Traditional membrane fouling models do not accurately describe the membrane bio-reactor (MBR) process. Since the major foulants in MBR are the activated sludges, the biological characteristics should be considered. Existing research finds that the activated sludge characteristic is closely related with membrane fouling. Many researches recognize that one of the important factors on fouling is the extracellular polymeric substances (EPS). However the existing models do not reflect the biological effects on membrane fouling. Little attempt has been made to integrate both the biological process and the membrane fouling simultaneously in an MBR model. The traditional fouling models do not accurately describe the fouling behavior of MBR, because these biological activities are not incorporated.

  In this study, the relationship between the solids retention time (SRT) and the amount of EPS is investigated in three lab-scale MBRs. Additionally, the EPS effect on membrane fouling is quantified by calculating the specific cake resistance using an unstirred batch cell test. By observing the sludge over a long period of time under various SRT scenarios, a wide range of EPS and membrane fouling data is collected. These multiple observations provide sufficient evidence of the functional relationship between SRT and EPS. A rigorous mathematical model reflecting the biological effects on membrane fouling was used to conduct this research. The model can predict the effluent quality and membrane fouling simultaneously. The model describes the dynamic behavior of the EPS concentration with the change of the biological operating factors such as the organic loading rate, the hydraulic retention time and SRT. The membrane fouling is simultaneously calculated using the modified resistance-in-series model. The model reflects all the experimental results from the lab-scale experiment and the unstirred batch cell test.

  There are four key empirical findings from this research. First, as SRT decreases, the amount of EPS bounded in sludge floc increases, however this is true only in low MLSS conditions. In contrast, in high MLSS condition (>5,000mg/L), the amount of bound EPS and SRT are not correlated, because EPS can be utilized as substrates by microorganisms. Second, EPS bounded in the activated sludge floc is correlated with membrane fouling. As the amount of bound EPS increases, the specific resistance of the cake layer is higher. Based on these results, a function between the amount of EPS and the specific cake resistance is established empirically. Third, from the simulation results, the research finds that the rate of flux decline tends to increase with increasing SRT for SRT less than 20 days. However, for SRT greater than 20 days, SRT does not influence the rate of flux decline. Fourth , the influences of SRT and food to microorganisms ratio (F/M) on EPS concentration shows that there
exists two distinctive regions. In the region where F/M is less than 1.0gCOD/gVSS/day, the amount of bound EPS decreases with increasing SRT. Conversely, in the region where F/M is higher than 1.0, the EPS concentration seems to be independent of SRT. EPS is the dominant factor affecting membrane fouling in short SRT with high F/M value, while total suspended solids is the most important factor in long SRT. Consequently, the model developed in this study can predict the effluent quality and membrane fouling, simultaneously. The model simulation can help MBR to be operated optimally.

Keyword :  Membrane Bio-Reactor (MBR), Extracellular polymeric substances (EPS), Fouling, Bio-fouling, Specific cake resistance, Modeling.