Can seawater desalination be a win-win fix to our water cycle?

A review of limitations and potentials of desalination as a sustainable source of water

For centuries, desalination, in one way or another, has helped alleviate water scarcity. Over time, desalination has gone through an evolutionary process influenced largely by available contemporary technology. This improvement, for the most part, was reflected in the energy efficiency and, in turn, in terms of the cost-effectiveness of this practice. Thanks to such advancements, by the 1960s, the desalination industry experienced notable exponential growth, becoming a formidable option to supplement conventional water resources with a reliable non-conventional resource. That said, often, there are pressing associated issues, most notably environmental, socioeconomic, health, and relatively recently, agronomic concerns. Such reservations raise the question of whether desalination is indeed a sustainable solution to current water supply problems. This is exceptionally important to understand in light of the looming water and food crises. This paper, thus, tends to review these potential issues from the sustainability perspective. It is concluded that the aforementioned issues are indeed major concerns, but they can be mitigated by actions that consider the local context. These may be either prophylactic, proactive measures that require careful planning to tailor the situation to best fit a given region or reactive measures such as incorporating pre- (e.g., removing particles, debris, microorganisms, suspended solids, and silt from the intake water prior to the desalination process) and post-treatments (e.g., reintroducing calcium and magnesium ions to water to enhance its quality for irrigation purposes) to target specific shortcomings of desalination.

Electrically conductive membrane distillation via an alternating current operation for zero liquid discharge

Membrane distillation (MD) shows promise for achieving high salinity treatment and zero liquid discharge (ZLD) compared to conventional water treatment processes due to its unique characteristics, including low energy consumption and high resulting water quality. However, performance degradation due to fouling and scaling under high recovery conditions remains a challenge, particularly considering the need to control both cations and anions for maximum scaling mitigation. Accordingly, in this study, alternating current (AC) operation for electrically conductive membrane distillation (ECMD) is newly proposed, based on its potential for controlling both cations and anions, in contrast to conventional direct current (DC) operation. Systematic experiments and theoretical analysis show that water recovery in ECMD can be increased by 27% through AC operation. The proposed modification and effective AC operation of ECMD increase the practicality of using MD in desalination for a high recovery rate, perhaps even for ZLD.

Production of N-acetylglucosamine with Vibrio alginolyticus FA2, an emerging platform for economical unsterile open fermentation

Members of the Vibrionaceae family are predominantly fast-growing and halophilic microorganisms that have captured the attention of researchers owing to their potential applications in rapid biotechnology. Among them, Vibrio alginolyticus FA2 is a particularly noteworthy halophilic bacterium that exhibits superior growth capability. It has the potential to serve as a biotechnological platform for sustainable and eco-friendly open fermentation with seawater. To evaluate this hypothesis, we integrated the N-acetylglucosamine (GlcNAc) pathway into V. alginolyticus FA2. Seven nag genes were knocked out to obstruct the utilization of GlcNAc, and then 16 exogenous gna1s co-expressing with EcglmS were introduced to strengthen the flux of GlcNAc pathway, respectively. To further enhance GlcNAc production, we fine-tuned promoter strength of the two genes and inactivated two genes alsS and alsD to prevent the production of acetoin. Furthermore, unsterile open fermentation was carried out using simulated seawater and a chemically defined medium, resulting in the production of 9.2 g/L GlcNAc in 14 h. This is the first report for de-novo synthesizing GlcNAc with a Vibrio strain, facilitated by an unsterile open fermentation process employing seawater as a substitute for fresh water. This development establishes a basis for production of diverse valuable chemicals using Vibrio strains and provides insights into biomanufacture.

Synergy of phosphate recovery from sludge-incinerated ash and coagulant production by desalinated brine

Wet-chemical approach is widely applied for phosphate recovery from incinerated ash of waste activated sludge (WAS), along with metals removed/recovered. The high contents of both aluminum (Al) and iron (Fe) in WAS-incinerated ash should be suitable for producing coagulants with some waste anions like Cl^- and SO₄^2- With acid (HCl) leaching and metals' removing, approximately 88 wt% of phosphorus (P) in the ash could be recovered as hydroxylapatite (HAP: Ca₅(PO₄)₃OH); Fe³⁺ in the acidic leachate could be selectively removed/recovered by extraction with an organic solvent of tributyl phosphate (TBP), and thus a FeCl₃-based coagulant could be synthesized by stripping the raffinate with the original brine (containing abundant Cl^- and SO₄^2-). Furthermore, a liquid poly-aluminum chloride (PAC)-based coagulant could also be synthesized with Al³⁺ removed from the ash and the brine, which behaved almost the same in the coagulation performance as a commercial coagulant on both phosphate and turbidity removals. Both P-recovery from the ash and coagulant production associated with the brine would enlarge the markets of both 'blue' phosphate and 'green' coagulants.

Beetle-Inspired Dual-Directional Janus Pumps with Interfacial Asymmetric Wettability for Enhancing Fog Harvesting

Fog-harvesting devices (FHDs) have been widely explored and applied to alleviate the shortage of fresh water. However, during the fog collection process, how to maintain a balance between fog capture and water removal behaviors to enhance the water collection rate still remains a challenge. Herein, inspired by the Stenocara beetle, we combined a beetle-like Janus surface and the conventional cross-sectional Janus structure together, developed a simple spray-and-dry strategy to obtain three types of biomimetic asymmetric meshes, and explored the working modes for atmospheric fog collection. The surface wettability could be carefully controlled, and various asymmetric meshes with different water transportation behaviors were obtained. Through a detailed study of the fog collection process, we concluded that there existed three main working modes: Janus mode, hybrid mode, and Janus and hybrid mode. It was noted that the dual-directional Janus pump with the Janus and hybrid working mode balanced the fog capture and water removal ability and exhibited the highest water collection rate of 2478.73 mg m^-2 h^-1, which was 2.61 times more than that of the corresponding superhydrophilic mesh. Furthermore, the prepared dual-directional Janus pump showed superior mechanical durability and antibacterial ability. In general, this work was considered instrumental in the reasonable design of biomimetic asymmetric meshes and could provide references for efficient atmospheric fog harvesting.

Creating coagulants through the combined use of ash and brine

Sludge incineration and seawater desalination are two approaches that can be used in the disposal of waste activated sludge (WAS) and for obtaining fresh water. As resource recovery from wastewater treatment and water purification is a topic of particular interest in these times, "water mining" has become a focus of research, with phosphate/P-recovery from WAS incineration ash, and extraction of useful elements from the brine of desalination being important steps in the pursuit of a circular/blue economy. However, P-recovery from ash involves removing metals, which need to be disposed of carefully, as does the brine collected. If cations in the ash and anions in the brine could be combined in order to produce coagulants/flocculants, a new circular model would be established. A preliminary experiment for this purpose has demonstrated that a liquid poly‑aluminum chloride (PAC) could be synthesized from the aluminum ion/Al³⁺ removed from the ash and the original brine. With this work, we synthesized the liquid PAC by a hydrothermal method, and the results from infrared spectrometer demonstrated that the synthesized PAC was similar to a commercial PAC. Moreover, the synthesized PAC was able to efficiently reduce the effluent turbidity of wastewater treatment plants (WWTPs), especially when compared with the commercial PAC. It is therefore important that research in this area be continued in order to improve the quality of synthesized coagulants and to produce different coagulants based on cations and anions in ash and brine.

A self-healing PVDF-ZnO/MXene membrane with universal fouling resistance for real seawater desalination

Seawater desalination is regarded as a possible way to overcome current shortages of fresh water, and membrane-based air humidification-dehumidification desalination (MHDD) represents a promising technique owing to its high-quality freshwater and cost-effectiveness; however, its development is restricted by membrane fouling. While a superhydrophobic membrane provides resistance to hydrophilic fouling, it remains susceptible to hydrophobic fouling. Here, a polyvinylidene fluoride-ZnO/MXene (PVDF-ZM) membrane, with a reversible conversion between superhydrophobicity and hydrophilicity was fabricated to achieve universal fouling resistance. It earned a competitive permeate flux (3.93 kg·m^-2·h^-1) and an excellent salt rejection (>99.5%). The membrane exhibited a strong anti-hydrophilic fouling ability, benefiting from its superhydrophobicity and rough surface. The adsorbed hydrophobic contaminants could desorb from the membrane surface under UV irradiation when transforming the surface wettability into hydrophilicity, exhibiting an anti-hydrophobic fouling ability. Subsequently, the membrane surface returned to the hydrophobic state under dark conditions. The membrane recovered 90% of the original permeation flux, while maintaining a salt rejection of >99.5%, thus realizing membrane self-healing. The PVDF-ZM membrane holds promise for sustainable desalination applications.

A Na+ ion-selective desalination system utilizing a NASICON ceramic membrane

Seawater is a virtually unlimited source of minerals and water. Hence, electrodialysis (ED) is an attractive route for selective seawater desalination due to the selectivity of its ion exchange membrane (IEM) toward the target ion. However, a solution-like IEM, which is permeable to water and ions other than the target ion, results in the leakage of water as well as extraction of unwanted ions. This degrades the productivity and purity of the system. In this study, A novel desalination system was developed by replacing the cation exchange membrane (CEM) with a Na super ionic conductor (NASICON) in ED. NASICON exceptionally permits Na⁺ ion migration, and this enhanced the productivity of desalted water by removing 98% of Na⁺ while retaining water and other cationic minerals. Therefore, the final volume of desalted water in N-ED was 1.36 times larger compared to that of ED. In addition, the specific energy consumption for salt (NaCl) extraction was reduced by ∼13%. Furthermore, the NASICON in N-ED was replaced into a two-sided NASICON-structured rechargeable seawater battery, thereby further conserving ∼20% energy by simultaneously coupling selective desalination with energy storage. Our findings have positive implications and further optimizations of the NASICON will enable practical and energy-effective applications for seawater utilization.

Towards sustainable saline agriculture: Interfacial solar evaporation for simultaneous seawater desalination and saline soil remediation

Interfacial solar steam generation is an efficient way to produce freshwater from saline water. This technology was further harnessed here for simultaneous saline soil remediation and enhanced agricultural sustainability. An interfacial solar evaporation and planting system was designed that uses treated seawater for saline soil washing and agricultural irrigation. In outdoor experiments the evaporator realized high freshwater production (10.95 kg m^-2 day^-1) with a soil washing efficiency 3 times greater than traditional distillation. Post treatment plant assays showed that initially highly saline soils could be restored to functional agricultural soils with germination rates of 65% after soil washing, where solar evaporation could continuously provide irrigation water for plant growth. This system is fully automated and uses only solar energy and seawater for saline soil remediation and irrigation. The development of this system provides a potentially useful solution to alleviate global problems associated with water scarcity, soil salinization, and desertification.

Analysis of Polyvinylidene Fluoride Membranes Fabricated for Membrane Distillation

The optimization of the properties for MD membranes is challenging due to the trade-off between water productivity and wetting tendency. Herein, this study presents a novel methodology to examine the properties of MD membranes. Seven polyvinylidene fluoride (PVDF) membranes were synthesized under different conditions by the phase inversion method and characterized to measure flux, rejection, contact angle (CA), liquid entry pressure (LEP), and pore sizes. Then, water vapor permeability (B_w), salt leakage ratio (L_w), and fiber radius (R_f) were calculated for the in-depth analysis. Results showed that the water vapor permeability and salt leakage ratio of the prepared membranes ranged from 7.76 × 10⁻⁸ s/m to 20.19 × 10⁻⁸ s/m and from 0.0020 to 0.0151, respectively. The R_f calculated using the Purcell model was in the range from 0.598 μm to 1.690 μm. Since the R_f was relatively small, the prepared membranes can have high LEP (more than 1.13 bar) even at low CA (less than 90.8°). The trade-off relations between the water vapor permeability and the other properties could be confirmed from the results of the prepared membranes. Based on these results, the properties of an efficient MD membrane were suggested as a guideline for the membrane development.

Cross-Linked Covalent Organic Framework-Based Membranes with Trimesoyl Chloride for Enhanced Desalination

The rational design of continuous covalent organic framework (COF)-based membranes is challenging for desalination applications, mainly due to the larger intrinsic pore size of COFs and defects in the crystalline film, which lead to a negligible NaCl rejection ratio. In this work, we first demonstrated a COF-based desalination membrane with in situ cross-linking of a COF-TpPa layer by trimesoyl chloride (TMC) to stitch the defects between COF crystals and cross-link the COF cavity with high-cross-linking degree networks to enhance NaCl rejection. With the addition of TMC monomers, both small spherical nodules and some elongated "leaf-like" features were observed on the membrane surface due to the appearance of nanovoids during cross-linking. The resulting COF-based desalination membrane had a water permeability of approximately 0.81 L m^-2 h^-1 bar^-1 and offered substantial enhancement of the NaCl rejection ratio from being negligible to 93.3% at 5 bar. Mechanistic analysis indicated that the amidation reaction of the secondary amine in keto COF with TMC induced the formation of a highly porous network structure both in the cavity and on the exterior of COF, thereby successfully forming a continuous and nanovoid-containing selective layer for desalination. In addition, the membrane exhibited excellent desalting performance for real industrial wastewater with both low and high salinity. This study proposed that the introduction of a cross-linker to react with the terminal amine group and secondary amine in the backbone of the keto form of COF or its derivatives could provide a facile and scalable approach to fabricate a COF-based membrane with superior NaCl rejection. This opens a new fabrication route for COF-based desalination membranes, as well as extended applications in water desalination.

Deep Learning Based Approach to Classify Saline Particles in Sea Water

Water is an essential resource that facilitates the existence of human life forms. In recent years, the demand for the consumption of freshwater has substantially increased. Seawater contains a high concentration of salt particles and salinity, making it unfit for consumption and domestic use. Water treatment plants used to treat seawater are less efficient and reliable. Deep learning systems can prove to be efficient and highly accurate in analyzing salt particles in seawater with higher efficiency that can improve the performance of water treatment plants. Therefore, this work classified different concentrations of salt particles in water using convolutional neural networks with the implementation of transfer learning. Salt salinity concentration images were captured using a designed Raspberry Pi based model and these images were further used for training purposes. Moreover, a data augmentation technique was also employed for the state-of-the-art results. Finally, a deep learning neural network was used to classify saline particles of varied concentration range images. The experimental results show that the proposed approach exhibited superior outcomes by achieving an overall accuracy of 90% and f-score of 87% in classifying salt particles. The proposed model was also evaluated using other evaluation metrics such as precision, recall, and specificity, and showed robust results.

Sustainable desalination: Long-term monitoring of brine discharge in the marine environment

The environmental impact of desalination is the most important concern related to its sustainable development. We present the results of a long-term environmental plan to monitor brine discharge (BD) from a desalination plant located in a high environmental value area in Spain. Generalized additive models were used to analyze the biological parameters of biological communities. Results of 17 years of BD monitoring show how its environmental impact can be minimized through well-planned decision-making between scientists and industry. The brine dilution prior to its discharge into an artificial channel of low ecological value significantly reduced the brine influence area. P. oceanica shoot production and echinoderms abundances were relatively stable across historical series and similar values in control and impacts locations were observed. Conversely, there was a higher abundance and species richness of fishes in the BD area. The important findings reported here should be considered for future applications in similar projects.

Why (Not) Desalination? Exploring Driving Factors from Irrigation Communities’ Perception in South-East Spain

Desalination for sustaining agricultural production is conceived as an alternative water source in some Mediterranean countries faced with climatological and hydrological constraints. Although high costs are often cited as limiting factors, how farmers discern desalinated water has not been discussed in-depth in the literature. This paper aims to deepen how desalination is perceived by irrigators, what driving factors are affecting irrigation communities’ decision-making processes, and what learnings can be drawn from their experiences regarding desalination acceptance or rejection. Eleven irrigation communities have been selected from Alicante and Murcia regions (South-East Spain), which account for more than 60,000 irrigators and 120,000 ha. Questionnaires were conducted between March and December 2019. Results highlighted the main advantages (water availability and supply security) and disadvantages (high price affecting profitable crop options, high-energy consumption, water quality standards, the production capacity of desalination plants, no seasonal variation in water production, and shortages due to technical problems) of using desalinated water. Additionally, through the analysis of regional and national press news, it can be concluded that socio-political aspects, such as corruption, cost overruns, and political disputes are also considered.