Study On Membrane Fouling Of Combined MBR-RO System

In combined MBR-RO process for advanced wastewater reclamation, the membrane fouling was studied systematically in four aspects: mechanism of membrane fouling in MBR, reduction of membrane fouling in MBR, mechanism of membrane fouling in RO, reduction of membrane fouling in RO.The parallel experiments show that the object on hollow fiber membrane is sludge cake, but the one on flat sheet membrane is secondary dynamic membrane. Sludge cake and pore block are thought to be the dominating fouling factors in hollow fiber membrane and flat sheet membrane, respectively, but secondary dynamic membrane can reduce pore block fouling. Further more, EPS only significantly affects the fouling of hollow fiber membrane because of the sludge cake, besides, secondary dynamic membrane largely reduces the correlation between SMP and fouling of flat sheet membrane. In long term operation, it is believed that secondary dynamic membrane can reduce membrane fouling in the beginning but accelerate membrane fouling at the last.The results of a pilot-scale flat sheet membrane MBR show that EPS is not uniformly distributed in the reactor. In the horizontal plane, the content of EPS is lowest in the center, and in the vertical plane, the content of EPS reduces with depth increasing. The distribution is effected by the located position of perforated pipe and the influent mode. In a bench-scale hollow fiber membrane MBR, dosing PAC and using granular sludge can both reduce membrane fouling. The filterability of granular sludge is better than that of floc sludge, but in operation, granular sludge is easily broken. It is appeared that the content of EPS in sludge cake is larger than that in the mixed liquor, both in floc sludge and granular sludge.Bovine serum albumin (BSA) and sodium alginate were used as models of protein and polysaccharides, which represent the organics in effluent of MBR in MBR-RO system. The experiments show that the impact of alginate on membrane fouling is more significant than that of BSA, and the synergistic influence between them can enhance membrane fouling notably. Fouling is observed to be more severe in the presence of Ca2+ compared to Mg²⁺ due to its ability to bridge organic substances that results in denser fouling layer. Increase in the ionic strength lead to weaker electrostatic repulsion, and hence more rapid fouling. And there are no synergistic influences between the three chemistries mentioned above. Lastly, the images of fouling layer illustrate that the gel of organic matter on membranes is thick and dense, which can induce severe membrane fouling.The effluent quality of MBR-RO system treating municipal wastewater is very good. In operation, the effect of periodical washing by salt water and using cation exchange resin (CER) as a pretreatment to reduce RO membrane fouling were studied. The results show that NaCl solution can effectively reduce membrane fouling in two ways. First, Na⁺ replaces the Ca²⁺ between organic substances. Second, the high osmotic pressure caused by high salt concentration can expand the fouling layer; CER process can remove divalent cation effectively, which alleviates the aggregation of organic matters, and as the result, the organic fouling of RO is reduced.