In the aeration tank of an activated sludge process, the mixture of activated sludge with primary effluent or raw wastewater and return sludge is called what, as long as it remains in the aeration tank?

• A. Mixed Liquor
• B. Activated Sludge
• C. Mixed Liquor Suspended Solids
• D. Return Sludge

Answer: (A)

In the aeration tank, the liquid that contains the wastewater plus the active microbial biomass and all suspended solids is called mixed liquor. This term describes the entire blend inside the aeration basin while it remains there. The biomass itself is often referred to as activated sludge, and the solids that settle in the downstream clarifier may be returned as return sludge or wasted, but inside the aeration tank the working mixture is the mixed liquor. The concentration of solids in this mix is defined as MLSS (mixed liquor suspended solids), which is a key parameter for controlling the process. So the best term for the mixture in the aeration tank is mixed liquor.