| Membrane bioreactors(MBRs) have been widely applied in wastewater treatment due to the advantages such as good effluent quality and small footprint. However, the fouling, which could shorten the membrane life and raise the operational cost, has become one of the most urgent problem to be solved. As an option, electric field was introduced into the MBR in order to suppress the fouling. In view of the insufficient research on the fouling suppression by electric field, and aiming at fouling suppression with comprehensive usage of electrical effects, a electro-membrane bioreactor(e-MBR) was designed and operated, and the performances in both fouling depression and pollutant removal were investigated. The effects of electric field were thoroughly investigated as well, using model substances and real MBR mixed liquor.The anodes used in the e-MBRwith comprehansiveelectrical effects were stainless steel meshes(Fe-MBR), and the cathodes were titanium sticks inserted into the water channels of a ceramic membrane module to make use of the electro kinetic properties. Under the voltage of 2.72 V(current of 0.10 A) and power-on time of 2 min/10 min(supplying electric field for 2 min every 10 min), lower transmembrane pressure and higher phosphorus removal efficiency were observed in the Fe-MBR. The analysis of properties of mixed liquor and the theoretical calculation of electric field force indicated that it was the joint force of electric field force and electrically released iron that suppressed fouling in Fe-MBR.The soluble microbial products in supernatant of MBR mixed liquor would form a gel-typefouling layer when filtered. The electric fields were applied on the fouling layers being formed and formed respectively, and higher fluxes were obtained under specific voltages, with disjunct gel pieces and porous fouling layers. Thus, the electric field could effectively suppress the gel-like fouling formed by soluble substances in the MBR supernatant. Additionally, a large number of precipitates and significant flux reduction were observed under a high voltage, which might be due to the reaction of metal ions and OH- generated at cathode.The specific resistance to filtration of synthetic MBR supernatant changed with the electrolysis time, following the trend of incensement first and then reduction. And to different extent, the existence of indigenous Ca2+in the MBR supernatant shortened the duration of incensement. Hence, the effect of electrically released iron on fouling propensity of MBR supernatant was in stages, and the critical iron demand to reduce the fouling propensity was related to the available binding sites on organic matter. It was obtained that the critical iron demand was negatively correlated to the concentration ratio of Ca2+and carboxyl group. Less critical iron demands than simulation results were obtained in the further investigation with real MBR supernatant and mixed liquor, which might be attributed to an enhanced aggregation by the small particles and colloids in real mixed liquor.The electrically released iron was found to be effective in soluble phosphorus removal, with kinetic properties varied from zero order to first order. The main mechanisms of phosphorus removal by electrically released iron were the co-precipitation of iron hydroxide with PO43-and adsorption of PO43-by amorphous iron oxides. Additionally, the phosphorous removal efficiency in real MBR supernatant was greater than simulation result, but less in real MBR mixed liquor, suggesting that the enhanced aggregation in supernatant strengthened the adsorption, while the microbial flocs in mixed liquor influenced the phosphorus concentration, iron ions and their hydrolysis products. |
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