| Among membrane processes, membrane bioreactor (MBR) technology is an innovative and promising option for wastewater treatment and reuse. MBR is a combination of membrane and activated sludge process, has gained considerable attention due to potential advantages over those of conventional biological treatment processes. In comparison with the activated sludge system, the MBR has merits with respect to the complete removal of solids from effluent, organic removals, a high loading rate capability, low excess sludge production and small land requirement. However, membranes are prone to fouling from interactions between the membrane and the mixed liquor. Membrane fouling significantly reduces membrane life, overall membrane performance and increases operating costs. Therefore, Membrane fouling has been the major limitation to further development and commercialization of the MBR process.In this study, the effects of the characteristics of mixed liquor on membrane fouling in MBR were investigated systematically. This work was performed as follow:(1) The behaviors of soluble microbial products (SMP) and SMP effects on an operational process of membrane bioreactor (MBR) were studied. During a 90-day period, total organic carbon (TOC), molecular weight distributions (MWDs), and specific oxygen uptake rate (SOUR) were periodically monitored. Experiment results showed that TOC of MBR supernatant increased and then slightly decreased. As the operation time was prolonged, the concentrations of SMP with MW>10 kDa remarkably increased in MBR supernatant. The SMP with MW>10 kDa obviously affected sludge activity and membrane fouling, but did not influence the MBR effluent.(2) In this study, the influence of microorganisms'metabolites, such as SMP and extracellular polymeric substances (EPS), on membrane fouling was examined during the operation of MBR. The activated sludge samples were taken from the MBR and used to investigate their filtration resistance in different phases. The results showed that accumulated SMP and EPS in the MBR had a negative effect on membrane filtration. The concentration of SMP, especially the SMP with molecular weight (MW) of 3-10 kDa, was positively correlated to the pore-blocking resistance. The cake layer formation was the main factor resulting in membrane fouling, and the amount of EPS and the SMP with MW>10 kDa had strong positive effect on cake layer resistance. By the Fourier transform infrared spectroscopy (FTIR), in cake layer the major components of the EPS were proteins, polysaccharides and humic substances, but the dominant components of the SMP were polysaccharides and humic substances.(3) The behavior of SMP and effects of SMP on membrane fouling under starvation of an activated sludge were studied. During a 16-day period, the experiment was investigated without nutrient addition to the closed system. Dissolved organic carbon (DOC) and MWDs of bulk solution were periodically monitored. The dead-end filtration tests were used to indicate the SMP effect on membrane fouling. Furthermore, the modified fouling index (MFI) was used to investigate the gel layer resistance of bulk solution closely related with the SMP. At last, an important increase in the concentrations of SMP with MW > 10 kDa in the reactor was observed, which may be related to the cellular component degradation from endogenous metabolism. The gel layer resistance was observed to make the highest contribution to flux decline. Specially, The MFI is directly related to the concentration of high MW fraction. The SMP with MW > 10 kDa were found to have strong correlation with the MFI.(4) In this study, the influence of aeration intensity on membrane fouling in MBRs was to investigate. Two lab-scale MBRs with aeration intensity of 500 and 100 L/h were operated for 60 days at a constant permeate flux. The dead-end filtration tests were conducted to confirm the mixed sludge filterability under each condition. The effect of aeration intensity on MW distributions of SMP, the particle size distributions (PSD) of flocs and the quantity of EPS in MBRs were also tested. The results show that large aeration intensity has a negative influence on the filterability of mixed sludge in MBR. Further research finds that the large aeration intensity results in the increase of SMP with MW >10 kDa in supernatant, which obviously affects the filtration of mixed sludge. It is also found that small particles in the range of 1-10μm and the concentration of EPS in flocs remarkably increase under high aeration intensity of 500 L/h.(5) This study was to investigate the effect of ferric chloride addition on retarding membrane fouling in MBR. The change of MFI and zeta potential of sludge flocs were used to determine the optimum dose of Fe (III). The optimal Fe (III) addition dose of 1.2 mmol/L significantly improved filterability of mixed liquor, and Fe (III) could supply more positive charges for soluble macromolecular substances and sludge flocs, and enhance the functions of charge neutralization. The effect of the optimum dose Fe (III) on the MW distributions of SMP and the PSD of flocs in hybrid MBR (HMBR) was also tested. Adding Fe (III) at the optimum dose reduced both SMP with MW > 10 kDa in supernatant and the fraction of small particles in the range of 1?10μm in flocs. Elemental analysis for EPS of activated sludge indicated that the additional Fe (III) in the HMBR system interacted with the negatively charged groups of EPS and enhanced the bioflocculation of small particles in the activated sludge. Additionally, the cleaning efficiencies of physical and chemical washing of the membrane for HMBR were compared with conventional MBR. De-ionized water cleaning followed by a metal chelating agent (ethylene diamine tetraacetic acid, EDTA) was highly effective for the fouled membrane of HMBR.
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