| Membrane bioreactor (MBR), a new high-efficient wastewater treatment technology combining biological treatment technology with membrane separation technology, has been widely used for wastewater treatment and reuse. However, membrane fouling has become the main obstacle for the wide applications of MBR. Therefore, it is significant to search for some methods to mitigate membrane fouling, thus helping increasing membrane life and reducing the operating cost. A laboratory-scale submerged flat-sheet MBR for the treatment of synthetic wastewater was operated in this study in order to verify the impacts of iron ion and Fe:Ca ratio on the membrane fouling and sludge properties of MBR. The main research contents and the results were summarized as follows:(1) The impacts of iron ion on membrane fouling were investigated in a flat-sheet membrane bioreactor (MBR) by changing the iron ion concentrations in influent water to 5 mg/L,15 mg/L,30 mg/L and 60 mg/L. The results showed that there was no significant effect of iron ion on effluent water quality. Membrane fouling could be mitigated at a certain concentration of iron ion, but it will become more seriously when the concentration was more than 30 mg/L. It could be seen from the correlartions between sludge properties and membrane fouling that soluble microbial products (SMP) and bound extracellular polymeric substances (BEPS) have significant impacts on membrane fouling. The decline of membrane fouling could be attributed to the decrease of EPS concentration through the adsorption of polysaccharides due to the strengthening bioflocculation of iron ion; however, high iron ion concentration could inhibit microbial activity, thus resulting in severe membrane fouling due to a significant increase in EPS concentration.(2) A bench-scale submerged MBR treating synthetic wastewater containing Fe3+ of 15 mg/L was operated under different sludge retention times (SRTs), i.e.10d,20d and 30d, in order to evaluate the influences of SRT on the performance of MBRs with the addition of Fe3+ The results showed that the effluent quality was slightly influenced by SRT, while membrane fouling was effectively mitigated by lengthening SRT under the condition of Fe3+ adding. The concentration of SMP significantly decreased as SRT extended from 10 d to 30 d, which correlated well with the variation of membrane fouling. The Fe3+ accumulated in MBR as the SRT prolonged, and the accumulation of Fe3+ enhanced bioflocculation and resulted in the increase of the average particle size. The accumulated Fe3+ in mixed liquor was mainly located in BEPS. It might be suggesting that the negatively functional groups in SMP were adsorbed by the bridging effects of Fe3+, which led to the reduction of SMP and increase of BEPS at longer SRT.(3) A bench-scale submerged MBR treating synthetic wastewater was operated under different ratios of Fe3+ to Ca2+, i.e. Fe3+:Ca2+=1:2, Fe3+:Ca2+=1:6, Fe3+:Ca2+=1:10. The results showed that there was no significant effect of the ration of Fe3+ to Ca2+ on the effluent quality. A certain ratio of Fe3+ to Ca2+ could be used to mitigate membrane fouling, which could be attributed to decline of reversible resistance due to the reductuion of SMP and increase of particle size by adsorption bridging of Fe3+; however, the lower ration with excess calcium ions would lead to the severe membrane fouling, which was mainly due to the increase of irreversible resistance due to the dramatic decrease of the sludge particle size. |
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