The difference in characteristics of foulants in submerged MBRs caused by the difference in the membrane flux

The main obstacle for a wider use of membrane bioreactors (MBRs) for wastewater treatment is membrane fouling, which increases operating costs. For a more efficient control of membrane fouling in MBRs, an understanding of the mechanisms of membrane fouling is important. We conducted two separate pilot-scale experiments using real municipal wastewater to investigate the influence of the membrane flux in MBRs on the characteristics of foulants, which were analyzed by Fourier transform infrared (FTIR) spectra, 13C nuclear magnetic resonance (NMR) spectra, monosaccharide composition and amino acid analyses. In each experiment, two identical membrane modules were submerged in the same MBR tank and were operated under different membrane fluxes. The results obtained in this study indicated that the membrane filtration flux significantly influenced membrane fouling in MBRs. Membrane fouling in the module operated with the higher flux was much greater than that of the other on the basis of the volume of filtered mixed liquor suspension. Analyses of the foulants desorbed from the fouled membranes revealed that the nature of the foulants significantly differed depending on the membrane flux despite the fact that the two modules filtered the same mixed liquor suspension at the same time. The difference in characteristics of the foulants caused by the difference in the membrane flux was similar in the two separate experiments, indicating that reproducibility of the data was sufficient. It was thought that different fractions of the mixed liquor suspension were transported to the surfaces of the membranes depending on the membrane flux and subsequently caused membrane fouling to different extents. The foulant desorbed from the membrane operated with a higher flux seemed to cause severer fouling than the foulant desorbed from the other membrane.

In this study, the influence of the membrane flux on the characteristics of foulants in submerged MBRs was investigated by conducting two separate pilot-scale experiments in which two membrane modules were submerged in the same MBR tank and filtered the same suspension at the same time with different membrane fluxes. Very similar results were repeatedly obtained in the two experiments. The results obtained in this study clearly showed that filtration resistance caused by filtering a unit volume of the suspension significantly increased when the membrane flux was high. The amounts of foulants in the fouled membranes could not explain the differences in the degrees of membrane fouling in the membranes operated with different fluxes. Chemical analysis of the foulants desorbed from the fouled membranes revealed that characteristics of the foulants were different when different fluxes were set in the submerged MBRs. When the membrane flux was set higher, the presence of rhamnose and anomeric carbon in the foulants was pronounced. Taking the result of FTIR analysis that indicated the pronounced presence of the amide group in the foulants in the case of the higher membrane flux into consideration, it is thought that contribution of the amino sugar component with the abundance of the rhamnose unit might become dominant when the membrane flux was high in the submerged MBRs. Particles/molecules that have features to cause severer membrane fouling can be transported to the membrane surface and eventually cause severe membrane fouling in the case of a high membrane flux. Such compounds are rarely transported to the membrane surface when the membrane flux is low. Investigation of the characteristics of organic matter with different sizes in suspensions in MBRs would be an important topic of research on fouling in MBRs.