Effect of sludge retention time (SRT) on nutrient removal in sequencing batch reactors

Abstract

Two bench‐scale activated sludge Sequencing Batch Reactors (SBR's), with working volumes of 100 liters each, operated on synthetic wastewater at sludge retention times (SRT) of 7 days (Reactor 1) and 15 days (Reactor 2) respectively. Biological phosphorus and nitrogen removals were applied in the systems to study nitrification, denitrification, and phosphorus removal kinetics during the start‐up phase. Excellent enhanced biological phosphorus removal was achieved in both reactors within 21 days. The maximum phosphorus release rate in each reactor was 10‐mg PO4‐P/ g.MLSS.h at 20°C and phosphorus contents of about 5% dry weight of biomass were achieved. In Reactor 2, very good nitrification was achieved with 100% ammonia removal within initial two weeks of operation. The maximum nitrification rate was 3.0 mg NO3‐N/g. MLSS.h, but later phosphorus release rates continued to decline, due to the elevated remaining nitrate concentrations from preceding cycles in the initial fill and mix phase. Remaining nitrate consumes influent organic during the fill and mix phase, decreasing the availability of organic matter for phosphorus removing bacteria, thus deteriorating their activity. For SBR 1, phosphorus release rates didnl shows any decline, as incomplete nitrification led to much lower nitrate during the initial fill and mix phase. The present study implied that higher SRT is beneficial for phosphorus removal. But later, SRT should be reduced to decrease the nitrification activity, if the target is phosphorus removal, otherwise remaining elevated nitrate concentration diminishes phosphorus removal.