| With the increasing demand for high quality reclaimed water,reverse osmosis(RO)has been widely used in the field of wastewater reclamation treatment due to its advantages of high-water quality and small footprint.However,in practical engineering,RO membrane treatment technology still faces bottlenecks such as membrane fouling and the difficulty to effectively recover membrane flux after chemical cleaning of the fouled RO membrane.In order to develop efficient chemical cleaning chemicals and optimize cleaning methods for fouled RO membranes,this study took a UF-RO wastewater reclamation treatment system in actual production operation as the research object,analyzed the water quality changes along the UF-RO treatment system under different seasonal conditions,explored the fouling characteristics of RO membranes and their relationship with raw water quality under different seasonal conditions.Based on this,the RO membrane was cleaned offline by pilot test system,the cleaning effect of different cleaning agents was analyzed,the cleaning agents and cleaning methods were optimized,and the RO membrane after cleaning by different chemical cleaning methods was autopsied,and the removal characteristics of the optimized cleaning agents on the fouled on the RO membrane surface were clarified.This research result can provide theoretical basis and technical support for the recovery of RO membrane flux after fouling in practical engineering.The results of the study showed that:Compared with other seasons,the water quality along the UF-RO system deteriorated significantly in winter,and the content of dissolved organic carbon,polysaccharides,and proteins in the RO feed water increased significantly.The anatomical analysis of the RO membranes at the winter influent end,the concentrated water end and the summer influent end showed that the content of organic-biological foulant on the RO membrane surface was above 90%,and the membrane fouling level at the influent end was higher than that at the concentrated water end,and the membrane fouling level in winter was higher than that in summer.The organic fouling characteristics on the RO membrane surface operating under both seasons were similar,with protein,polysaccharide and microbial metabolites dominating,and the content of polysaccharide was significantly higher than that of protein.The inorganic fouling elements on the RO membrane surface were similar in both seasons,with Al,Ca,Mg,Na,and K dominating,among which the content of Al in the RO feed water was low,but could be deposited on the membrane surface in large quantities.The thickness analysis of biofilm and organic fouling layer composed of polysaccharides and proteins on the RO membrane surface in both seasons revealed that the organic fouling on the RO membrane was more serious than the biofouling in both seasons.The microbial community structure of RO membrane surface and RO feed water was analyzed by Illumina-MiSeq sequencing technology,and it was found that the microbial community structure of RO membrane surface and RO feed water were significantly different in different seasons.Although the microbial community structure on the RO membrane surface is different in winter and summer,the relative abundance of chlorine-resistant bacteria on the RO membrane surface is very high,among which Mycobacterium(9.72%-73.42%),Methylobacterium-Methylorubrum(5.65%-24.71%),Sphingomonas(0.31%-9.11%)and Pseudomonas(0.04%-3.92%)were the main chlorine-resistant bacteria on the RO membrane surface.The results of the study on the optimization of RO membrane chemical cleaning conditions through pilot tests show that the repeated short-time soak-cycle cleaning method can effectively shorten the cleaning time under the premise of achieving better cleaning effect.When the cleaning temperature is 35℃,the flow rate of cleaning solution is 9.6 m~3/h,first Na OH alkaline cleaning,after HCl acid cleaning conditions,the use of repeated short soak-cycle cleaning method,in the total cycle time of 2 h,the total immersion time of 4 h,the cleaning RO membrane flux can be increased by 31.98%,with the cycle of 2 h-soak 15 h cleaning method cleaning effect is similar.The alkaline cleaning agent and acid cleaning agent were optimized,and 0.1%sodium hydroxide(Na OH)+0.025%sodium dodecylbenzene sulfonate(SDBS)+1%tetrasodium ethylenediaminetetraacetate(EDTA-4Na)+2%sodium chloride(Na Cl)was used as the alkaline cleaning agent(p H=12),and 0.2%hydrochloric acid and 2%citric acid in equal volume were used as the acid cleaning agent(p H=2).The effective removal of foulants on RO membrane can be achieved.After cleaning,the recovery rate of RO membrane flux can reach 45.74%.The anatomical analysis of RO membranes cleaned by different chemical cleaning methods revealed that the RO membrane surface cleaned by different chemical cleaning methods was still dominated by organic-biological fouling,and the optimized chemical cleaning method could remove more foulants than the conventional chemical cleaning method.The optimized chemical cleaning method has significantly improved the removal rate of Ca,Mg and Fe elements,but the removal of Al elements is not high for both chemical cleaning methods.In addition,the conventional chemical cleaning method was not effective in the removal of organic matter,while the optimized chemical cleaning method could achieve efficient removal of polysaccharides and microbial metabolites.The microbial community structure of the RO membrane surface changed significantly after chemical cleaning,and bacteria such as Mycobacterium,Chryseobacterium,Cloacibacterium and Brevundimonas are more resistant to chemical cleaning and difficult to be removed.In addition,Mycobacterium can not only resist chemical cleaning with strong acid and alkali,but also survive in the harsh environment of high salt and become the dominant microorganism. |
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