What is the term for the mixture inside the aeration tank?

• A. Mixed liquor
• B. Mixed liquor suspended solids
• C. Activated sludge
• D. Return liquor

Answer: (A)

In the aeration tank of an activated sludge system, the liquid is a blend of wastewater and the active microorganisms that treat it. This blend is called mixed liquor. The term mixed liquor suspended solids refers specifically to the concentration of solids in that blend, not the blend itself. Activated sludge describes the biomass or the treatment process, but it isn’t the name for the entire mixture inside the tank. Return liquor is the recycled liquid from the clarifier back to the aeration tank, not the contents of the tank during aeration.