Effect of sludge retention time on sludge properties and membrane fouling of different hydrophobic PTFE membranes

Membrane bioreactor (MBR) systems have shown excellent removal performance under different process settings (lab-scale, pilot-scale and real-time situations). Membrane fouling is still highlighted as one of the important draw-back of MBR process. The sludge retention time (SRT) is an influential factor that significantly affects the fouling propensity. In this study, the effects of various SRTs (2, 5, 10, 30 d) on the biomass properties, and on the performance of three polytetrafluoroethylene (PTFE) membranes (0.22 μm pore size) with different hydrophobicity (contact angle, CA = 70°, 90°, and 130°) were investigated, using long-term MBR studies.The results revealed that, the trans-membrane pressure (TMP) variations of three different hydrophobic membranes were significant at SRT of 2 d. However, there were no significant differences on TMP, flux and total resistances observed at SRTs of 5, 10 and 30 d, respectively. The analysis of extra-cellular polymeric substance (EPS) concentration and cake resistance at different SRTs, revealed that the composition of cake layer and its resistances were closely related to sludge properties. The SRT significantly influenced the average particle size, EPS concentration and protein to carbohydrate (P/C) ratio of mixed liquor. The operation at higher SRTs resulted in the change of sludge properties and reduced the fouling potential of the membrane. The effect of membrane hydrophobicity on MBR performance was only present during the initial stage. Thus, the variation of operational conditions in MBR greatly influences the sludge characteristics and the fouling, which in turn, affects the membrane performance.