This critical review investigates the potential for membrane distillation to desalinate high-salinity waters using low-grade heat at the water-energy nexus.

Desalination membranes are essential for the treatment of unconventional water sources, such as seawater and wastewater, to alleviate water scarcity. Promising research efforts on novel membrane materials may yield significant performance gains over state-of-the-art thin-film composite (TFC) membranes, which are constrained by the permeability–selectivity trade-off. However, little guidance currently exists on the practical impact of such performance gains, namely enhanced water permeability or enhanced water–solute selectivity. In this critical review, we first discuss the performance of current TFC membranes. We then highlight and provide context for recent module-scale modeling studies that have found limited impact of increased water permeability on the efficiency of desalination processes. Next we cover several important examples of water treatment processes in which inadequate membrane selectivity hinders process efficacy. We conclude with a brief discussion of how the need for enhanced selectivity may influence the design strategies of future membranes.

With more than a billion people lacking accessible drinking water, there is a critical need to convert nonpotable sources such as seawater to water suitable for human use. However, energy requirements of desalination plants account for half their operating costs, so alternative, lower energy approaches are equally critical. Membrane distillation (MD) has shown potential due to its low operating temperature and pressure requirements, but the requirement of heating the input water makes it energy intensive. Here, we demonstrate nanophotonicsenabled solar membrane distillation (NESMD), where highly localized photothermal heating induced by solar illumination alone drives the distillation process, entirely eliminating the requirement of heating the input water. Unlike MD, NESMD can be scaled to larger systems and shows increased efficiencies with decreased input flow velocities. Along with its increased efficiency at higher ambient temperatures, these properties all point to NESMD as a promising solution for household-or community-scale desalination.our billion people around the world face at least 1 month of water scarcity every year (1, 2). To meet increasing water demand, it has become necessary to exploit saline water, abundant in the ocean and in brackish aquifers, and convert it to potable water (3, 4). Presently, there are more than 18,000 water desalination plants operating in 150 countries, producing 86.8 × 10 6 m 3 of water per day, enough for 300 million people (5, 6). The annual energy consumed by these plants is nominally 75 TWh, accounting for 50% of their operating costs (7-9) and 0.4% of the world electric power consumption (10). The possibility of directly using renewable energy would reduce this highly demanding cost of operation and make affordable clean water more accessible around the world.Many of the current desalination techniques involve phase change, and thus are inherently energy intensive. Among these, membrane distillation (MD) has gained recent attention because it can distill water at lower temperatures than conventional distillation (i.e., boiling) and lower pressures than reverse osmosis (RO) (11-16). In the conventional direct-contact MD process, hot saline water (feed) and cold purified water (distillate) flow on opposite sides of a hydrophobic membrane (Fig. 1A). The temperature difference between the two flows produces a vapor pressure difference across the membrane, leading to (salt-free) water vapor transporting through the membrane from the warmer feed to the colder distillate, where it condenses. However, MD suffers from several inherent limitations. Heat transfer reduces the cross-membrane temperature difference, resulting in lower vapor flux across the membrane and thus lower efficiency. This temperature difference is further decreased along the length of the membrane module, resulting in a maximal usable length of a single module.When no recirculation or heat recovery is used, energy is also lost when hot feed water exits the membrane module. Heating the volume of feed wat...

We review pressure-retarded osmosis focusing on the net energy extractable from the process and the ultimate viability of various configurations.

We reveal the insignificance of advanced materials in further enhancing the energy efficiency of desalination and suggest more impactful approaches.

Water scarcity, expected to become more widespread in the coming years, demands renewed attention to freshwater protection and management. Critical to this effort are the minimization of freshwater withdrawals and elimination of wastewater discharge, both of which can be achieved via zero liquid discharge (ZLD), an aggressive wastewater management approach. Because of the high energetic cost of thermal desalination, ZLD is particularly challenging for high-salinity wastewaters. In this review, we discuss the potential of high-pressure reverse osmosis (HPRO) (i.e., reverse osmosis operated at a hydraulic pressure greater than ∼100 bar) to efficiently desalinate hypersaline brines. We first discuss the inherent energy efficiency of membrane processes compared to that of conventional thermal processes for brine desalination. We then highlight the opportunity of HPRO to reduce energy requirements for desalination of key high-salinity industrial wastewaters. The current state of membrane materials and processes for hypersaline brine desalination is also discussed, emphasizing several process design considerations unique to HPRO. Lastly, we discuss the most pressing research needs for the development of HPRO, notably the development of membranes and modules suitable for high pressures as well as fundamental studies of compaction and transport under HPRO conditions.

No abstract

Energy savings in reverse osmosis (RO) are highly constrained by the design of conventional processes, for which the minimum practical energy of desalination substantially exceeds the thermodynamic minimum. Batch processes can theoretically approach the thermodynamic minimum, suggesting the possibility for further energy savings. In this study, we aim to quantify what energy reductions may be possible for batch-like processes when process inefficiencies such as frictional losses and concentration polarization are included. We first introduce a practical batch process that utilizes energy recovery devices and an unpressurized feed tank. We also consider a less practical pressurized-tank scenario, as well as semi-batch (closed-circuit) RO. We then derive analytical approximations and conduct numerical modeling to compare the energy requirements of batch, semi-batch, and staged RO processes under realistic conditions. Through this analysis, we find that practical batch-like processes and processes with increased staging offer comparable and significant energy savings. For example, semi-batch RO and twostage RO would save 13% and 15% energy, respectively, over one-stage seawater RO at 50% recovery. We conclude with a discussion of other important factors, such as capital costs and process robustness and flexibility, that will affect the implementation of batch, semi-batch, and staged processes.