Which term refers to the total mass of organisms in a given area or volume?

• A. Biomass
• B. Biosolids
• C. BOD
• D. CBOD

Answer: (A)

Biomass is the total mass of living organisms in a defined area or volume. In wastewater treatment, that living mass comprises the microbes that biologically degrade organic matter in the treatment process, such as the microbes in activated sludge. This living material drives the biological treatment and is often estimated in practice by measures like mixed liquor suspended solids or volatile suspended solids, which approximate the amount of active microbial mass.

Biosolids, by contrast, are stabilized solids that come from the treatment process and are not defined by the current living microbial mass. BOD and CBOD quantify how much oxygen is needed to biologically oxidize organic matter, not how much living mass is present.