Evaluation of simultaneous nutrient removal and sludge reduction using laboratory scale sequencing batch reactors

The treatment and disposal of excess sludge has been a rising challenge for wastewater treatment plants worldwide. In this study, simultaneous sludge reduction and nutrient removal was evaluated in laboratory scale sequencing batch reactors (SBRs). Two SBRs were operated alongside for a duration of 370 d. One SBR was operated to achieve nutrient removal (control-SBR) at 10 d solids retention time (SRT), while the other (modified-SBR) was operated to achieve nutrient removal along with sludge reduction. Sludge reduction in the modified-SBR was accomplished by subjecting the recycled biomass to feasting and fasting at sufficiently high SRT close to infinity (phase I and II) and finite SRT (phase III). The observed biomass yield in the modified-SBR was estimated to be 0.17 mg TSS mg−1 COD, representing 63% sludge reduction compared to the control-SBR. The NH3 levels in the effluents from both SBRs always remained below detection limit. The average dissolved phosphorus removal efficiencies in the control-SBR and the modified-SBR were 87% and 84%, respectively, during phase II. However, the biomass of the modified-SBR increased during phase II. To control this, biomass wastage was initiated directly from the modified-SBR during phase III at a rate equivalent to the observed biomass accumulation rate in the system in phase II. This resulted in an overall 100 d SRT for the modified-SBR system. Following this change, biomass accumulation in the modified-SBR was controlled, and a net 63% sludge reduction could be sustained along with 90% phosphorus and 100% NH3 removal. Consistent denitrification activities were also noticed in both SBRs despite the absence of any carbon source during the anoxic phase of every cycle.