Study On The Mechanism Of Ferrate(Ⅵ) Enhanced Ultrafiltration For Secondary Effluent Water Quality Improvement And Membrane Fouling Mitigation

As an advanced wastewater treatment technology,ultrafiltration technology has attracted more and more attention.However,the membrane fouling problem caused by ultrafiltration membrane in the long-term operation process has also become an important factor limiting the membrane development.Therefore,coagulation,adsorption,oxidation and other pretreatment technologies to alleviate membrane fouling have also been studied by many scholars.Its essence is to remove part of the pollutants that cause membrane pollution through coagulation,adsorption,oxidation decomposition,so as to alleviate membrane pollution and improve the water quality of effluent.However,sometimes a single pretreatment technology can not well alleviate the membrane fouling problem,so some pretreatment processes are often combined.In this paper,the secondary effluent was taken as the treatment object.In the experiment,ferrate with multiple functions such as oxidation,coagulation,adsorption and disinfection was selected as the pretreatment agent before membrane,and ferric chloride was selected to enhance the oxidation and coagulation effect of ferrate.The effects of Fe（Ⅵ）、different molar ratios of Fe（Ⅵ）/Fe（Ⅲ）on the fouling alleviation of membrane were investigated respectively.In the experiment,the membrane fouling mitigation effect was evaluated by membrane flux curve,membrane fouling resistance distribution,membrane fouling surface characterization and membrane fouling model fitting analysis.In addition,the purification efficiency of these membrane pretreatment technologies coupled with ultrafiltration on water quality was also investigated.The decontamination efficiency was evaluated from the water quality indexes,such as DOC,UV₂₅₄,polysaccharides,proteins,EEM,molecular weight distribution and nutrients（TN,TP）.In the process of adding Fe（Ⅵ）,it is found that the floc has small particle size and poor settlement performance,which causes the rise of turbidity and seriously affects the results of UV₂₅₄and polysaccharide.However,the ultrafiltration can block the floc and the effluent water quality is good,but the removal rate of organic matter in the secondary effluent by ultrafiltration itself is not high.The pretreatment of Fe（Ⅵ）in front of the membrane can significantly alleviate the membrane flux decline,and has a significant removing effect on the hydraulic reversible resistance and hydraulic irreversible resistance.It can slightly alleviate the standard blockage and effectively alleviate the membrane fouling.When Fe（Ⅵ）concentration was100μmol/L,the removal rates of DOC,UV₂₅₄,polysaccharide and protein could reach 36.58%,39.51%,17.75%and 36.71%,respectively.The removal effect of fluorescent organic compounds and macromolecular organic compounds detected by UV was also the best,but it had almost no effect on TN,and the removal rate of TP reached 92.37%.Both oxidation and coagulation of Fe（Ⅵ）can remove organic matter,and coagulation has better effect on DOC removal than oxidation.When Fe（Ⅵ）dosage is 100μmol/L,oxidation and coagulation account for 30.85%and 69.15%of DOC removal contribution.However,oxidation is the main function of removing all fluorescent organic compounds,and coagulation only has a certain removal effect on aromatic proteins and tryptophan-like substances,but the removal of UV absorbent macromolecular organic compounds is mainly by oxidation.For membrane fouling control,with the increase of Fe（Ⅵ）dosage and operating cycle,coagulation gradually becomes the main mechanism to alleviate membrane fouling,in which oxidation can better alleviate the hydraulic irreversible resistance,coagulation is the key to alleviate the hydraulic irreversible resistance,but oxidation can only relieve standard blockage.The combination of Fe（Ⅵ）/Fe（Ⅲ）can obviously promote the formation of flocs and the settlement performance of flocs,and can obviously improve the water quality,so as to avoid the influence on the ultraviolet detection of organic indicators.The coupling ultrafiltration with Fe（Ⅵ）/Fe（Fe）dosage ratio of100/50μmol/L has the best removal effect on organic pollutants The removal rates of DOC,UV₂₅₄,polysaccharide and protein could reach 40.08%,48.15%,23.67%and41.67%respectively.However,the Fe（Ⅵ）/Fe（Ⅲ）dosage ratio is 50/100μmol/L,and the specific flux is 0.6 at the end of the membrane fouling,and the specific flux is restored to 0.91 at the beginning of the third cycle after the hydraulic backwashing,and the hydraulic irreversible resistance and hydraulic irreversible resistance are mitigated to the greatest extent.By contrast,the specific flux was 0.54 at the end of the 100/50μmol/L Fe（Ⅵ）/Fe（Ⅲ）dosage ratio,and recovered to 0.77 at the beginning of the third cycle after hydraulic backwashing.In this paper,Fe（Ⅵ）and the combination of Fe（Ⅵ）/Fe（Ⅲ）were respectively used to systematically explore the mechanism of membrane fouling alleviate and water quality improvement.The research results can provide a new idea for the membrane pretreatment process and the advanced wastewater treatment technology.