| Forward osmosis(FO)technology,with the advantages of high solute rejection,low membrane fouling propensity,high water recovery,and relatively low energy consumption,has been considered as a promising technology for wastewater treatment.However,FO technology still faces some challenges,including severe membrane fouling when treating complex wastewater,efficiency issues of FO hybrid processes,and the lack of highperformance draw solutes,which restrict further applications of FO technology.In this thesis,an organic phosphonate scale inhibitor is used as the draw solution to alleviate the membrane fouling in the FO application of oilfield produced water treatment,and further investigated as the feed additive for membrane fouling control in the ultralfiltration-FOmembrane distillation(UF-FO-MD)application of oilfield produced water treatment.In addition,the treatment of dye wastewater with FO-extraction hybrid process is explored,and the effects of various factors such as the composition,p H,and temperature of dye wastewater on dye recovery are investigated.Detergent and polyanionic electrolyte(carboxymethyldextran sodium salt)are also selected as draw solutes,and their feasibility in FO applications of dye or biogas wastewater treatment,protein concentration are systematically studied.The details are as follows.(1)An organophosphonate scale inhibitor,ethylenediamine tetra(methylenephosphonic acid)sodium salt(EDTMPA-Na)draw solution,is adopted for alleviating the membrane fouling in the forward osmosis(FO)application of oilfield produced water treatment.Compared with the typical Na Cl draw solution,the employment of EDTMPA-Na draw solution can alleviate the membrane fouling effectively and therefore achieve a higher water recovery rate(60.6%)after a continuous 48-h test.And the mechanism of membrane fouling control with EDTMPA-Na draw solution has been systemically explored by varying the feed or draw solution compositions.The results turn out that the leakage of EDTMPA-Na draw solute to the feed solution can alleviate the formation of inorganic scales on the membrane surface effectively,ascribed to its excellent scale inhibition ability and the unique diffusion route of the draw solute(resulting in a relative higher concentration of EDTMPA-Na near the membrane surface)in FO process.And the alleviation of membrane scaling can further mitigate the accumulation of organic foulant on the surface of FO membrane.This study may provide an efficient strategy for alleviating the membrane fouling in the FO application of oilfield produced water treatment.(2)An UF-FO-MD hybrid process is utilized to treat oilfield produced water.The dense FO membrane can reject most organic and inorganic substances in the oilfield produced water,while MD process can recover the diluted draw solution and obtain high-quality product water simultaneously.UF pretreatment can remove most of the organics in the oilfield produced water,which effectively alleviates the membrane organic fouling in the subsequent FO process,but the problem of inorganic fouling still exists.On this basis,using EDTMPA-Na scale inhibitor as the feed additive can control the inorganic fouling of the FO membrane easily and effectively,which is superior to the common chemical cleaning method.The efficiency of this method is also satisfactory in the cycle test.This work has certain reference significance for the control of membrane fouling in the UF-FO-MD application of oilfield produced water treatment.(3)A FO-extraction hybrid process is applied for dye wastewater treatment and resource recovery simultaneously.FO process is utilized to concentrate the dye wastewater,to achieve a higher concentration factor and lower energy consumption compared with NF process.The FO pre-treatment can increase the dye concentration and reduce the stream volume,thereby reducing the reagent consumption in the subsequent extraction process and lowering the processing cost.In the extraction process,the compositions of the extractant and stripping agent are also optimized,and the recovery rates for congo red,fuchsin acid,orange acid and methyl blue are 97.8%,82.1%,80.1% and 76.1%,respectively.In addition,the FO process is driven by the simulated seawater(0.6 M Na Cl)draw solution,which avoids the energy-intensive operation for the draw solution regeneration.Therefore,this study provides a sustainable strategy for the dye wastewater treatment in the coastal areas.(4)Commercial detergents with the main composition of surfactants are also employed as the FO draw solutions.When the concentration of surfactant exceeds its critical micelle concentration,micelles with the larger size will form,which reduces the reverse solute flux in the FO process significantly.The effects of detergent composition and concentration on physicochemical properties and FO performances are systemically investigated.Furthermore,detergents are employed as draw solutions in FO treatment of dye wastewater or biogas wastewater from chicken farms.Water flux stability,membrane fouling and flux recovery after cleaning are also investigated during the long-term cycle test.After FO process,the diluted detergent draw solution can be used for daily washing,avoiding the energy-intensive operation of draw solution regeneration.(5)Polyanionic electrolytes,carboxymethyl dextran sodium salt with three different molecular weights(CM-dextran-1000/5000/20000),are synthesized by one-step method and used as FO draw solutes.The chemical structure,degree of substitution and cytotoxicity of the product are characterized.The effects of solution concentration,solute molecular weight and degree of substitution on the physicochemical properties(relative viscosity,osmotic pressure,electrical conductivity)of the solution and the FO performance are investigated.CM-dextran draw solution has the advantages of non-toxicity,high osmotic pressure,less reverse solute leakage,etc.,which are suitable for the application of protein concentration.The theoretical calculation also confirms that FO process shows lower energy consumption than UF process in the application of protein concentration. |
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