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

Two bench-scale activated sludge Sequencing Batch Reactors (SBR's), with wo rking volumes of 100 liters each, operated on synthetic wastewater at sludg e retention times (SRT) of 7 days (Reactor 1) acid 15 days (Reactor 2) resp ectively. Biological phosphorus and nitrogen removals were applied in the s ystems to study nitrification, denitrification, and phosphorus removal kine tics during the start-up phase. Excellent enhanced biological phosphorus re moval was achieved in both reactors within 21 days. The maximum phosphorus release rate in each reactor was IO-mg PO4-P/ g.MLSS.h at 20 degrees C and phosphorus contents of about 5% dry weight of biomass were achieved. In Rea ctor 2, very good nitrification was achieved with 100% ammonia removal with in initial two weeks of operation. The maximum nitrification rate was 3.0 m g 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 org anic 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 didn't shows any decline, as incomplet e nitrification led to much lower nitrate during the initial fill and mix p hase. The present study implied that higher SRT is beneficial for phosphoru s removal. But later, SRT should be reduced to decrease the nitrification a ctivity, if the target is phosphorus removal, otherwise remaining elevated nitrate concentration diminishes phosphorus removal.