What does MLSS stand for in wastewater treatment?

• A. Waste Activated Sludge (WAS)
• B. Return Activated Sludge (RAS)
• C. Mixed Liquor Suspended Solids (MLSS)
• D. Mixed Liquor Volatile Suspended Solids (MLVSS)

Answer: (C)

In wastewater treatment, MLSS stands for mixed liquor suspended solids—the concentration of solids that are suspended in the mixed liquor of the aeration tank, which is the blend of wastewater and biomass inside the reactor. This value captures both organic and inorganic solids present and is a key parameter for controlling how the biological reactor operates, affecting mixing, oxygen transfer, and settling behavior. It’s typically measured in mg/L using a gravimetric method (filtering a sample and drying the residue to constant weight).

A related concept is mixed liquor volatile suspended solids, which specifically targets the organic, volatile portion of MLSS and serves as an estimate of active biomass. The other terms refer to sludge flows (waste activated sludge and return activated sludge) rather than the concentration of solids in the mixed liquor.