The mass of bugs in the system is referred to as what?

• A. MLSS
• B. MLVSS
• C. TSS
• D. COD

Answer: (A)

In an activated sludge system, the organisms that drive the treatment are the biomass or “bugs” in the mixed liquor. The standard way to quantify that biomass is the mass of suspended solids in the mixed liquor, called mixed liquor suspended solids (MLSS). This measure includes all suspended solids in the aeration tank, which encompasses the microbial mass as well as any inert solids.

A closely related term is MLVSS, which represents the organic portion of MLSS and is a better proxy for active biomass. However, when the question asks for the mass of the bugs in the system, MLSS is the best single label because it denotes the total suspended solids in the mixed liquor, including the microbial component. TSS is the total suspended solids in general (not specific to the biomass vs inert solids), and COD is a chemical measure of organic matter, not the mass of living biomass. The given answer reflects the typical way the total suspended solids in the mixed liquor are referred to as the mass of the bugs.