Evaluation of Methods for Assessing Biomass Activity in Biological Treatment Processes

The volatile solids content of the mixed liquor in an activated sludge process typically is considered to represent the mass of microorganisms that is available for waste treatment. However, not all volatile solids are active microorganisms. Inert cell matter and volatile but non-biodegradable influent solids also contribute to the mixed liquor volatile solids. Oxygen uptake occurs when active microorganisms consume biodegradable constituents of the wastewater. A method is presented whereby oxygen uptake measurements can be used to provide estimates of the active biomass in mixed liquors. Case studies show that mixed liquor volatile solids typically range from 5 to 25% active microorganisms. The method also can be used to estimate the active nitrifier biomass in nitrification systems and the active methanogenic biomass in anaerobic processes.