Preparation And Application Of Forward Osmosis Membrane

Novel forward osmosis (FO) technology has the advantages of low energyconsumption, anti-pollution, zero emission and it is expected to be the next generationof desalination technology. Currently, it has been further developed in utilizingosmotic energy. Meanwhile, it has been widely applied in dealing with complex anddifficult wastewater. But, there are still several technical challenges that impede FO’slarge-scale industrial applications, for example: membrane preparation, selection andpreparation of draw solute, concentration polarization, reverse solute diffusion, aswell as membrane fouling.In the aspect of the membrane preparation, preferable FO membrane should havelower membrane structural parameter. Therefore, in this paper, high-performancehollow fiber and flat sheet thin film composite (TFC) FO membranes were made byreducing membrane structure parameter from proceed with membrane structure andmaterial. The morphology and performance of substrates and TFC membranes werecharacterized fully, and their FO performances were compared with membranesreported from the recent literatures.By adjusting the parameters of membrane preparation, series of polysulfonesubstrates with different structure and thickness had been made. Then, hollow fiberthin film composite (TFC) FO membranes were prepared on top of substrates byinterfacial polymerization. The result shown that air gap, bore fluid flow rate anddope flow rate mainly affect the thickness of substrates which is one of the mainfactors affecting the FO performance. The osmotic efficiency of membrane withthickness of0.08mm is0.53, and it has a higher FO performance but lacks ofmechanical strength. When the thickness is0.129mm, the load and osmotic efficiencyof hollow fiber membrane are2.48N and0.42respectively which meet therequirement of the application.By blending method, flat sheet hydrophilic PSF/SPEEK membrane substrateshad been made and TFC FO membranes were then prepared on top of them through interfacial polymerization. Although the addition of SPEEK resulted in weakermechanical strength and lower water permeability, it significantly increased theporosity and hydrophilicity of membrane and reduced the structural parameter of FOmembrane (S value is1.34×10-4m) which is far less than commercial CTAmembrane (S value is4.27×10-4m). In addition, the hydrophilicity of PSF/SPEEKmembrane substrate benefits the recovery of dehydrated membrane by immersing indeionized water instead of organic solvent such as ethanol which would damage themembrane.For the selection of draw solute, the FO performances of draw solute not onlyrelate to the nature of the solute, but also the property of the membrane. First of all,the FO performances of branched organics salts such as EDTA/EGTA/DTPA indifferent membrane orientations had been studied using hollow fiber membrane as anevaluating medium and comparing with rod-like substance, such as NaCl/EDA. Weresearched the influence of osmotic pressure, hydrated radius and diffusion coefficientof solutes on FO performance. Secondly, commercial RO-like and NF-like TFCmembranes were used as an evaluating medium, and compared with CTA membrane.The FO performance was tested for NaCl, Na2SO4and MgCl2solutes in differentmembrane orientations, in order to assess influence of membrane structure, pore sizeand electrical charge on FO performances of draw solutes. Combining with modelinganalysis, we provide a reference for the selection of draw solute.The result shown that branched organic salts such as EDTA/EGTA/DTPA withgreater water solubility and dissociation number generated higher osmotic pressurethan NaCl/EDA at the same concentration. Meanwhile, these substances had lessreverse solute diffusion due to large size, and they can be effectively recycled bynanofiltration or DCMD process as well. Further，through experiment and modelinganalysis, we found that both diffusion coefficient and permeability of draw solute areimportant parameters that determined FO performances and shown unequal effects indifferent membrane orientations. In AL-FS mode, the FO performances of drawsolutes were mainly effected by diffusion coefficient, so EDTA/EGTA/DTPA withminor diffusion coefficient will lead to less efficiency of osmotic pressure than NaCl/EDA due to serious ICP in the porous support layer；in AL-DS mode, bothdiffusion coefficient and permeability played important roles in FO performances ofdraw solutes, balancing and offsetting each other, so osmotic pressure efficiency ofEDTA/EGTA/DTPA slightly larger than NaCl/EDA.The nature of membrane also has some influence on the choice of solute: Unlikecommercial CTA membrane, non-woven fabric layer of traditional TFC membranegreatly increases the membrane asymmetry, affects the free diffusion of solutes, andbe harmful to FO performance. Both surface pores size and electric charge ofmembrane effect the reverse salt diffusion in FO process: in terms of the solutepermeability (B value), three kinds of membrane surfaces are negatively charged,therefore B value of Na2SO4is minimum; for the CTA and RO-like TFC membraneswith low surface charges or dense active layer, the reverse salt diffusion is mainlydetermined by screening effect, but for NF-like TFC membrane with high surfacecharges and the loose active layer, electrostatic effect is the main factor. Comparingwith CTA membrane of weaker surface charge, the TFC membranes had different Bvalues for MgCl2in different membrane orientations due to Donnan effect. In addition,suitable FO model was established for TFC membranes, and the simulated valuesshow good agreement with experimental data. So the selection of draw solute needs toconsider the natures of the solute and membrane synthetically.Finally, FO+MD system was used to treat low and high concentration shale gaswastewater. The result shown that PSF/SPEEK membrane with less structureparameter had greater water flux than CTA membrane in FO section, it increased thequality of the final produce water and reduced the lost of draw solute due to highersalt rejection. The final produce water achieved drinking water standards, the wholesystem is simple and easily operated and has reasonable energy consumption.