Response of a sludge-minimizing lab-scale BNR reactor when the operation is changed to real primary effluent from synthetic wastewater

The activated sludge process is the most widely used treatment method for municipal wastewater. However, the excessive amount of biomass generated during the process is a major drawback. Earlier studies using the activated sludge process running in a biomass fasting and feasting mode demonstrated both nutrient removal and a minimization of biomass production. However, these studies were conducted using synthetic wastewater. In this study, we report findings from a lab-scale sludge-minimizing biological nutrient removing (BNR) reactor when its operation was changed from synthetic to real wastewater (primary effluent). Two lab-scale sequencing batch reactors, one in sludge minimization mode (hereafter called modified-SBR), and the other in conventional activated sludge mode (referred as control-SBR), were operated for more than 300 days. Both reactors were started and operated with synthetic feed. Gradually the feed to both reactors was changed to 100% primary effluent collected from a local full-scale wastewater treatment plant. Irrespective of the feed composition, more than 98% NH3-N removal was recorded in both SBRs. However, while 89% of the total dissolved phosphorus was removed from the 100% synthetic feed, only 80% of the total dissolved phosphorus was removed from the 100% primary effluent in both SBRs. The overall observed sludge reduction in the modified-SBR as compared to the control-SBR also decreased from 65% to 39% when the feed was changed from 100% synthetic to 100% primary effluent. The specific oxygen uptake rate for the modified-SBR was 80% higher than that for the control-SBR when the SBRs were fed with primary effluent wastewater. The modified-SBR showed a greater diversity of ammonia-oxidizing bacteria (AOBs) with synthetic wastewater as well as during the transition period than the control-SBR. Yet when the reactors were running on 100% real wastewater, only Nitrosomonas europaea/eutropha were identified in both SBRs. The nitrite-oxidizing bacterial community and the polyphosphate accumulating organisms (PAOs) responded in a similar way in both SBRs.