Bench-scale study on zero excess activated sludge production process coupled with ozonation unit in membrane bioreactor

In this bench-scale study, two promising processes for minimizing excess activated sludge (EAS) production, i.e., membrane bioreactor (MBR) and sludge ozonation (SO), were coupled in this study into the MBR-SO process to treat domestic wastewater for 80 days, and the vital related operational factors were also investigated. Mathematical models were developed to elucidate the relationships among process control parameters and the actually operational effects of these parameters on the performance of MBR-SO process. As a consequence, the ratio of flow-rate draining to ozonation unit (q) to influent wastewater flow-rate (Q) was the mainly operational parameter, which was significantly affected by the sludge lysing ratio in ozonation unit (ξ), produced COD per unit mass lysed MLSS (η), observed sludge yield coefficient for wastewater (Yobs) and intrinsic yield coefficient for COD produced by lysed sludge (Y2). To keep the mixed liquid suspended solid concentration (MLSS) in MBR around 8000 mg/L, the ratio of q/Q and ξ for each batch ozonation was set at 0.0067 and 0.72, respectively. The generated EAS was continuously drained into ozonation unit at a frequency of 2 batch/d for lysing cells, and almost constant MLSS concentration with zero observed sludge yield coefficient (Yobs) and excellent effluent quality could be achieved in MBR except for TP concentration (only approximately 3.62% TP removal efficiency rate obtained in Test stage). The calculation of sludge disintegration number (SDN) and the maximum SDN (SDNmax) indicated that the higher ξ could reduce apparently the sludge amount needed for ozonation. The low input ozone gas concentration and high flow-rate could enhance the sludge lysing effects at same ozone dosage, and therefore lower energy consumption of 0.041Yuan (US$0.0053)/m3 wastewater was obtained. Overall, mass balance showed that the preset value of operation parameters listed in mathematical models matched well with trends of sludge reduction found in this experimental result.