Effect of Process Configurations and Alum Addition on EBPR in Membrane Bioreactors

Bench-scale studies were undertaken to explore the relative efficiency of three different process configurations for enhanced biological phosphorus (EBPR) removal; the University of Cape Town (UCT) process, the Sammamish Biological Nutrient Removal (SmBNR) process, and the University of Washington Membrane Biological Nutrient Removal (UW-MBNR) process. In this study, they all had the same anaerobic, anoxic, and aerobic volumes. The UCT process had the highest phosphorus (P) removal efficiency when the amount of P removed per unit of COD consumed was considered. It was found that addition of alum to the anaerobic zone of the process resulted in improved P removal. Critical to EBPR performance was the finding that phosphorus uptake rates are slower than anaerobic release rates and that the uptake rate is slower for lower reactor P concentrations. This encourages consideration of staged reactors to achieve lower effluent P concentrations. For all the systems, significant quantities of phosphorus accumulating organisms (PAO) and tetrad-forming glycogen accumulating organisms (GAO) were observed. Simulations using the commercial BioWin™ software overpredicted measured P removal. This may have been the result of GAO growth in the bench-scale reactors, which is not modeled in BioWin™. The BioWin™ models were calibrated by reducing the influent readily biodegradeable COD in the model. Although the BioWin™ models confirmed that the UCT process had the highest P removal efficiency, it was found in simulations of a full-scale plant with lower relative nutrient concentrations than were used in the bench-scale tests that both the UCT and SmBNR process configurations resulted in near complete P removal.