Using Sand Filter As Pretreatment Of Reverse Osmosis System For Iron(?) Removal In Seawater: Performance And Mechanism

Seawater desalination has becomes an important way to solve global water resources crisis.Reverse osmosis?RO?is commonly used as seawater desalination process.The iron concentration in the influent of RO process shall be less than 0.05mg/L.After flocculation and sedimentation pretreatment,the total iron concentration exceeds the standards of RO influent.Therefore,it is urgent for seawater desalination plant to find a pretreatment process which can meet the requirements of RO influent.The result will provide the basis for the selection of pretreatment process of RO system.It adopted the sand filter as the iron removal process.The effects of total iron concentration,organic matter concentration in the influent,filtration rate and backwashing period on iron removal performance were studied.The mechanism of iron removal was discussed through the comparison of adsorption ability of different medias,distribution of different forms of iron,sterilization experiments of sand filter and characterization by XPS,XRD,FTIR on filter materials and backwash mud sample.Finally,high-throughput sequencing was used to analyze the microbial community structure on the filter media.After 47 days of start-up,the total iron concentration in the effluent of filter was lower than 0.05 mg/L,and the media was mature.When total iron concentration was1.5-4.5 mg/L,COD_Mn concentration 5-20 mg/L,filter rate was 4-10 m/h and backwashing cycle was 2-3 d,the total iron concentration in the effluent could meet the requirement of reverse osmosis system?less than 0.05 mg/L?.Freundlich model could well fit the adsorption process of the different medias in freshwater.The iron adsorption capacity of filter media without biofilm,sterilized filter media and filter media with biofilm in seawater were 0.0386 mg/kg,0.0614mg/kg and 0.1466 mg/kg,respectively,while the iron adsorption capacity of these medias in freshwater are 0.3527 mg/kg,0.4395 mg/kg and 0.5053 mg/kg,respectively.It could be concluded that the iron adsorption capacity of filter media with biofilm is the largest,besides,the presence of other metals in seawater has a competitive inhibition effect on the adsorption of iron.It was found that 67.51%of the iron were intercepted and adsorbed on the media,while 27.24%of iron could get into the cell to become non-eluted iron according to the measurement of the different forms iron of backwashing mud samples.The contribution of filter filtration interception and microbial adsorption on iron removal rate could reach 94.89%.The microbial uptake and utilization was key to the total iron in the effluent being less than 0.05 mg/L.Through the XPS and XRD analysis of media without biofilm,media with biofilm and backwashing mud sample,it could be inferred that iron chemical compounds that were intercepted and adsorbed on the media with biofilm were present as Fe???.Through the FTIR analysis of backwashing mud sample,the presence of extracellular polymeric substances are confirmed.The microorganisms in the filter did not involve the iron oxidizing and reducing bacteria,they were mainly composed of the proteobacteria and a few salt resistant bacteria.