MAINTENANCE AND DECAY IN ENHANCED BIOLOGICAL PHOSPHOROUS REMOVAL IN ACTIVATED SLUDGE DURING LONG-TERM STARVATION

Operational problems in plants performing Enhanced Biological Phosphorus Removal (EBPR) under dynamic loading are often linked to an overall decay of active biomass or to an unbalanced decay of storage products in phosphorus accumulating organisms (PAO). Decay processes occurring during starvation of biomass decrease the amount of active PAO and their organic (PHA) and inorganic (poly-P) storage products. In this study, the effects of long-term starvation periods on EBPR activity of activated sludge were examined. The main objectives were: i) quantification of the change in EBPR activity due to starvation under anaerobic and aerobic conditions and ii) identification of the mechanisms occurring under starvation conditions and their relation to currently used mathematical modeling approaches. An anaerobic-aerobic Sequencing Batch Reactor (SBR) was operated with synthetic wastewater under steady state loading. Sludge was withdrawn from the SBR and deprived from external substrate under anaerobic and aerobic conditions for 1, 7, 14, 21 or 28 days. Phosphorus release and uptake rates were measured before and after starvation, together with viability staining. A first order decay rate for PAO of 0.15 d−1 was obtained from the aerobic starvation experiments. Approximately 25% of EBPR activity was measured after 28 days of anaerobic starvation, although the reason for this decreased activity was not completely clear. Further studies showed that cell lysis and protozoan predation are enhanced under aerobic starvation. Decay of PAO is greatly reduced under anaerobic storage as compared to aerobic storage conditions.