| Over the past decades,the deep treatment and resource reclamation of wastewater in chemical industry have attracted extensive attentions,due to increasing demand on clean water production and the concept of achieving minimal or zero liquid discharge(MLD/ZLD).Particularlly for the textile industry,the dyeing processes are characterized by high water consumption and extremely polluted effluents containing high concentrations of various dyes and salts.Therefore,developing efficient,low-cost technologies for wastewater treatment and reuse has become an important goal but remained great challenging.To address this issue,we designed,for the first time,a novel membrane-based technology combining a“nanofiltration-forward osmosis-membrane distillation”hybrid process.In general,this novel approach showed a great promise in effectively fractionating of the dye and the salt,recovering valuable dye molecules,and producing large amounts of clean water,which offers a new strategy and option for the promotion MLD/ZLD in textile industry.Meanwhile,the study on membrane innovation and hybrid processing are also fundamentally essential to provide an insightful understanding and technical support.In this thesis,three critical aspects have been considered,involving:(1)the development of a novel NF membrane with an excellent dye/salt fractionation efficiency.(2)the conceptualization and design a novel symmetric FO membrane without internal concentration polarization,and the tentative application in concentrating textile wastewater.(3)the construction of a robust hybrid membrane process,and the evaluation of the system compatibility,stability,and ecomomic analysis.Details are presented as follows:(1)A novel sulfonated loose nanofiltration membrane was fabricated using 2,2'-benzidinedisulfonic acid(BDSA)and trimesoyl chloride(TMC)by conventional interfacial polymerization.Due to a nascent barrier layer with a loose architecture,the obtained TFC-BDSA-0.2 membrane showed an ultra-high pure water permeability of 48.1±2.1 L m-2 h-1 bar-1,and a considerably low Na Cl retention ability of<1.8%over a concentration range of 10-100g L-1.The membrane displayed an excellent rejection of over 99%towards Congo red(CR)and rather high“solute-solute”selectivity,allowed the fast fractionation of high-salinity textile wastewater.Then,the fractionation performance of the simulated textile wastewater were tested,the TFC-BDSA-0.2 membrane required less water addition and less time consumption to accomplish the fractionation of dye and salt in comparison with NF270(Dow Company).Furthermore,by using a 20%alcohol solution as a back-washing medium,a flux recovery ratio of 95.6%was achieved with TFC-BDSA-0.2,and the dye rejection ability remained the same,whereas the active layers of NF270 membrane would be damaged by the cleaning solution.(2)Considering the severe fouling of the TFC-BDSA-0.2 membrane in the deep-concentration of the separated dye solution,the FO technology,with an inherently lower fouling propensity,was introduced.However,due to the limitation of the polymer choices and the film-fabrication processes,commercial and lab-made FO membranes are mostly constructed with an asymmetric structure.Such a conformation readily experiences the severe membrane flux decline,due to the significant reduction of the osmotic pressure utilization caused by internal concentration polarization(ICP).To address this issue,we have reported the synthesis of the COOH-derived polyoxadiazole copolymer for the fabrication of a self-standing selective thin film without a support layer.The thickness of the membrane was controlled at merely a few micrometers to achieve a high rate of rejection of the Na2SO4 draw solution,while maintaining acceptable water permeability.Because of the symmetric architecture,the membrane exhibited excellent and identical FO performance at both of its sides.Its water flux and reverse solute flux both exhibited a linear relationship with the concentration of the draw solution.The structural parameter of the fabricated membranes was zero because of the absence of internal concentration polarization in the symmetric FO membranes.Our results highlight the potential of support-free membranes for the further development of FO technology.(3)Further,we investigated the feasibility of our symmetric FO membrane(PTAODH-1.0membrane,8?m)for deep concentrating the separated dye solution,as mentioned above.Due to its dense architecture,ultrasmooth surface,and high negative surface charge,the PTAODH membrane exhibits excellent FO performance with minimal fouling,low reverse salt flux,and negligible dye passage to the draw solution side.Cleaning with a 40%alcohol solution,after achieving a concentration factor of 10,resulting in a high flux recovery ratio(98.7%)for the PTAODH membrane,whereas significant damage to the active layers of two commercial FO membranes was observed.Moreover,due to the existence of cytotoxic oxadiazole and triazole moieties in the polymer structure,our PTAODH membrane exhibited an outstanding antibacterial property with two model bacteria(E.coli and S.aureus).Our results demonstrate the promising application of the symmetric PTAODH membrane for the concentration of textile wastewaters and its superior antifouling performance compared to state-of-the-art commercial FO membranes.(4)However,as the periodical replenishment of the draw solute,for maintaining a constant osmotic pressure,would inevitably produce large amounts of draw solution(DS)during the FO process.Therefore,the necessity of DS recovery/reuse has remained as one of the greatest drawbacks of the FO technology.To address this issue,we propose an FO-MD hybrid process using our symmetric membrane and a hydrophobic polytetrafluoroethylene membrane in the FO and MD units,respectively.Three types of operation modes were investigated to systematically study the process performance in the concentration treatment of model textile wastewater;two commercial FO membranes were also tested for comparison.Owing to its low fouling propensity and lack of an ICP effect,the water transfer rate of our symmetric FO membrane quickly reaches equilibrium with that in the MD unit,resulting in continuous and stable operation.Consequently,the hybrid process using the symmetric FO membrane was found to consume the least energy,as indicated by its lowest total cost in both lab-and large-scale systems.Overall,our study provides a new strategy for using a symmetric FO membrane in the FO-MD hybrid process and highlights its great potential for use in the treatment of textile wastewater.In summary,we have developed a novel“NF-FO-MD”technology for use in the deep treatment and resource reclamation of textitle wastewater.Through membrane innovation and process optimaztion,our approach provided a new strategy in effectively fractionating of the multiple solutes,recovering dye molecules,and producing extra clean water,successively,which is highly promising to meet MLD/ZLD in textile industry. |
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