| Membrane bioreactors (MBRs) are effective and advantageous in treating municipal and industrial wastewaters over the conventional process because of the advantages including small space occupation, better effluent qualities, higher sludge concentration/low yield of surplus sludge, the sludge residence time and the hydraulic residence time separated. The major drawback and restriction of MBRs come from the high cost in preventing performance deterioration caused by progressive membrane fouling and the high cost of membrane, which are the most challenging issues for the further development of MBRs.In this work, by blending with poly(vinylidene fluoride)(PVDF) powders, the strong hydrophilic graphene oxide (GO) nanosheets and antibacterial cuprous oxide (Cu2O) nanoparticles were chosen as nanofillers to prepare the highly antifouling composite membranes.(1) The GO nanosheets were prepared with natural graphite powder through a modified Hummers method. It was found that, the GO nanosheets were fully exfoliated with a thickness of about1nm and an average platelet diameter of about several μm. The main structure of GO was a graphene sheet or overlapped by several sheets bonded to oxygen in the form of carboxyl, hydroxyl or epoxy groups, which improve O/C ratio up to38.97%. The contact angle of GO sheets was only21.6°, indicating strongly hydrophilic properties. Using GO as hydrophilic additives, increased the exchange rate between the casting solvent (DMAC) and non-solvent (water), leading to the development of finger-like voids and thin-walled sponge pores along with the increasing porosity and mean pore size, which enhanced the water permeability of GO modified membranes. In the permeation experiments, all of the composite membranes showed improvement of water permeability and separation capability, compared to the control PVDF membrane. Furthermore, the higher flux recovery ratio (FRR) and lower flux decay rate indicated that GO modified PVDF membranes had better antifouling performances due to the changes of surface hydrophilicity and morphologies.(2) Taguchi method was used as the experimental design to determine the optimum conditions for the preparation of PVDF/GO antifouling MF membranes. PVDF content, solution type, GO amount, and PVP content were chosen and tested as important effecting parameters. It was found that the combination of PVDF=12wt%, solution type=DMAC, G0=3wt%and PVP=5wt%was the optimal condition for preparation antifouling membranes. A confirmation experiment performed under the optimum conditions showed good agreement with those of the optimal operating condition. The as-prepared composite membranes were successfully used in a submerged MBR system for long-term tests. The PVDF/GO composite membrane demonstrated higher critical flux, sustained permeability, lower cleaning frequency, and filtration time that was four times longer than that of the PVDF membrane (GO=3wt%). In terms of anti-EPS accumulation, the PVDF/GO composite membrane showed lower membrane resistance, particularly, lower pore plugging resistance than the PVDF membrane. Fewer amounts of EPS, specifically LB-EPS accumulated on the composite membrane surface due to the introduction of GO nanosheets. The SEM and confocal laser scanning microscope images showed that the cake layer on composite membrane was looser and thinner, exposing part of membrane surface, which facilitated maintenance of stable permeability.(3) The highly antifouling PVDF/Cu2O/GO composite membranes coupled with hydrophilcity and antibacterial property were prepared via immersion precipitation phase inversion process. The modified membranes presented more rough surface morphologies due to the spontaneously aggregated Cu2O nanoparticles. Besides the developed finger-like voids the cellular pores of these sponge structures become larger and thin-walled even interconnected with each other after the addition of Cu2O and GO nanoparticles, which was very beneficial to the membrane permeation. The XPS results revealed that the solid-state cuprous compounds contained Cu2O and CuO in as-prepared membrane, but the Cu2O nanoparticles was dominant in cuprous compounds and accounted for about79.73%, thus, the modified membrane by Cu2O nanoparticles specifically exhibited the efficient antibacterial activity. Due to the hydrophilic and bactericidal membrane surface, the composite membranes showed excellent antifouling performances, including higher flux recovery rate (FRR), more resistant against accumulated contaminants, and lower filtration resistance, especially lower irreversible resistance. |
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