Study Of Draw Solution Selection And Membrane Fouling In Forward Osmosis

Over the last few decades,fresh water scarcity issues have become more and more severe due to industrial ization,population growth and climate change.Forward Osmosis(FO),as a green membrane separation technology,gains more and more attentions.FO process is based on the principle of natural osmotic process,driven by the osmotic gradient between two solutions of different osmotic concentrations when they are separated by a semi-permeable membrane.When the highly concentrated draw solution(DS)and the feed solution(FS)are separated by a semi-permeable membrane,pure water moves from the feed side to the concentrated DS due to the osmotic difference generated across the membrane,leading to the concentration of FS and dilution of DS.FO technology has several advantages,such as low energy consumption,low pollution,and high recovery rate.FO can be widely used in seawater desalination,power generation,industrial wastewater treatment,food production,aerospace,pharmaceutical industrial production andmany areas of daily life.However,the selection of DS and membrane fouling are still two main challenges of FO recently.It is very necessary to study further about the draw principle and the membrane fouling.Based on the study of positive permeability at home and abroad,this paper selected two kinds of FO membranes(Triacetate film(CTA film)and high-performance thin polyamide compound film(TFC film)produced from American HTI Company).To find the best anion and ion,eight common inorganic draw solutes,NaCl,NaNO3,KCl,KNO3,NH4Cl,NH4NO3,CaCl2,and Ca(NO3)2,were selected and systematically studied the influence of different cations(K+,Na+,NH4+,and Ca2+)and different anions(Cl-and NO3-)on water flux under two membrane orientation modes(FO mode:FS facing active analysis layer,PRO mode:FS facing porous support layer).Next,the effects of salinity on the concentration polarization on both sides of the membrane were studied under the unchanged osmotic pressure difference.Then,the removal efficiency of surfactant(sodium dodecylbenzenesulfonate,SDBS),heavy metal Ni2+,and antibiotic(ciprofloxacin,CIP)were investigatedunder different operating conditions.According to the principle of draw solute selection,the feasibility of using polyacrylamide(HPAM),a polyelectrolyte substance that is widely used in the oil field,as a driving agent is studied by comparing with common inorganic driving agents,and then it is applied to the treatment of dye wastewater.The efficiency of long-term operation under different membrane orientations was studied.In addition,in order to investigate the situation of forward osmosis membrane fouling,bovine serum albumin(BSA)and octanoic acid(OA)were selected to represent the widespread presence of secondary effluent.Proteins and organic acids were studied for their effects on membrane fouling under different conditions such as pH,Ca2+ concentration,ionic strength,membrane orientation and initial water flux.The main findings are as follows:1.Through comparison of the driving effect of inorganic drivers,it was found that different types of cations had different impacts on water flux.Na+ and K+ ions had higher water fluxes among the five cations.Chloride had more water flux with less reduction than nitrate.In addition,the solution properties had different effects on water flux at different membrane orientations.In a certain range of viscosity,the water flux in the PRO mode was dominated by osmotic pressure;and in a certain range of osmotic pressure,the water flux in the FO mode was dominated by viscosity.Among the eight inorganic salt drivers selected,KCl was the best performance driver and NaCl was the best cost-effective driver.It provided theoretical support for the selection of inorganic driver in the future.2.Under the condition of that the solution osmotic pressure difference of the two sides of membrane was the same,the effect of increasing salinity of the solution on both sides of the membrane was determined.The increase of salinity on both sides of the membrane enhanced the concentration polarization,and then caused the water flux to decrease.In addition,under low salinity conditions,the water flux under FO mode increased significantly with increasing temperature,but under high salinity conditions,the increase rate of water flux declined with increasing temperature.The increase of the flow rate effectively alleviated the external concentration polarization and promoted the water flux in the PRO mode,but it had little effect on the internal concentration polarization,and cannot effectively increase the water flux under the FO mode.With the increase of salinity,the role of flow velocity on water flux enhanced.In PRO mode,high salt wastewater caused severe fouling of the membrane,which could be removed by simple physical cleaning.This section clarified the effect of salt salinity changes on the water flux and membrane fouling on both sides of the membrane,and discussed the influence of temperature,flow rate,and membrane orientation changes on water flux and membrane fouling in FO process.It provided some important technical support to the application of FO in treating waterwaters of different salinities.3.FO was applied to treat various wastewaters containing surfactant SDBS,heavy metal Ni2+,and antibiotic ciprofloxacin CIP using both TFC and CTA membranes.The results revealed that TFC membrane had high rejection efficiency for the three pollutants,and the main rejection mechanisms were steric hindrance and membrane adsorption.The retention rate of SDBS by the TFC membrane was above 98%under all cases.The rejection rate of Ni2+ for both CTA and TFC membranes was above 93%.Under low salinity conditions,the surfactant SDBS increased the rejection of Ni2+ ions,while under high salt conditions,the effect of SDBS was not obvious because of the accumulation of a large number of Na+ ions on the membrane surface.The rejection rate of CIP for CTA membrane was greater than 80%.In addition,pH had a great influence on the rejection rate of CIP.With the increase of pH,the retention rate of membrane to CIP continued to decrease,from 97.4%to 86.7%.The electrostatic repulsion contributed less in the rejection of CIP.In addition,the increase of salinity enhanced the retention of all three pollutants.It had vital guiding significance for the practical application of FO technology in different wastewaters.4.The organic polymer HPAM solution had stable water flux and low reverse salt flux.After the dilution by FO process as DS,HPAM solution maintained high viscosity and met the requirements of the three oil production.Using FO process,HPAM solution was applied as DS to treate the dye wastewater.After 50 hours of operation,dye wastewater was well concentrated and HPAM was well diluted.In the PRO mode,the membrane fouling went through 3 different steps:a.HPAM formed the cake layer;b.Dilution of the HPAM cake layer;c.Membrane pores blocking;while the membrane fouling was mild in the FO mode.In the combined membrane orientations of FO and PRO mode,HPAM and dye molecules interacted and cross-linked to form a complex fouling layer,which was hard to move only by the simply physical cleaning.Comparing with KCl,which had the best performance among the inorganic salts,the driving effect of organic polymer HPAM was studied and applied to treat dye wastewater,which enriched DS system of FO technology.The diluted HPAM could maintain a high viscosity and be directly applied to oilfields for polymer flooding,saving a large amount of freshwater resources,which had a very positive significance for the oil field.5.The membrane fouling process and mechanism of BSA and fatty acid OA in FO process were studied.The study found that the presence of Ca2+ enhanced the BSA membrane fouling since that Ca2+ and BSA combined and formed a complex BSA-Ca2+,and at the same time,Ca2+ occupied the adsorption site of the membrane surface and reduced the adsorption amount of OA into the membrane surface,and thus lightened the membrane OA fouling.The increase of FS ionic strength of FS weakened the electrostatic repulsion,which made the accumulation of BSA on the membrane surface increase,led the more severe membrane fouling of BSA.Both BSA and OA had the most serious contamination at isoelectric point(BSA pH 4.7,OA pH 3.56).It indicated that membrane fouling caused by neutral molecules was most serious.The high initial water flux led to the formation of filter cake layer and the plugging and compaction of membrane pore,resulting in more severe membrane fouling.The interaction of OA and BSA lightened the combined membrane fouling.