| At present,the shortage of water resource is becoming increasingly serious,it`s urgent to develop economical and efficient water pollution control and water reuse technology.As an advanced wastewater treatment technology,forward osmosis receives widespread attention because of the superiority of low energy consumption,cost saving,relatively small membrane fouling,easy cleaning,high recovery rate and so on.However,as with all membrane treatment technologies,membrane fouling of forward osmosis during separation process also affects the membrane treatment efficiency and membrane lifetime.The research object of this paper was the secondary effluent of sewage treatment plant.Through the method of the contrast test(The comparison system of feed solution contains the secondary effluent of low concentration of organics and nutritive salt,microorganisms,calcium and magnesium ions;And the test system of feed solution contains the secondary effluent of high concentration of the above substances),this paper analyzes the total retention rate of forward osmosis of ammonia nitrogen,phosphate,nitrate,TOC and so on in the feed solution as the presence of different pollutants,the difference of the retention rate on the membrane surface and of the reverse salt diffusion in draw solution,we explored the influence of three major pollutants(organics and nutritive salt,microorganisms,calcium and magnesium ions)on the forward osmosis membrane separation performance.Through analyzing change rule of forward osmosis membrane flux in the process of separation,the difference of membrane surface morphology between before and after the separation and the degree of recovery of membrane flux under different cleaning methods,we explored the impact of these three major pollutants on membrane fouling.The main results are as follows:(?)The influence of organics and nutritive salt,microorganisms,calcium and magnesium ions on the retention property of forward osmosis:(1)The total retention rate of ammonia nitrogen,phosphate,nitrate and TOC of the contrast system and the test system were 62.2%~86.6%,86.0%~99.2%,80.1%~86.4%and 69.2%~90.5%,the membrane surface retention rate of the test system respectively compared with the contrast system increased by 1.5,1.7,2.8,3.6times.It indicates that the presence of these three kinds of pollutants in the feed solution will exacerbate the retention rate of conventional water quality indicators on the membrane surface,and makes a large number of pollutants accumulate on the forward osmosis membrane surface,which is not conducive to the long-term operation of forward osmosis penetration.(2)In three groups of test system,the reverse diffusion of draw solution increased4.143g/(m~2·h),4.236g/(m~2·h),2.482g/(m~2·h)thancontrastsystem,respectively,the reverse diffusion of Cl~-increased 75mg/L?90mg/L?210mg/L.The result showed that three kinds of main pollutants in the feed solution would exacerbate the reverse diffusion of draw solution.(?)The influence of organics and nutritive salt,microorganisms,calcium and magnesium on forward osmosis membrane pollution.(1)The membrane flux was decreased with the extension of the separation time both in the test system and the contrast system and the membrane flux of three groups of test system decreased by 5.9%,7.4%,9.5%than the contrast system,respectively,which illustrated the three main pollutants accelerated the decline of membrane flux,increasing membrane the degree of pollution.(2)The results of scanning electron microscopy(SEM)analysis of the forward osmosis membrane before and after forward osmosis separation showed that these three main pollutants would make the film cake layer thicker and denser,and the area of clean and exposed membrane is reduced,and the degree of contamination on the surface of forward osmosis membrane is intensified.Using EDS to analyze the membrane after two system running with calcium and magnesium ions as the research factors.The results show that the pollution of the membrane is mainly organic pollutants,the percentage of the total weight of C and O were 87.6%,89.7%respectively,and Ca,Mg,Si,Fe and Al are the main inorganic fouling substances in the cake layer,the concentration of these element in the membrane surface cake layer is low,but the greater contribution to the membrane surface pollution.(3)After the polluted forward osmosis membrane cleaning by high pressure spray,membrane flux recovery rate of the test system is about 4%less than the corresponding contrast system.After ultrasonic cleaning,the membrane flux could be restored to about 92%,the test system is about 2%less than the corresponding contrast system membrane flux recovery rate.(4)The contribution of the three kinds of pollutants on the total membrane resistance of the contribution rate was sorted from high to low were microbial,calcium and magnesium ions,organics and nutritive salt.However,for the forward osmosis membrane,the most of membrane fouling was a reversible pollution and easy to be removed by physical cleaning. |
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