Which term refers to the mixture of microorganisms and wastewater in an aeration tank?

• A. MLSS
• B. Sludge age
• C. Biofilm index
• D. Activated sludge

Answer: (A)

In an aeration tank, the mixture of wastewater and microorganisms is known as the mixed liquor, and the amount of suspended solids in that mixture is called Mixed Liquor Suspended Solids, or MLSS. MLSS measures how much solid material—primarily the microbial biomass as flocs—exists per liter of mixed liquor. This concentration is a key control parameter because it tells you how densely the biomass is present to treat the organics and how it will affect oxygen transfer, mixing, and settling characteristics.

Activated sludge refers to the biomass and the treatment process itself, not just the solids concentration in the tank. Sludge age is about how long solids stay in the system, and biofilm index pertains to attached-growth systems, so neither best describes the suspended solids content of the aeration-tank mixture.