Respirometric anatomy of the OSA process: microbial basis of enhanced sludge reduction mechanism

BACKGROUND: Extensive research effort has been devoted to the performance of the OSA system. Despite all studies focused on the assessment of observed sludge yield values, their explanations of the mechanisms causing sludge reduction remained highly speculative. RESULTS: A laboratory-scale OSA system fed with peptone mixture was operated together with a system consisting only of a sequencing batch reactor. The experimental program focused on the assessment of the nature and characteristics of all streams by calibrating an appropriate model by oxygen uptake rate profiles obtained under different conditions. Results indicated that the interchange stream contained a significant fraction of heterotrophic biomass. Therefore, interpretations of the microbial functions in the OSA/oxic reactor should account for active heterotrophic input through the interchange stream. As also justified by corresponding mass balance expressions, the OSA/oxic reactor sustained a substantially higher active biomass concentration as compared with the control reactor. In this way, the relative magnitude of the endogenous decay was significantly increased with respect to microbial growth and the observed experimental results totally accounting for the excess sludge reduction in the tested lab-scale OSA system. CONCLUSION: The experimental results in this study provided conclusive proof that enhanced endogenous decay due to higher active biomass level sustained in the OSA/oxic reactor should be regarded as the major cause of excess sludge reduction in the OSA system.