Effects of hydraulic retention time on behavior of start-up submerged membrane bioreactor with prolonged sludge retention time

High strength synthetic wastewater was treated using submerged ceramic membrane bioreactors (MBRs) at sludge retention time (SRT) of 200 d. It was found that the MBRs could achieve both COD and TOC overall removal efficiency at 96% in all the 3 experimental reactors. The MBRs could also be operated at higher solids concentration of up to 20 g/l. The sludge concentration, which was inversely proportional to the hydraulic retention time (HRT), yielded excellent nutrient removal and low F/M ratio and high sludge age conditions. The particle size of sludge was found to decline and eventually attained a constant mean size of approximately 50 μm. This led to an intriguing discovery that aeration at 8 l/min was the underlying factor contributing to the decline in the mean size of sludge particles. It was also found that the particle size of sludge played an influential role on the specific permeate flux rate. Small particles like soluble organic particles would deteriorate the permeability of the ceramic membrane by directly absorbing into the 0.2 μm pores. An improvement in the flux rate was observed when sludge floc breakage occurred and thus bringing down the mean sludge particle size. These observations brought along the conclusion that aeration was a significant parameter governing both the size of the sludge floc and the filtration performance.