Sludge characteristics and membrane fouling in membrane bioreactors with various sludge retention times

Long-term sludge filterability and membrane fouling were investigated in a membrane bioreactor (MBR) with different sludge retention times (SRTs). The effect of SRT was evaluated systematically in terms of biomass evolution and sludge filterability such as modified fouling index (MFI) and specific cake resistance (a). The membrane fouling rate induced by the activated sludge mixed liquor in each MBR was also analyzed based on transmembrane pressure profiles and resistance-in-series model. Food to microorganism ratio gradually decreased close to 0.30 kg COD/(kg VSS d) with increasing SRT. There were no significant differences of the sludge growth rate, organic degradation rate, and the sludge yield for the MBRs. Sludge filterability significantly improved, described by the decreased MFI and a of the mixed liquor in the MBR at longer SRTs (above 20 d). The reduction of the soluble microbial products (SMP) content was relatively small as compared with a decrease in a. Lower filtration resistances were measured at SRT of 40 d coincided with lower polysaccharides and protein concentration in SMP and higher ratio of protein/polysaccharides. MBR at appropriate SRT (40 d) with a stable and high-quality permeate as well as membrane fouling mitigation could be adopted in the full-scale application.