Effects of bio-carriers on membrane fouling mitigation in moving bed membrane bioreactor

Two moving bed membrane bioreactors (MBMBR) were compared with a conventional membrane bioreactor (MBR) to investigate the effects of bio-carriers on membrane fouling treating real domestic wastewater. Under the experimental conditions, membrane filtration operating time of the MBMBR (without the scouring of bio-carriers) was nearly 1.5 times extended when compared to the MBR. Furthermore, with the scouring of membrane by bio-carriers in the MBMBR (denoted as MBMBRSC), the membrane filtration operating time was 8 times further extended. From the characterization of the mixed liquor suspensions and the membrane foulants of the three bioreactors, the results showed that the MBMBR could reduce membrane fouling due to the 58.8% lower concentration of biopolymers especially lower concentration of carbohydrates, and 15.6% lower concentration of low molecular weight (LMW) compounds than the MBR. Scouring of the membrane surface by bio-carriers was sufficiently effective in controlling membrane fouling, and the effects of bio-carriers on mitigation of membrane fouling depended more on mechanical effects of bio-carriers scouring than physio-chemical effects of mixed liquor suspension. The fouling resistance distribution showed that the dominant fouling mechanism in the MBR and the MBMBR was cake layer fouling which contributed to 84.8% and 79.4% of the total fouling resistance, respectively, while the resistance of cake layer was significantly reduced by 40.5% via the scouring of bio-carriers in the MBMBRSC. It suggested that scouring of bio-carriers could enhance the shear force on the membrane surface. As a result, the selective enrichment of Nitrospirae in the cake layer of the MBMBRSC was observed. Also, total suspended solids (TSS), organic matters and the inorganic matters in the fouling cake layer of the MBMBRSC could be largely reduced by the scouring of bio-carriers and resulted lower membrane fouling rate. The effective mitigation of the cake layer fouling contributed to the remarkable enhancement of the membrane filtration performance in the MBMBRSC, while the slowly accumulation of the dissolved organic matters and the inorganic matters in the pore blocking and constriction (PBC) as well as their combined bridging effects resulted in the final membrane fouling.