WAS) to raise or lower the biomass.
- What units is MLSS in?
- Milligrams per liter, mg/L.