Study On Membrane Fouling Caused By Different Sludge Fractions From A Membrane Bioreactor With Long Solid Retention Time

Compared with the conventional activated sludge process, the membrane bioreactor (MBR) can minimize sludge disposal and reduce the bioreactor size with higher biomass loading. However, Membrane fouling limits the industrial application of MBRs with long SRT. Moreover, the mixed liquor composition varies with operating conditions, and the contribution of each sludge fraction to membrane fouling is changed along with different operating conditions. Therefore, it is necessary to investigate the influences of MLSS concentration, trans-membrane pressure and cross-flow velocity on membrane fouling to have a better understanding of the fouling behaviours in MBRs with long SRT conditions.A 300-day SRT MBR and 10-day SRT SBR were set up for the study. Activated sludge taken from these systems was fractioned into suspended solids, colloids and solutes; and their relative contributions to membrane fouling during crossflow filtration were calculated. Furthermore, sub-divided filtration resistances were analysed using resistance-in-series model. Results from this study indicated that the suspended solid was the main fouling fraction, with the relative contribution to the total resistance of 70% and 85% in MBR and SBR. On the other hand, the importance of colloids and solutes could not be neglected. Moreover, sub-divided resistances were analysed to have a deep understanding about the fouling mechanism. It was demonstrated that the resistances of concentration polarization (Rcp, 31~73%) and cake layer (Rc, 23~63%) mainly contributed to the total resistance, while the internal fouling resistance (Rif, 0.7~2.4%) and membrane resistance (Rm, 1.2~6.4%) were negligible during the filtration of activated sludge.Furthermore, suspended solids remained the main contributor to membrane fouling while the dominant filtration resistance varied according to different MLSS concentration, TMP and CFV. The membrane fouling increases with the increasing MLSS concentration, rising TMP and reducing CFV respectively. The optimun operating conditions in this experiment are: MLSS=320mg/L,TMP=0.5bar,CFV=0.438m/s。Lower cleaning frequency and longer membrane life would be achieved, resulting in lower operation cost and wider application of MBRs with long SRTs.