•Additional hydraulic pressure is proposed to compensate water flux loss in OD.•ICP determines the availability of HP method at both membrane orientations.•Water flux decline can be mitigated by constant pressure under limited conditions.•CDHP is relevant to variational concentration difference in HP processes.•3-Stage HP leads to 12.79% increase in final water flux at AL-FS orientation.Forward osmosis has motivated practical applications in seawater desalination and agricultural irrigation due to its potential advantage of osmotic dilution. However, water flux decline accompanies with continuous dilution of the DS, which will cause extra membrane expenditure, until final osmotic equilibrium. Without the help of additional driving force, it is impossible to reduce driving force loss in OD. In this study, concentration-dependent hydraulic pressure is exactly introduced as an auxiliary driving force. Investigations on water flux decline behavior in OD showed that water fluxes at lower initial concentration difference, lower initial solution volume and AL-DS orientation suffered more severe decline; furthermore, it implied that additional hydraulic pressure could alleviate adverse effects of greater concentration difference variation generated by pressure-induced water flux increment on water flux. For given dilution of the DS, minimized change in bulk FS concentration was conducive to ensure the effectiveness of constant hydraulic pressure on reducing water flux decline. Validation experiments demonstrated that current model equations were more appropriate under lower hydraulic pressures, and stable water flux also relied on concentration difference variation corresponding to applied hydraulic pressure. Potential implications were highlighted in the context of technical progress of membrane preparation and application potential of OD.Download high-res image (104KB)Download full-size image

This work describes the covalently binding of 2-hydroxyethyl methacrylate (HEMA) and 2-(dimethylamino)ethyl methacrylate(DMAEMA) brushes onto the poly(vinylidene fluoride) (PVDF) membrane surfaces via surface-initiated atom transfer radical polymerization (ATRP). Prior to ATRP, PVDF was coated with 3,4-dihydroxyphenylalanine (DOPA). The hydroxyl groups on the polyDOPA-coated PVDF membrane surface and pore surface were used for the immobilization of alkyl halide ATRP initiator. The grafting yield of poly(hydroxyethyl methacrylate) (PHEMA) and poly((dimethylamino)ethyl methacrylate) (PDMAEMA) was determined by weight gain which was linearly increased with the polymerization time. Fourier transform infrared spectrometer (FT-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and atomic force microscope (AFM) were used to characterize the chemical composition and surface morphology of PVDF membrane and modified membrane, respectively. Water contact angles and water intake measurements indicated that the introduction of PHEMA graft chains promoted remarkably the surface hydrophilicity of PVDF membranes. It was also found that PHEMA graft chains provided higher pure water flux and better anti-protein absorption ability to PVDF membranes. Water flux decreased with increasing polymerization times, while the BSA rejection curves shifted to lower molecular weight cutoff values. The quaternized PVDF-g-PDMAEMA, PVDF-g-PDMAEMA-b-PHEMA membranes exhibited excellent antibacterial properties against Staphylococcus aureus. This study not only introduces a modification approach to obtain a PVDF membrane grafting hydrophilic PHEMA, but also provides the antibacterial properties for PVDF membrane with PDMAEMA.Graphical abstractSchematic diagram illustrating the process of SI-ATRP from the PVDF membrane.Highlights► Modification of poly(vinylidene fluoride) membranes via surface-initiated atom transfer radical polymerization. ► Alkyl halide ATRP initiator is immobilized by hydroxyl groups on polyDOPA-coated PVDF membrane surface and pore surface. ► PHEMA graft chains promote remarkably the surface hydrophilicity and antifouling property of PVDF membranes. ► The quaternized PDMAEMA-grafted membranes exhibit excellent antibacterial properties against S. aureus.

The antifouling property of poly(vinylidene fluoride) (PVDF) ultrafiltration (UF) membrane incorporating the amphiphilic copolymer additive, poly(vinylidene fluoride)-graft-poly(hydroxyethyl methacrylate) (PVDF-g-PHEMA) synthesized via atom transfer radical polymerization (ATRP), has been investigated. During casting, the PVDF-g-PHEMA additive segregates to form a PHEMA brush layer on all blend membrane surfaces, including internal pores. Polymerization time and monomer content are used as the independent variables to manipulate the amount of grafted PHEMA on PVDF, respectively. The grafting yield of PHEMA is determined by molecular number (Mn) which is increased with the polymerization time and monomer content. The polydispersity index of PVDF-g-PHEMA remains narrow Mw/Mn at around 1.21–1.45 throughout the reaction. The chemical compositions of copolymer and blend membrane are investigated by Fourier transform infrared spectrometer (FT-IR) and X-ray photoelectron spectroscopy (XPS). The morphology of blend membrane is investigated by scanning electron microscopy (SEM). It is found that PHEMA brushes provide higher pure water flux and better anti-protein absorption ability to PVDF membranes. Water permeability of blend membrane using PEG20000 as pore-forming agent is excellent than that using PEG400 due to formation of larger pore sizes on the PVDF membrane surface and some PEG20000 tangling up with copolymers in PVDF matrix. The exceptional anti-fouling performance holds promise for extending UF membrane lifetimes without need for aggressive cleaning procedures.Highlights► Modification of PVDF polymers via atom transfer radical polymerization. ► ATRP initiator is immobilized on the PVDF-OH polymer via Fenton reaction. ► PHEMA amphiphilic chains promote surface antifouling property of PVDF blend membrane. ► Hydrophylicity and BSA adsorption resistance of blend membrane are increased.

•We proposed prepressing operation in spacer-filled channels for water flux surge.•Deformation-induced water flux increase rate reached up to 65.41%.•Slight water flux decline was observed in stability experiments of FO.•Water flux decline could be compensated via repeated prepressing operation.In this paper, a simple and feasible prepressing method in spacer-filled channels was applied to improve the membrane performance for commercial TFC-FO membranes. Deformation-induced water flux increase rate reached up to 65.41%, meanwhile reverse solute flux had just increased by 24.90%. Such results showed that the membrane performance was effectively improved without complex chemical modification. Stability experiments also demonstrated that the viscous forces in the water permeation process had exerted slight adverse effect on the permeability–selectivity performance of deformed membranes compared to hydraulic pressure. As such, repeated prepressing operation turned out to be effective in terms of stable water flux in long-term filtration process. Therefore, the prepressing operation in spacer-filled channels can be an alternative method to enhance the initial membrane performance and, more significantly, improve the process efficiency in FO.Download high-res image (76KB)Download full-size image

•Using SI and S&DSI to predict scaling in NF–SWRO process at high RNF.•CaSO4 scale was more easy to form on an NF membrane surface than CaCO3 scale.•CaCO3 scale forms at NF recovery above 40% without pH adjustment.•An antiscalant dosage could inhibit CaSO4 scale and increase NF recovery up to 65%.•The specific energy consumption of the loosen NF membrane is lower than 0.95 kWh · m− 3.Pilot-scale tests were carried out on a nanofiltration (NF)–seawater reverse osmosis (SWRO) integrated membrane system (IMS) using a kind of ultra-low pressure and high selectivity NF membrane (ESNA3, Hydranautics). Three different schemes were investigated for the NF seawater softening processes. The effect of increasing NF permeate recovery rate (RNF) on the potential of scaling in the NF and RO modules was investigated in term of concentration polarization modulus (CP) of scalant ions, Stiff and Davis Stability Index (S&DSI), and Supersaturation Index (SI) of CaCO3 and CaSO4. The results show that within the test range, high RNF, large permeate flux and low specific energy consumption (Es) could be achieved simultaneously for the loosen NF membrane. The Es for ESNA3 membrane could be lower than 0.95 kWh · m− 3. CPSO42 − was larger than CPCO32 − in the NF module under the NF retentate recycling operating conditions. The SI data indicated that at RNF of higher than 65% and with antiscalant addition and pH adjustment, CaSO4 would preferentially precipitate on NF membrane surface. However, with pH adjustment, the S&DSI values on the NF and SWRO membrane surface remained negative, which indicated that CaCO3 scaling could not form in the pilot test operating range.

•We propose a hybrid RED-ED system for high-salinity wastewater.•We investigate effects of operation conditions on ED and RED-ED systems.•We provide two indexes to design and improve the hybrid system by economic analysis.•We point out control conditions for preliminary screen of applicable wastewater.•The hybrid system can lower overall operating costs and recover SGP.Reverse electro-dialysis (RED) is a potentially available technology to harvest salinity gradient power, which can be extracted from high-salinity wastewater in industrial processes. This study proposes a hybrid RED/ED system to simultaneous osmotic energy recovery and complete desalination in the phenol-containing wastewater treatment process. Experimental investigation on standalone ED system reveals that the high stack resistance, resulting from the salinity difference between both feed streams, accounts for the inefficient desalination performance in early stages, which implies potential implication of RED as a pre-desalination process with natural driving force to reduce the salinity difference. Pre-desalination rate is related to power generation in RED stage and energy saving in ED stage under optimal operation conditions. Compared with artificial seawater, greater power generation is produced by using phenol-containing wastewater as the high-salinity stream. Economic analysis suggests that average power generation and limiting wastewater treatment capacity can provide insightful guidelines for the design and improvement of the hybrid system. Control conditions are listed as a fundamental criterion to search for potentially applicable wastewater systems in the hybrid system. Therefore, hybrid RED/ED system can realize triplex advantages of salinity energy usage, high-valued resource reclamation, and low-energy desalination in high-salinity wastewater treatment processes.Download high-res image (69KB)Download full-size image