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

We present a simple and rapid methodology to characterize the water and solute permeability coefficients (A and B, respectively) and structural parameter (S) of forward osmosis (FO) membranes. The methodology comprises a single FO experiment divided into four stages, each using a different concentration of draw solution. The experimental water and reverse salt fluxes measured in each stage are fitted to the corresponding FO transport equations by performing a least-squares non-linear regression, using A, B, and S as regression parameters. Hand-cast thin-film composite (TFC) FO membranes and commercial TFC FO, TFC reverse osmosis (RO), and cellulose acetate-based asymmetric FO membranes are evaluated following this protocol. We compare the membrane properties obtained with our FO-based methodology with those derived from existing protocols based on an RO experiment followed by an FO experiment. For all membranes, the FO-based protocol gives more accurate predictions of the water and salt fluxes than the existing method. The numerical robustness of the method and the sensitivity of the regression parameters to random errors in the measured quantities are thoroughly analyzed. The assessment shows that confidence in the accuracy of the determined membrane parameters can be enhanced by simultaneously achieving close fitting of the predicted fluxes to experimental measurements (i.e., high R 2 values) and constant water to salt flux ratios in each stage. Additionally, the existing and proposed approaches yield consistently dissimilar results for some of the analyzed membranes, indicating a discrepancy that might be attributed to the different driving forces utilized in RO and in FO that should be further investigated.

Solar-thermal desalination (STD) is a potentially low-cost, sustainable approach for providing high-quality fresh water in the absence of water and energy infrastructures. Despite recent efforts to advance STD by improving heat-absorbing materials and system designs, the best strategies for maximizing STD performance remain uncertain. To address this problem, we identify three major steps in distillation-based STD: (i) light-to-heat energy conversion, (ii) thermal vapor generation, and (iii) conversion of vapor to water via condensation. Using specific water productivity as a quantitative metric for energy efficiency, we show that efficient recovery of the latent heat of condensation is critical for STD performance enhancement, because solar vapor generation has already been pushed toward its performance limit. We also demonstrate that STD cannot compete with photovoltaic reverse osmosis desalination in energy efficiency. We conclude by emphasizing the importance of factors other than energy efficiency, including cost, ease of maintenance, and applicability to hypersaline waters.

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