Behavior of activated sludge systems with an active heterotrophic biomass inflow – a novel perspective for sludge minimization

BACKGROUND: This study evaluated the microbial behavior of activated sludge systems operated with an active heterotrophic biomass inflow. The evaluation focused on the possibility of sludge minimization. Thus, related process stoichiometry was derived on the basis of mass balance for essential parameters. RESULTS: Stoichiometric relationships showed that the reactor holds a larger amount of biomass without the controlling impact of the sludge age, which decreases the magnitude of microbial growth compared with endogenous decay. Consequently, the observed yield, Y_OBS becomes much lower than what may be observed in conventional activated sludge systems. This stoichiometry exactly defines the microbial basis of the OSA (oxic/settling/anaerobic) system and identifies the interchange stream connecting the anaerobic reactor to the aerobic main reactor as the source of the active biomass inflow. Therefore, biomass inflow, largely overlooked in related studies, offers a novel microbial perspective for the merit of the OSA process in minimizing sludge generation. CONCLUSION: The impact of lower Y_OBS, while explaining lower active biomass production, cannot be extrapolated to cover inert particulate COD fractions, namely a similar reduction in the initial inert COD and the residual particulate microbial products. Future studies should be directed to explore the fate of residual particulate compounds.