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Numerical simulations were carried out to assess the technical and economic feasibility of a solar water desalination system that has a novel hybrid Concentrating Photovoltaic Thermal (CPVT) collector coupled with a Vacuum Membrane Distillation (VMD) process. A special characteristic of this CPVT is its triangular receiver with PV cells facing the reflecting surface. This type of receiver has the advantage of generating more electricity with less PV surface area and great potential to be used to hybridize conventional parabolic thermal collectors. TRNSYS was employed to analyze the annual performance of the CPVT-VMD system evaluating parameters such as solar fraction, specific permeate production and specific energy production for different coastal cities. In the dynamic simulations, local annual weather data and specific information about the characteristics and operating conditions of a real CPVT collector and a VMD module were considered. From the parametric analysis the optimal surface area of collectors and the input temperature of the VDM module were determined. A maximum specific permeate of 218.410 m 3/m2VMD for Acapulco, MX, and a minimum of 170.365 m 3/m2VMD for Singapore, SG, were achieved for the proposed CPVT-VMD system of four solar collectors with an operating set temperature of 55 °C. An economic profit was found after 7 years for Acapulco city, which showed great potential to use solar energy from hybrid CPVT collectors for a VMD process to provide freshwater in coastal cities. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) can concentrate non-volatile solutes or remove volatiles and dissolved gases from an aqueous feed. A microporous hydrophobic membrane provides a barrier between the hot feed and cold distillate. Although MD can operate at ambient pressure and moderate temperatures, use waste heat, and treat wastewater via an MD-bioreactor, it has problems such as temperature polarization, liquid weeping to the distillate side, and membrane fouling. Prior studies speculated that fouling can add a heat- or mass-transfer resistance, or cause a vapor-pressure reduction owing to the Kelvin effect, but did not isolate these effects. This study confirms that the vapor-pressure depression owing to the concave interface in the small pores of the fouling layer is a dominant cause of the 25–63% flux reduction observed for humic-acid fouling on PTFE and PVDF membranes. This study underscores the importance of selecting MD membranes based on their pore-size distribution rather than just their nominal diameter in order to maximize the contribution of Knudsen diffusion. It suggests the development of dual-layer membranes having a thin hydrophilic layer with relatively large pores overlying a hydrophobic layer with a typical MD membrane structure in order to mitigate the vapor-pressure reduction owing to membrane fouling. [ABSTRACT FROM AUTHOR]

Abstract: Membrane distillation (MD) is a continuous process whereby a hot aqueous feed provides the latent heat to vaporize water that diffuses through a hydrophobic microporous membrane and condenses. MD is attractive for saline and wastewater treatment as well as concentrating heat-sensitive solutions such as fruit juices and biological fluids since it operates at moderate temperatures and atmospheric pressure, rejects particulates and nonvolatile solutes, uses non-selective membranes, and has low capital costs. However, MD and MD bioreactor (MDBR) performance is compromised by fouling. The effect of fouling layers with reasonably large pores (>50nm) is reasonably well-understood and has been incorporated into MD models. However, the effect of fouling layers with very small pores or free volume (<50nm) owing to macromolecular- and bio-fouling (MMBF) is not well-understood. While MMBF introduces an additional resistance to heat transfer, it can also reduce the vapor-pressure driving force. This latter effect is unique to MD fouled by MMBF. In this analysis the vapor pressure reduction has been incorporated into a model that indicates the large flux declines observed in prior studies can be explained by the vapor-pressure reduction associated with pore diameters ranging from 3.9 to 8.5nm, which agree well with evapoporometry characterization of MMBF sludges. The predicted flux decline and temperature polarization coefficient indicate that the vapor-pressure reduction increases markedly for effective pore diameters less than 10nm, but can be mitigated by increasing the thermal resistance of the membrane and by fabricating a dual-layer membrane for which the feed side is hydrophilic with relatively large pores. [Copyright &y& Elsevier]

Abstract Despite membrane distillation (MD) being well acknowledged as a green technology due to the capability to treat water using waste heat, the efficient use of this heat remains one of the biggest challenge for MD to be cost-effective. In this study, we harnessed the high thermal conductivity of metallic spacers to improve the energy efficiency in DCMD, with the performance studied via both simulations and experiments. The frequently used polypropylene spacer with each mesh dimension of 3 mm was used as a benchmark for comparison. Two metals were investigated, (namely, nickel and copper), along with three spacer densities (namely, 3 mm mesh, 1.5 mm mesh and foam). The noteworthy results include: (i) the metallic spacers placed on the feed side of the membrane gave higher temperatures that were more uniform along the membrane surface; (ii) whereas the surface-averaged membrane temperature for the polypropylene material decreased with spacer density, that for the metallic materials decreased then increased with spacer density; (iii) although the experimental distillate fluxes were similar, the heater input energy per unit volume distillate was lower for the metallic foam spacers by up to 16%; and (iv) platinum-coated Ni foam gave a remarkable reduction in energy input of 28% under light irradiation. This study demonstrated the feasibility of using metallic spacers with inherent high thermal conductivity to improve energy efficiency, providing a new platform for further improvements through other commercially available metallic foams or spacer modifications. Highlights • Harness high thermal conductivity of metallic spacers for energy-efficient MD. • Compared nickel and copper foams against conventional polymeric spacers. • Metallic spacers gave higher and more uniform feed-side temperatures. • Energy per unit volume distillate for the metal foam spacers reduced by up to 16%. • Photocatalytic coat on Ni foam reduced energy required by 28% under light irradiation. [ABSTRACT FROM AUTHOR]

The escalating volume of oily wastewater, particularly produced water, necessitates efficient means of treatment for the recovery of both the water and oil constituents. Towards this end, membrane distillation (MD) is a promising candidate. Unfortunately, studies on the MD of oil emulsions are scarce, in large part because of membrane pore-wetting issues, which undermines the separation of oil and water. An earlier experimental study has indicated that membrane pore-wetting is caused by the presence of surfactant (namely, SDS) and salt (namely, NaCl), rather than oil, but the mechanisms underlying the poor performance remains unknown. Therefore, in order to improve the feasibility of MD, this molecular dynamics study is targeted at revealing the complex interactions that are detrimental to MD performance. The interactions of the three key constituents in produced water (namely, surfactant, oil and salt) among themselves and with the membrane are studied. The worst membrane pore-wetting experimentally observed for feeds containing SDS and NaCl is because NaCl increased the SDS-PVDF affinity without decreasing the mobility of SDS, and thereby leads to decreased surface tension and the increased likelihood of exceeding the liquid entry pressure (LEP). Accordingly, in order to mitigate pore-wetting in the membrane distillation of such feeds, means towards reducing the surfactant-membrane affinity, rather than oil-membrane affinity, and surfactant mobility near the membrane are recommended. [ABSTRACT FROM AUTHOR]

To optimize the Laser-Induced Graphene (LIG) Janus membrane, this study investigated the effects of membrane pore structure, polydimethylsiloxane (PDMS) coating sequence and addition of silver (Ag) nanoparticles on membrane distillation (MD) performance. This study aimed to enhance the photothermal characteristics of graphene while using the intrinsic electrical conductivity for simultaneous photo- and electrothermal MD. Operating at the same photo- and electro-thermal power input, the LIG Janus membrane made by treating the membrane face with smaller pores (i.e., shiny side) gave an improved flux performance of up to 53.6% and a decrease in specific energy of 35.4% compared to that by treating the membrane face with larger pores (i.e., dull side). The effect of the PDMS coating sequence also depended on the pore structure. For the face with smaller pore structures, coating PDMS before laser irradiation (PDMS-BLSS) gave a flux improvement of up to 24.5% and a decrease in specific energy of 19.7%, compared to coating PDMS after laser irradiation (PDMS-ALSS). As for the face with larger pore structures, coating PDMS before laser irradiation (PDMS-BLDS) resulted in a flux reduction of up to 20.8% and an increase in specific energy of 27.1%, compared to coating PDMS after irradiation (PDMS-ALDS). The LIG Janus membranes embedded with Ag nanoparticles led to improved photothermal heating properties, improving flux by 43.1–65.8% and decreasing specific energy by 15.2–30.5% while maintaining similar electrothermal heating properties. Carrying out simultaneous photo- and electro-thermal MD indicates that only the Ag-doped Janus LIG membrane gave a synergistic effect whereby the flux of the combined heating mode was higher than the summation of the fluxes obtained when operating in the individual heating modes. [Display omitted] • Simultaneous photothermal and electrothermal membrane distillation (MD). • Formation of laser-induced graphene (LIG) on different membrane structures. • Effect of coating sequence on MD performance. • Effect of doping silver nanoparticles on MD performance. • Evaluate the synergistic effect of photothermal and electrothermal heating in LIG. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) is a promising green technology that can harness waste or solar heat to treat water. Since such heat sources suffer from temporal variations, a complementary, efficient means is necessary. Induction heating is attractive in providing localized heating at the feed-membrane interface where the heat matters, but the typical conductive materials (e.g., metal, alloy) corrode readily in the high-salinity, high-temperature environment. In this work, a superior induction material was proposed, namely, reduced graphene oxides (rGO) coated onto a porous Nickel foam by Nafion (rGO/Nafion/Ni), which exhibited superior and sustained performance for membrane distillation. Results indicate that rGO/Nafion/Ni achieved up to 28.1% higher water flux (6.42 ± 0.36 Lm−2h−1) and 37.5% lower energy consumption (3.13 kWhL−1) compared to other carbon materials coated on Ni or the bare Ni foam. The mechanism underlying the enhanced MD performance was the higher absorption of the electromagnetic waves via multiple internal reflections and larger eddy currents generated by rGO, leading to higher temperatures at the feed-membrane interface that increased the driving force for distillation and thereby the energy efficiency of MD. Our work demonstrated that the induction heating of rGO has high potential for augmenting MD performance in water treatment. Image 1 • Modification of spacer rather than membrane to improve MD performance. • Induction heating of rGO/Nafion/Ni spacer reduced MD energy consumption by 37.5%. • rGO was superior to bare Ni spacer and two other carbon materials coated on Ni. • rGO gave the highest temperature via induction heating due to marked Joule effect. • rGO nanosheets absorbed more electromagnetic waves via internal reflections. [ABSTRACT FROM AUTHOR]

Highlights • Novel thermoelectric coupled sweeping gas membrane distillation (T-SGMD) hybrid. • Hybrid membrane distillation for decentralized water treatment, dehumidification and space cooling. • Latent heat cooling provides for water treatment simultaneously. • Impact of membrane area, cool air recycle and module orientation on T-SGMD efficiency. • Interplay among power consumption, space cooling capacity and condensate rate. Abstract The current concept for cooling the indoors is far from ideal with respect to the total energy consumed and waste discharged. A novel concept for improving the energy efficiency is proposed via hybridizing the heat pump with a membrane distillation (MD) unit for simultaneous space cooling and water treatment. MD is well-acknowledged for utilizing low-quality waste heat for water treatment, which makes it feasible for coupling with a heat pump to make use of both the hot and cold reservoirs of the pump. Accordingly, the objective of the current effort was to investigate via experiments the efficacy of a thermoelectric heat pump coupled with a sweep-gas MD system (T-SGMD) by measuring the cooling capacity, condensate production and power consumption. The results from this study can be extended to other heat pumps. Three key highlights emanated from this study. Firstly, condensate production per unit energy consumed can be doubled with the T-SGMD system relative to thermoelectric dehumidification alone. Secondly, cool air recycle affected the condensate flux the most without a drastic loss of cooling compared to other tested parameters during the operation of the T-SGMD. Lastly, the T-SGMD system was able to provide an increase in condensate produced per unit energy without a loss in cooling capacity per unit energy input. These advantages of coupling heat pumps with MD, leveraging on the current advancements in MD, is promising for a hybridized system for decentralized water treatment, dehumidification and space cooling. [ABSTRACT FROM AUTHOR]

To improve the mechanistic understanding for advancing the design and engineering of the membrane distillation (MD) modules, the objective of the current study was to investigate via both experiments and simulations the impact of (i) module orientation, (ii) module geometry, and (iii) an oily feed on the permeate flux and pore wetting propensity of direct contact membrane distillation (DCMD). Three module orientations and four feed channel geometries were investigated via experiments and simulations for oily feeds. Two key highlights emanated from this study. Firstly, module orientation mattered for DCMD, particularly in view of the formation of natural thermal convective currents and when the particle density of the particulate foulants varied. Particulate foulants with density much lesser and greater than water only deposited when the membrane was oriented respectively atop and beneath the feed. Secondly, the lack of consideration of convection currents, oil coalescence and the corresponding cake-enhanced temperature polarization in the simulations caused disagreement with the experimental results, which underscores the importance of these factors. This highlights that the optimization of MD modules particularly for treating oily feeds requires more mechanistic studies, especially in view of the thermal gradients, rather than relying on analogy with pressure-driven filtration processes. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) is a promising means for high-purity separations like those needed for potable water. The benefits of MD have not been realized for the treatment of surface or ground water, whereby the presence of both humic acid and calcium are detrimental to other membrane-based processes. Accordingly, this study investigated the efficacy of MD in rejecting various contaminants, namely, ibuprofen, boron and arsenic, in the presence of typical feeds comprising humic acid, calcium chloride and sodium chloride. Feeds investigated ranged from DI water to synthetic feeds mimicking surface water to NEWater brine (i.e., reverse osmosis concentrate from a local wastewater treatment plant in Singapore). Results consistently indicate constant permeate fluxes and low conductivities throughout the experiments with varying concentrations of humic acid and calcium, along with varying pH values, which implies negligible membrane fouling and wetting. Also, complete rejections of boron and arsenic were achieved, while rejection of ibuprofen was approximately 90%. The detection of the non-volatile ibuprofen and humic acid in the permeate suggests some hydrophobic interactions with the membrane. Finally, a cost analysis was carried out to evaluate MD against the conventional nanofiltration (NF) process. Although MD is currently at a higher cost than NF, (i) the availability of improved membranes tailored for MD and low-cost heat would decrease costs to at least those of NF, and (ii) the performance of MD is superior in terms of sustained flux over prolonged periods, capable to treat feeds with higher concentrations of foulants, and better permeate quality. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) is an emerging green technology, but very few reports are available on its use for treating oily feeds, despite produced water representing a significant source of oily wastewater. Accordingly, this study was targeted at understanding the primary impediments of oily water MD by systematically investigating the influence of the key components (namely, oil-in-water emulsion, surfactant and salt) in such feeds. When the feed contained all three components typical in produced water, the MD performance severely deteriorated in terms of permeate flux and quality. Interestingly, for feeds containing oil or SDS or NaCl alone, the MD performance was reasonable, which indicates each component on its own did not impact the MD process significantly. Furthermore, the performance was also reasonable when the feed contained oil and NaCl, and improved when the feed contained oil and SDS. The adverse impact on the MD process was thereby traced to the combined presence of SDS and NaCl. Even in the absence of oil, increasing the concentrations of SDS and NaCl in the feed progressively deteriorated the permeate flux and quality. Therefore, to use MD for treating oily feeds necessitates a pre-treatment step to remove or significantly dilute either the surfactant or the salt. More understanding of the interaction between NaCl and SDS via molecular dynamics simulations would be useful to not only provide information on the interaction strength but also on means to circumvent such issues. [ABSTRACT FROM AUTHOR]

To improve the energy efficiency of membrane distillation (MD), localized heating of the feed-membrane interface is advantageous relative to heating of the bulk feed. This motivated the current study, which targeted at developing a simple two-step way of fabricating Laser-Induced Graphene (LIG) Janus membrane for electrothermal membrane distillation. Specifically, the Janus membrane had an electrically conductive graphene side and a polydimethylsiloxane (PDMS) coated polyethersulfone (PES) side. The PES membrane was first coated with PDMS in limonene (a green solvent) and the active layer exposed to low-powered laser. Joule heating was carried out on the LIG surface by connecting two electrodes to the opposite ends of the LIG surface and providing power with an alternating current power supply at regular household frequency. This Janus membrane can be heated on the LIG surface while insulated from heat loss on the PDMS-coated PES surface, resulting in a marked improvement in the MD performance. Relative to pre-heated feed at the same power input, Joule heating of the Janus membranes gave a flux increase of up to 119.5%, specific heating energy decrease of up to 53.9% and single-pass heat utilization efficiency increase of up to 119.5%, while maintaining excellent material stability. • Two-step modification of PES membrane to improve MD performance • Photothermal conversion of membrane surface to graphene with laser • Laser-induced graphene (LIG) surface can be heated via Joule heating. • Improvement over other surface heating methods • Lower laser exposure time gave better surface heating performance. [ABSTRACT FROM AUTHOR]

This study mainly focused on enhancing oil-membrane electrostatic repulsion towards anti-oil-fouling via electrically conductive hydrophobic membrane in electricity-assisted membrane distillation (MD). Carbon nanotubes (CNTs) were coated on commercial membranes to fabricate membrane cathode. For concentrated hexadecane-in-saline water emulsions, the modified membrane exhibited significantly less flux decline, < 5% in dealing with extreme high-concentration oil emulsion (2000 ppm) at cell potential of 3.0 V. The anti-oil-fouling robustness was further confirmed over a continuous three-cycle operation with in-situ DI rinse. The anti-fouling mechanism was systematically discussed regarding the hydrophilicity of the membrane interface, charge repulsion between the oil and membrane cathode as well as slippery property at the liquid-gas-solid triple-phase interface. A modeled fouling rate constant was negatively associated with the calculated capacitive surface charge of the membrane cathode. Thermodynamics analysis suggested enhanced foulant-membrane electrostatic repulsion leads to a significant energy barrier which favored anti-fouling performance. Interestingly, the sliding dynamics of oil droplet along the interface of membrane cathode was found, tuned by the herein weak cell potential, which could also contribute to the fouling mitigation. Our results insight that the electrically conductive membrane cathode could modulate the foulant-membrane interaction which plays an important role in mitigating fouling/wetting occurred at the triple-phase interface. [Display omitted] • Conductive membrane cathode consisting of CNT on PVDF substrate is proposed. • Electrostatic repulsion between oil droplet and membrane is firstly studied in MD. • Negligible flux decline was found for hexadecane emulsion of 2000 ppm. • Sliding of oil droplet along the membrane cathode is found. • Foulant-membrane interacting dynamics is modulated. [ABSTRACT FROM AUTHOR]

Despite membrane distillation (MD) being well acknowledged as a green technology for water treatment, it remains unattractive for commercial implementation due to limited energy efficiency, and temporal fluctuations in sustainable energy sources like waste heat and solar. The induction heating of metallic spacers addresses these shortcomings. In this study, we harnessed the high thermal conductivity of nickel (Ni) spacers and their capacity to be inductively heated to improve the energy efficiency of direct contact MD, and benchmarked the performance against the conventional external heating of the bulk feed and the use of polypropylene (PP) spacers. Both simulations and experiments were performed. Noteworthy results include: (i) induction heating gave higher flux and energy efficiency than the conventional external heating and use of PP mesh, particularly at lower flowrates; and (ii) induction heating was able to supplement external heating without a decrease in energy efficiency, which makes it a promising method to circumvent the problem of temporal fluctuations of waste or solar heat. This study demonstrated the feasibility of localized induction heating of metallic spacers in MD, which has potential for further improvements through optimizations of the thermal conductivity and geometry of the spacer, and energy efficiency of the induction heater. • Metallic spacer benefits MD via thermal properties and induction characteristics. • Induction heating of metallic spacer gave higher flux and energy efficiency. • Improvement over conventional method greater at lower feed flowrate. • Induction heating feasible to supplement fluctuating waste or solar heat input. • Nickel foam exhibited long-term durability by giving similar fluxes over 168 h. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) is promising for treating the vast amounts of oily wastewater, but unfortunately membrane fouling and wetting remain major impediments. This study aimed to firstly assess a novel means of modifying membranes to mitigate fouling/wetting by grafting the hydrophobic PVDF membrane with zwitterionic sulfobetaine methacrylate (SBMA) polymers, which have been shown to offer significant fouling resistance in ultrafiltration, but their efficacy has not been verified in MD. Characterization studies indicate that, relative to the pristine membranes, the modified membranes had enhanced hydrophilicity/oleophobicity, elements corresponding to SBMA according to the XPS spectra, a denser layer at the surface corresponding to the SBMA layer per the SEM images, nearly constant porosity, less negative zeta potential values and slightly higher fluxes for 5 g/L NaCl feeds. These characteristics enabled the modified membranes to give lesser flux declines and conductivity increases than the pristine membranes for the range of foulants investigated. The second aim of the study was to further understand the mechanisms underlying fouling and wetting. One suite of experiments involved high concentrations (2000 ppm) of two surfactant-free oil emulsions (namely, hexadecane and soy bean oil), while another suite involved hexadecane emulsions stabilized by three surfactants of different charges (namely, SDS, CTAB and Tween 20). Results indicate that the relative extents of fouling/wetting among the different foulants cannot be explained by electrostatic interactions or surface tension of the feed (tied to liquid entry pressure) or reduction of hydrophobic affinity alone, but the enhanced hydrophilicity and greater absolute ratios of the zeta potential of the oil emulsion to the membrane conferred by the zwitterionic SBMA layer were keys to mitigating fouling and wetting. • Novel grafting of zwitterionic SBMA on hydrophobic PVDF membrane for MD applications. • Modified membranes consistently out-performed pristine ones for five foulants. • Reduction of hydrophobic-hydrophobic affinity underlie superior performance. • Electrostatic interactions deficient to explain fouling/wetting trends among foulants. • Greater ratios of zeta potentials of foulant to membrane gave better performance. [ABSTRACT FROM AUTHOR]

Membrane distillation (MD) is a promising green technology that can utilize waste heat to concentrate non-volatile solutes, or remove volatiles and dissolved gases from an aqueous feed. The primary reason for the limited commercial implementation of MD is membrane pore-wetting, which deteriorates the separation performance. For oily wastewater, the surfactants are particularly detrimental due to the inherent nature to lower surface tension, which reduces the liquid entry pressure (LEP) and thereby increases the likelihood for pore-wetting. Unfortunately, the understanding on the influence of surfactants on MD performance is limited. To contribute to the knowledge base, we systematically investigated five surfactants with different hydrophobicities and charge types, in the absence and presence of salt, both via experiments and molecular dynamics simulations. The results show that surfactants adversely affect MD performance, and different surfactants exert different influences. Among the anionic surfactants of different hydrophobicities, the most hydrophobic exhibited the worst MD performance, which is tied to greatest surfactant-membrane attraction. However, when comparing surfactants of different charge types, the greatest surfactant-membrane attraction is not linked to the worst MD performance, suggesting factors like water-membrane affinity that underlies wetting propensity have to be considered too. This study provides more mechanistic insights into the role of surfactants in affecting MD performance. • Five surfactants with different hydrophobicities and charge types were investigated. • Molecular dynamics simulations were carried out to explain experimental MD results. • Most hydrophobic surfactant gave worst performance due to surfactant-PVDF affinity. • For different charges, worst MD performance was tied to water-membrane affinity. • Effect of NaCl depended on surfactant type. [ABSTRACT FROM AUTHOR]