Increased effluent phosphorus (P) concentrations have been reported for enhanced biological phosphorus removal (EBPR) systems after periods (days) of low organic loading. A reduction of poly-β-hydroxybutyrate (PHB) and the decay of polyphosphate accumulating organisms (PAOs) due
to excessive aeration during low loading periods have been suggested as mechanisms leading to decreased EBPR performance. These decay processes result in an imbalance between P-release and subsequent P-uptake resulting in increased effluent concentrations. The purpose of this study was to
evaluate the importance of maintaining both high levels of PHB and high levels of active biomass to prevent process disturbance after low organic loading conditions. The following operational adjustments during low loading periods were tested: (a) unchanged operation, (b) reduced aeration,
(c) reduced sludge wastage, and (d) combination of reduced aeration and reduced sludge wastage. Experiments were performed in a laboratory scale anaerobic/aerobic sequencing batch reactor (SBR). It was observed that P-release rates decreased during low loading periods but recovered rapidly
after increasing the loading to normal levels again. However, P-uptake rates often required several days to increase to normal levels after low loading periods. Reducing aeration and sludge wastage during low loading periods allowed to maintain high levels of PHB and active biomass, respectively.
Mathematical simulations using the Activated Sludge Model No.3 combined with the EAWAG Bio-P module allowed to demonstrate these mechanisms of maintaining PHB and PAO concentrations. However, current mathematical models deviate from P-uptake rates measured during the first 24 hours after low
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