How Input Active Biomass Affects Sludge Age and Process Stability

Input of active biomass into suspended-growth biological processes occurs when biological processes are in series, during bioaugmentation, and when input wastewaters contain active bacteria. This paper extends mathematical modeling to include steady-state processes that receive significant inputs of biomass active in degrading substrates present in the system. The equations for concentrations of substrate, active biomass, and inert biomass are of the same form as for the conventional solutions with no input of biomass. However, the true sludge age for the active biomass uses the net growth rate as its denominator. The true sludge age is greater than the conventionally computed sludge age when inputs of active biomass are significant. Several practical examples show that the input of active biomass can reduce effluent substrate concentrations, allow stable operation of systems otherwise in washout, and shift the competitive advantage to otherwise less-competitive species.