Long-term human Space missions will rely on regenerative life support as resupply of water, oxygen and food comes with constraints. The International Space Station (ISS) relies on an evaporation/condensation system to recover 74-85% of the water in urine, yet suffers from repetitive scaling and biofouling while employing hazardous chemicals. In this study, an alternative non-sanitary five-stage treatment train for one "astronaut" was integrated through a sophisticated monitoring and control system. This so-called Water Treatment Unit Breadboard (WTUB) successfully treated urine (1.2-L-d with crystallisation, COD-removal, ammonification, nitrification and electrodialysis, before it was mixed with shower water (3.4-L-d(-1)). Subsequently, ceramic nanofiltration and single-pass flat-sheet RO were used. A four-months proof-of-concept period yielded: (i) chemical water quality meeting the hygienic standards of the European Space Agency, (ii) a 87- +/- -5% permeate recovery with an estimated theoretical primary energy requirement of 0.2-kWh p -L-1, (iii) reduced scaling potential without anti-scalant addition and (iv) and a significant biological reduction in biofouling potential resulted in stable but biofouling-limited RO permeability of 0.5 L-m(-2)-h(-1)-bar(-1). Estimated mass breakeven dates and a comparison with the ISS Water Recovery System for a hypothetical Mars transit mission show that WTUB is a promising biological membrane-based alternative to heat-based systems for manned Space missions.

Membrane distillation (MD) is a thermally driven separation process, operated at moderate temperature, allowing for the use of waste heat as driving force. While the literature is saturated with lab-scale models, almost none exist for designing a complete MD system. Based on previously published and thoroughly validated models, this work demonstrates a graphical user interface tool, capable of designing a complete membrane distillation system, including all of the supporting equipment and able to predict the price of the obtained distillate for the most commonly studied and used membrane distillation configurations. The user can also optimize the module geometry based on specific requirements. Four different case studies are discussed, ranging from 2 to 1000 m(3) of distillate per day, with final brine salinity up to 20 wt%, feed temperature up to 80 degrees C. The optimal system design for each case is demonstrated. The distillate price varied from 25(sic)/m(3) of distillate for the smallest scale to 2.1(sic)/m(3) for the largest scale. Finally, the reader is presented with a simplified cost model that can be used to quickly estimate the price of the produced distillate at different production scales and concentration factors.

Membrane distillation is an emerging membrane technology, commonly used for desalination. The development of high performing and cheap membranes is one of the concerns for increasing the industrial applicability of the technology. In this paper, an economically interesting alternative has been found through the deposition of an atmospheric plasma coating on an inexpensive commercial hydrophilic membrane with a microporous structure. This coating enables obtaining the required hydrophobicity in order to retain the feed solution and preventing wetting of the membrane. The effect of the plasma discharge time on the membrane properties and membrane distillation performance was investigated. It was found that by using atmospheric plasma technology, very thin hydrophobic layers of a few μm can be obtained on top of a hydrophilic membrane structure. The thin hydrophobic layer obtained using this technology reduces the mass transport resistance, which results in very high fluxes at seawater salinity. This flux is equal to the flux of more expensive and supported thin PTFE membranes.

ABSTRACTHighly porous macrovoid‐free polyethersulfone membranes have been prepared using the phase‐inversion process with water as the non‐solvent. These membranes are of great interest for membrane distillation (MD) after application of a hydrophobic/oleophobic coating. The membrane structure was controlled by optimizing the process conditions and dope composition. Counter intuitively, increasing the polymer concentration favors the formation of larger surface pores under similar process conditions. A symmetric membrane is obtained when a sufficient amount of high‐molecular‐weight polyvinylpyrrolidone was added to the dope solution, which appears to play an important role in the structure formation process. The final membrane shows similar performance compared to commercial MD membranes. However, the membranes developed in this study show an oleophobic character, broadening the applications of MD. Moreover, the compressibility of these membranes is severely reduced compared to stretched membranes, which is expected to result in an improved MD performance at full scale. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45516.

Membrane distillation is a thermal separation technique using a microporous hydrophobic membrane. One of the concerns with respect to the industrialization of the technique is the development of novel membranes. In this paper, a commercially available hydrophilic polyethersulfone membrane with a suitable structure for membrane distillation was modified using available hydrophobic coatings using ORMOCER® technology to obtain a hydrophobic membrane that can be applied in membrane distillation. The surface modification was performed using a selection of different components, concentrations, and application methods. The resulting membranes can have two hydrophobic surfaces or a hydrophobic and hydrophilic surface depending on the application method. An extensive characterization procedure confirmed the suitability of the coating technique and the obtained membranes for membrane distillation. The surface contact angle of water could be increased from 27° up to 110°, and fluxes comparable to membranes commonly used for membrane distillation were achieved under similar process conditions. A 100 h test demonstrated the stability of the coating and the importance of using sufficiently stable base membranes.

Water management is becoming increasingly challenging and several technologies, including membrane distillation (MD) are emerging. This technology is less affected by salinity compared to reverse osmosis and is able to treat brines up to saturation. The focus of MD research recently shifted from seawater desalination to industrial applications out of the scope of reverse osmosis. In many of these applications, surfactants or oil traces are present in the feed stream, lowering the surface tension and increasing the risk for membrane wetting. In this study, the technological boundaries of MD in the presence of surfactants are investigated using surface tension, contact angle and liquid entry pressure measurements together with lab-scale MD experiments to predict the wetting resistance of different membranes. Synthetic NaCl solutions mixed with sodium dodecyl sulfate (SDS) were used as feed solution. The limiting surfactant concentration was found to be dependent on the surface chemistry of the membrane, and increased with increasing hydrophobicity and oleophobicity. Additionally, a hexadecane/SDS emulsion was prepared with a composition simulating produced water, a waste stream in the oil and gas sector. When hexadecane is present in the emulsion, oleophobic membranes are able to resist wetting, whereas polytetrafluoretheen (PTFE) is gradually wetted by the feed liquid.

Membrane distillation is a thermal membrane technology, commonly used for desalination. The industrial breakthrough of the technology requires high performing and inexpensive membranes. In this paper, an alternative for the traditional hydrophobic membrane materials has been explored through the deposition of a hydrophobic coating on a commercial hydrophilic membrane with a microporous structure. The coatings enable obtaining the required hydrophobicity and prevent membrane wetting. The effect of the surface chemistry and structure on the membrane distillation performance was investigated for 5 different coatings. The coatings were applied only on the top surface or on the entire membrane cross-section and the effect of this difference in morphology was investigated. The atmospheric toplayer coating and vacuum plasma coating on the entire cross-section were selected for further optimization. The stability of the coating and the relation of the membrane structure and performance at different salinities were also investigated using direct contact membrane distillation.

Air gap membrane distillation (AGMD) is one of the most widely discussed membrane distillation configurations at the moment and has been regarded as more thermally efficient than direct contact membrane distillation (DCMD), due to the insulation properties of the air gap. Several AGMD models are available in the literature. However, most of the models developed to date are either missing validation or are only validated at lab-scale. A major hurdle in modelling membrane distillation is the lack of information about the condensation that is occurring inside the gap. Often, major parameters such as the average condensate thickness are taken from semi-empirical formulas or are simply estimated based on educated guesses. Moreover, some studies had shown that at certain conditions the gap can be completely flooded with condensate, which raises the question whether the module can be modelled as air gap altogether. In this study, a previously developed and thoroughly validated DCMD model is extended by adding the air gap compartment. In this way the model only needs to be adjusted for the gap-related parameters. A simple technique is demonstrated for observing the condensation in real time, which also allows to experimentally obtain the value of the average condensate thickness parameter and the flooding of the gap. The model is subsequently thoroughly and simultaneously validated with experimental data from two commercially available modules with areas of 7.2 and 24 m(2), showing an excellent fit to the experimental data. Moreover, this work shows a direct comparison between AGMD and DCMD in terms of flux and thermal efficiency at full-scale using modules with identical geometries from the same manufacturer.

While many models exist in the literature for description of lab-scale direct contact membrane distillation (DCMD) performance, only a handful of modelling attempts at full-scale have been described. In this article a method is presented for the simulation of the flux and energy efficiency of a full-scale, counter-currently operated, spiral-wound DCMD module. The model is based on a previously calibrated lab-scale model. The geometry of the full-scale, spiral-wound module is discretized into small sections and the lab-scale model is applied in each section. It was found that the membrane used in this module compacts significantly under the operational conditions. This effect cannot be neglected, therefore the model had to be extended to account for it. It is noteworthy that, apart from this extension, no additional model calibration at full-scale was needed. Given its validity at full-scale the model can be safely used for extended scenario analysis with regard to the optimization of the module design. The developed model and method are powerful tools to decide on system design and operation.

•A literature review is given on the different methods used to produce membrane distillation membranes.•An critical overview is given on how to evaluate the membrane performance in membrane distillation.•Novel techniques are compared with the classical phase inversion and stretching techniques.•The membrane performance is assessed and the benefits and drawbacks are given.

Membrane distillation separates liquids and solutes using a hydrophobic microporous membrane. Different configurations have been investigated at lab scale, among which direct contact membrane distillation (DCMD) and air gap membrane distillation (AGMD). Lab scale studies comparing different configurations show higher flux for DCMD, while AGMD is more energy efficient. However, no straightforward directions are given yet on how to translate these results to pilot scale. As membrane distillation is currently shifting from lab environment towards pilot experiments, a good understanding of pilot scale modules and the differences with lab modules are important. In this study, DCMD and AGMD were compared both at lab and pilot scale using the same membrane. At lab scale, it was found that the flux of DCMD is a factor 4 higher compared to the flux of AGMD. However, at pilot scale, the AGMD modules showed a higher flux and lower energy consumption compared to DCMD. As it is important for further upscaling of the technology, this study focusses on the explanation behind these unexpected higher fluxes of AGMD modules. Since the AGMD modules showed better performance and do not require an additional recuperating heat exchanger, this configuration is preferred over DCMD for larger scale applications.

In membrane distillation, the liquid phase including dissolved components is retained by a hydrophobic membrane, while the microporous structure allows transport of vapor through the membrane. The vapor pressure difference over the membrane is the driving force and is applied using a variety of configurations. This paper directly compares the flux and energy efficiency of a lab scale direct contact membrane distillation (DCMD), air gap membrane distillation (AGMD), permeate gap membrane distillation (PGMD) and vacuum membrane distillation (VMD) using the same bulk driving force. The highest flux was observed for VMD, followed by DCMD>PGMD>AGMD. Furthermore, it was observed that the different configurations are not equally sensitive to the applied process conditions, including temperature difference, flow velocity and salinity. For the first time, also the importance of the specific requirements for the membrane for each configuration was investigated.

Membrane distillation (MD) uses a microporous hydrophobic membrane to separate dissolved molecules from a liquid stream. Notwithstanding the great potential, membrane distillation is not applied on an industrial level yet, because of the lack of specifically developed membranes, modules, and techno-economic data at full scale. This review gives a comprehensive overview of the optimal membrane properties and can serve as a guideline for the development of new membranes, specifically for membrane distillation. Optimization of the membrane is needed to sufficiently resist wetting. Generally, a pore diameter of 0.3 mu m is recommended to balance between a high liquid entry pressure and flux. Since vacuum membrane distillation is more sensitive to wetting, a smaller pore diameter could be appropriate for this configuration to avoid membrane wetting. An optimal membrane thickness is found between 10 and 700 mu m, depending on process conditions, balancing between mass transport and energy loss. To improve the mass transfer and energy efficiency, membrane porosity should preferably be as high as possible (>75%), while low tortuosity (1.1-1.2) and thermal conductivity (>0.06 w-m(-1)K(-1)) are recommended as well.

Membrane distillation is an emerging technology to separate non-volatile components from an aqueous feed stream. Mathematical models have proven useful to pursue breakthrough in the economics of the technology and for further improvement through module design and operational optimization. However, before this can be done, all of the resistances in the system must be identified correctly and the model must be carefully calibrated to ensure its predictive power. In this work the typical structure of a direct contact membrane distillation (DCMD) model is studied, where the mass transfer inside the membrane is simulated using the Dusty Gas Model and Nusselt type equations are used to simulate the heat transfer inside the channels. We demonstrate that an off-the-shelf Nusselt equation cannot directly be applied to simulate the heat transfer in the spacer filled channels. Instead, the equations should be calibrated to match the behaviour of the particular spacer. A Monte Carlo filtering method was applied to calibrate and study the structure of the DCMD model for the membrane region. The method proved useful to identify which parameters need to be included in the calibration as it highlighted parameter correlations. Additionally, a submodel selection was performed for the heat and mass transfer inside the membrane. A simple, yet physical method for the simulation of supported membranes was tested and validated on 3 supported membranes, resulting in an excellent fit.

Membrane distillation (MD) uses a microporous hydrophobic membrane for the separation of non-volatile solutes from liquid streams. The microporous structure should be designed for optimal vapor transport through the membrane, whereas the hydrophobicity is required to retain the liquid phase. Currently, different types of commercially available hydrophobic microfiltration membranes are used for membrane distillation. However, no comparison is available between these membranes, complicating the selection of a proper membrane and the evaluation of new membranes. In this study, over 20 (semi-)commercial hydrophobic membranes are characterized and tested in a lab scale direct contact membrane distillation set-up. These membranes include the standard PTFE, PVDF and PP membranes, but also less known PE and PES membranes. These membranes are synthesized using the phase inversion technique, stretching or electrospinning, resulting in a wide variety of membrane structures. In this study, a method is proposed to evaluate the suitability of membranes. The membrane performance in MD is evaluated with a performance chart including flux and energy efficiency using realistic process conditions. From this chart a benchmark performance is proposed, which depends on the salt concentration.

One of the major challenges for long-term manned Space missions is the requirement of a regenerative life support system. When minimizing the amount of water available per astronaut to 13 L d-1, a mission of 6 crew members requires almost 30 ton of fresh water supplies per year, i.e. the International Space Station (ISS) weighs approximately 400 ton. Therefore, the development of an efficient water recovery system is essential to future Space exploration. A Water Treatment Unit Breadboard (WTUB) has been developed to recover 90% of the water in urine, condensate and shower water produced by one crew member and this life support testbed facility was inspired by the MELiSSA loop concept, ESA's Life Support System. In the WTUB, a water recovery of >85% was achieved by an integrated bio-physicochemical urine treatment combined with NF/RO filtration.

Membrane distillation is an emerging thermal membrane technology for the separation of salts and other non-volatile inclusions from water streams. The process offers a solution for the treatment of concentrated solutions, which are not viable for reverse osmosis. However, only few studies focused on the optimal membrane properties and operational conditions in the high concentration regime. In this paper, membranes with variations in thickness, porosity and structure are experimentally investigated in direct contact membrane distillation (DCMD); in addition, the performance is simulated using the Dusty Gas Model. Operational conditions, including the temperature difference over the membrane, the flow velocity and the feed stream salinity up to saturation were varied. It was confirmed that for pure water, thinner membranes show higher fluxes, while energy efficiency is unaffected by membrane thickness. At higher salinities, an optimal membrane thickness depending on membrane parameters and process conditions exists. The optimal membrane thickness computed in this article ranges from 2 to 739μm for concentrations of NaCl ranging from 0 up to 24wt% and variations in bulk temperature difference and flow velocities for four different membrane structures.