Effects of hydraulic retention time on system performance of a submerged membrane bioreactor

To investigate the effect of hydraulic retention time (HRT) on system performance in a submerged membrane bioreactor (MBR) with a prolonged sludge retention time (SRT) for the treatment of industrial wastewater, six runs of a laboratory scale MBR with HRTs of six days, three days (two runs), one day, 12 hours, and 6 hours, respectively, were conducted. The MBR process was capable of achieving over 90% COD removal, on average, almost independent of HRT. Membrane rejected 70-90% of residual COD in bioreactor to ensure high quality of effluent even if the biological treatment process mal-functioned. With declining HRT, sludge concentration in the bioreactor increased accordingly, while the ratio between mixed liquor volatile suspended solid (MLVSS) and mixed liquor suspended solid (MLSS) remained constant in each run. The governing equation in activated sludge process was re-examined and found applicable in MBR systems with a prolonged SRT. The mean particle size at different HRTs were in the range of 24.4 to 58.18 mum, lower than that in activated sludge process. Filtration performance was found to be associated with the stability of mean particle size. Constant or superior filtration performance was studied during the experiment period to search the cues of the potential solution for the fouling problem. Based on the system performance at different HRT, an optimal HRT of 12 hours was suggested to optimize MBR system performance-to achieve economy in design and a constant and superior filtration performance in operation.