Membrane distillation for wastewater reverse osmosis concentrate treatment with water reuse potential

High-efficiency Ti/TiO2-NTA/PbO2-Nd anode electrochemical oxidation of coking reverse osmosis concentrated (ROC)

In this study, a Ti/TiO2-NTA/PbO2-Nd anode was fabricated for the treatment of coking reverse osmosis concentrate (ROC). The TiO2-NTA interlayer reduced β-PbO2 grain size and increased the PbO2 surface area. Compared to the conventional Ti/PbO2 electrode, the Ti/TiO2-NTA/PbO2-Nd anode demonstrated enhanced electrochemical performance, including higher oxygen evolution potential (OEP), improved hydroxyl radical (•OH) generation, reduced charge transfer resistance, and longer service life. Under optimal conditions (20 mA/cm2, pH 6), COD and TOC removal efficiencies reached 88.0 % and 70.8 %, respectively, with high degradation efficiency, current efficiency, and low energy consumption (0.23 kWh/g COD). The degradation followed pseudo-first-order kinetics. Reactive oxygen species contributions to ROC degradation were identified through quenching experiments. Recyclability tests (6 and 80 cycles) and accelerated service life tests confirmed the anode's excellent performance and stability. 3D EEM and GC-MS analysis further demonstrated the effective removal of refractory organics from ROC. These results highlight its potential for long-term wastewater treatment applications.

Advances in membrane distillation for wastewater treatment: Innovations, challenges, and sustainable opportunities

Water pollution and the growing demand for zero liquid discharge solutions have driven the development of advanced wastewater treatment technologies. Membrane distillation (MD) is a promising thermal-based process capable of treating high-salinity brines and wastewater. This review provides an in-depth analysis of MD configurations, operating principles, and membrane characteristics while addressing key challenges such as fouling and pore wetting which hinder large-scale implementation. To overcome these limitations, various membrane fabrication and modification strategies, including physical and chemical approaches, have been explored. The integration of MD with other processes (hybrid MD) for wastewater treatment is also examined. A comprehensive discussion on the mechanisms of organic, inorganic, and biological fouling and their impact on MD performance is presented. Additionally, recent advancements in antifouling strategies, including surface modifications, novel materials, and operational optimizations, are reviewed. Furthermore, the review critically analyzes membrane wetting, its governing mechanisms, and mitigation techniques. By summarizing the current challenges and future prospects, this work provides valuable insights into improving MD performance for practical applications. The findings serve as a foundation for further research and technological advancements in the field of wastewater treatment using MD.

The role of membrane technology in palm oil mill effluent (POME) decontamination: Current trends and future prospects

This article reviews the role of membrane systems in treating palm oil mill effluent (POME), a waste generated by the palm industry. The review focuses on various membrane systems such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), highlighting their effectiveness in removing pollutants and recovering water. Special attention is given to hybrid systems integrating membrane bioreactors (MBRs) and other advanced processes to enhance fouling control, improve water quality, and promote sustainability. Several case studies and quantitative data have demonstrated the reduction of chemical oxygen demand (COD), total suspended solids (TSS), and biological oxygen demand (BOD), illustrating the impact of these technologies. This comprehensive review also explores recent advancements, such as the integration of Zero Liquid Discharge (ZLD) processes, providing insights into the benefits and challenges of membrane technology for POME treatment. This article aims to inform future research and guide industrial applications toward more sustainable and efficient wastewater management in the palm oil industry.

Review of Hybrid Membrane Distillation Systems

Membrane distillation (MD) is an attractive separation process that can work with heat sources with low temperature differences and is less sensitive to concentration polarization and membrane fouling than other pressure-driven membrane separation processes, thus allowing it to use low-grade thermal energy, which is helpful to decrease the consumption of energy, treat concentrated solutions, and improve water recovery rate. This paper provides a review of the integration of MD with waste heat and renewable energy, such as solar radiation, salt-gradient solar ponds, and geothermal energy, for desalination. In addition, MD hybrids with pressure-retarded osmosis (PRO), multi-effect distillation (MED), reverse osmosis (RO), crystallization, forward osmosis (FO), and bioreactors to dispose of concentrated solutions are also comprehensively summarized. A critical analysis of the hybrid MD systems will be helpful for the research and development of MD technology and will promote its application. Eventually, a possible research direction for MD is suggested.

Modification of the distribution of humic acid complexations by introducing microbubbles to membrane distillation process for effective membrane fouling alleviation

Membrane fouling caused by inorganic ions and natural organic matters (NOMs) has been a severe issue in membrane distillation. Microbubble aeration (MB) is a promising technology to control membrane fouling. In this study, MB aeration was introduced to alleviate humic acid (HA) composited fouling during the treatment of simulative reverse osmosis concentrate (ROC) by vacuum membrane distillation (VMD). The objective of this work was to explore the HA fouling inhibiting effect by MB aeration and discuss its mechanism from the interfacial point of view. The results showed that VMD was effective for treating ROC, followed by a severe membrane fouling aggravated with the addition of 100 mg/L HA in feed solution, resulting in 45.7% decline of membrane flux. Analysis using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and zeta potential distribution of charged particles proved the coexistence of HA and inorganic cations (especially Ca2+), resulting in more serious membrane fouling. The introduction of MB aeration exhibited excellent alleviating effect on HA-inorganic salt fouling, with the normalized flux increased from 19.7% to 37.0%. The interfacial properties of MBs played an important role, which altered the zeta potential distributions of charged particles in HA solution, indicating that MBs adhere the HA complexations. Furthermore, this mitigating effect was limited at high inorganic cations concentration. Overall, MBs could change the potential characteristics of HA complexes, which also be used for other similar membrane fouling alleviation.

Nanofilament-Coated Superhydrophobic Membranes Show Enhanced Flux and Fouling Resistance in Membrane Distillation

Membrane distillation (MD) is an important technique for brine desalination and wastewater treatment that may utilize waste or solar heat. To increase the distillation rate and minimize membrane wetting and fouling, we deposit a layer of polysiloxane nanofilaments on microporous membranes. In this way, composite membranes with multiscale pore sizes are created. The performance of these membranes in the air gap and direct contact membrane distillation was investigated in the presence of salt solutions, solutions containing bovine serum albumin, and solutions containing the surfactant sodium dodecyl sulfate. In comparison to conventional hydrophobic membranes, our multiscale porous membranes exhibit superior fouling resistance while attaining a higher distillation flux without using fluorinated compounds. This study demonstrates a viable method for optimizing MD processes for wastewater and saltwater treatment.

Application of Machine Learning to Characterize the Permeate Quality in Pilot-Scale Vacuum-Assisted Air Gap Membrane Distillation Operation

Membrane distillation (MD) is a thermal desalination technique proposed for the valorization of residual brines that other operations such as reverse osmosis cannot treat. Previous studies have shown that vacuum-assisted air gap (V-AGMD) operation in commercial multi-envelope modules improves the performance of MD noticeably. However, the permeate quality at pilot scale has not been thoroughly characterized so far. The aim of this study is, therefore, to assess and model the effect of the main operating conditions (feed flow rate, inlet temperatures, and feed salinity) on the permeate quality. Results from different steady-state experiments allowed to estimate descriptive metrics such as the salt rejection factor (SRF) and the membrane leak ratio (MLR). Given their non-linear behavior, these metrics were subsequently modeled using artificial neural networks (ANN) to estimate the permeate quality in the whole scope of operating conditions. Acceptable SRF results with MLR values lower than 0.2% confirmed the validity of MD as an operation for the treatment of concentrated brines, although the salinity of the resulting permeate does not comply in all cases with that permitted for human consumption.

Membranes in Water Reclamation: Treatment, Reuse and Concentrate Management

In this article, an extensive examination is provided on the possible uses of membranes and hybrid processes in wastewater treatment. While membrane technologies face certain constraints, such as membrane fouling and scaling, the incomplete elimination of emerging contaminants, elevated expenses, energy usage, and brine disposal, there are approaches that can address these challenges. Methods such as pretreating the feed water, utilizing hybrid membrane systems and hybrid dual-membrane systems, and employing other innovative membrane-based treatment techniques can enhance the efficacy of membrane processes and advance sustainability.

A review on hybrid membrane-adsorption systems for intensified water and wastewater treatment: Process configurations, separation targets, and materials applied

In the era of rapid and conspicuous progress of water treatment technologies, combined adsorption and membrane filtration systems have gained great attention as a novel and efficient method for contaminant removal from aqueous phase. Further development of these techniques for water/wastewater treatment applications will be promising for the recovery of water resources as well as reducing the water tension throughout the world. This review introduces the state-of-the-art on the capabilities of the combined adsorption-membrane filtration systems for water and wastewater treatment applications. Technical information including employed materials, superiorities, operational limitations, process sustainability and upgradeing strategies for two general configurations i.e. hybrid (pre-adsorption and post-adsorption) and integrated (film adsorbents, low pressure membrane-adsorption coupling and membrane-adsorption bioreactors) systems has been surveyed and presented. Having a systematic look at the fundamentals of hybridization/integration of the two well-established and efficient separation methods as well as spotlighting the current status and prospectives of the combination strategies, this work will be valuable to all the interested researchers working on design and development of cutting-edge wastewater/water treatment techniques. This review also draws a clear roadmap for either decision making and choosing the best alternative for a specific target in water treatment or making a plan for further enhancement and scale-up of an available strategy.

Virus rejection and removal in pilot-scale air-gap membrane distillation

Membrane distillation (MD) is a thermally-driven process that can treat high concentration streams and provide a dual barrier for rejection and reduction of pathogens. Thus, MD has potential applications in treating concentrated wastewater brines for enhancing water recovery and potable water reuse. In bench-scale studies, it was demonstrated that MD can provide high rejection of MS2 and PhiX174 bacteriophage viruses, and when operating at temperatures greater than 55 °C, can reduce virus levels in the concentrate. However, bench-scale MD results cannot directly be used to predict pilot-scale contaminant rejection and removal of viruses because of the lower water flux and higher transmembrane hydraulic pressure difference in pilot-scale systems. Thus far, virus rejection and removal have not been quantified in pilot-scale MD systems. In this work, the rejection of MS2 and PhiX174 at low (40 °C) and high (70 °C) inlet temperatures is quantified in a pilot-scale air-gap MD system using tertiary treated wastewater. Both viruses were detected in the distillate which suggests the presence of pore flow; the virus rejection at a hot inlet temperature of 40 °C for MS2 and PhiX174 were 1.6-log₁₀ and 3.1-log₁₀, respectively. At 70 °C, virus concentrations in the brine decreased and were below the detection limit (1 PFU per 100 mL) after 4.5 h, however, viruses were also detected in the distillate in that duration. Results demonstrate that virus rejection is lower in pilot-scale experiments because of increased pore flow that is not captured in bench-scale experiments.

Nutrient recovery of the hydrothermal carbonization aqueous product from dairy manure using membrane distillation

Phosphorus is a crucial resource for the agricultural industry, but its limited supply requires recovery from waste materials before it is lost and leads to eutrophication. Dairy manure is rich with phosphorus, and the growth and consolidation within the dairy industry has led to dairy manure management becoming a significant concern. Hydrothermal carbonization (HTC) and membrane distillation (MD) were investigated as an alternative to treat dairy manure and recover nutrients, specifically phosphorus and nitrogen. HTC is a thermal treatment process that converts organic matter into a hydrochar analogous to a low-grade coal, and MD is a thermally-driven separation process that can utilize low-grade waste heat from HTC, thus the two processes are synergetic. A byproduct of the HTC process is the aqueous product (HAP) that contains the water-soluble nutrients and organic components of dairy manure. In this work, the efficacy of MD to concentrate the nutrients in the presence of dissolved organic carbon was assessed. Samples included synthetic nutrient-rich streams as well as HAP produced at HTC temperatures ranging from 200 °C to 260 °C. In each case, the nutrients were successfully concentrated in the feed loop with rejections >99%. Dissolved carbon was found to foul the MD membrane at levels proportional to its hydrophobicity, with little fouling observed for glucose and substantial fouling observed for HAP solutions created at higher temperatures.

Film Distillation with a Porous Condenser for Seawater Desalination: Evaluation of Materials' Stability in the Tropical Climate of Vietnam

Desalination and treatment of wastewater has become critical for Asia regions with water scarcity. In this work, the concept of thin-film distillation equipped with a porous condenser (FDPC) was considered for its implementation in a tropical climate of Vietnam. It was found that samples with a concentration of biocide of 0.5 wt.% possessed lower biofouling, in contrast to the neat membranes. The FD-PC module was developed and water desalination experiments were conducted in Russia and Vietnam. The experiments showed high reproducibility of the results; in particular, the evaporation rate was (4.9/3.0) kg/m²h in Russia and (4.1/2.0) kg/m²h in Vietnam. In addition, as part of this work, the optimal configuration of the installation was calculated using solar collectors as the main energy source. The calculation showed high energy efficiency: specific energy consumption 0.1-0.5 kWh/m³.

Treating reverse osmosis concentrate to address scaling and fouling problems in zero-liquid discharge systems: A scientometric review of global trends

Currently, reverse osmosis concentrate (ROC) treatment is one of the most promising techniques for its disposal because it produces freshwater with high recovery and valuable materials such as salts and reduces waste volume and environmental pollution. Public attention to the severe consequences of water pollution and strict environmental regulations on wastewater discharge has pushed water-polluting industries toward zero-liquid discharge (ZLD). However, scaling and fouling problems increase energy consumption and limit permeate flux at high salt concentrations, mainly due to calcium, magnesium, and silica precipitation, ultimately decreasing ZLD performance. Therefore, this study discusses drivers and ROC pretreatment technologies to improve ZLD efficiency and presents a scientometric review of global trends. The advantages, disadvantages, and economic and environmental aspects of conventional and emerging pre-treatment technologies were studied. Traditional treatment of chemical processes combined with precipitation removes a large amount of scaling ions; however, high operation and maintenance costs and limited full-scale plant experience are the main drawbacks. Softening and coagulation are most commonly applied to treat large volumes at a moderate cost; however, substantial sludge production and increased conductivity are major operational issues. Moreover, emerging technologies efficiently remove scale-forming ions with high capital and operating costs. New variations in standard reverse osmosis technologies have improved ZLD efficiency; nonetheless, scaling and fouling are of concern. Therefore, this review presents the studies on ROC pre-treatment technologies for removing scaling ions to enhance ZLD efficiency, which can help in future research.

Unique Behaviors and Mechanism of Highly Soluble Salt-Induced Wetting in Membrane Distillation

Scaling-induced wettinggreatly limits the application of membrane distillation (MD) for the desalination of high-salinity feed. Although highly soluble salts (e.g., NaCl) have high concentrations in this water, their scaling-induced wetting remains overlooked. To unravel the elusive wetting behaviors of highly soluble salts, in this study, we systematically investigated the scaling formation and wetting progress by in situ observation with optical coherence tomography (OCT). Through examining the influence of salt type and vapor flux on the wetting behavior, we revealed that highly soluble salt-induced wetting, especially under high vapor flux, shared several unique features: (1) occurring before the bulk feed reached saturation, (2) no scale layer formation observed, and (3) synchronized wetting progress on the millimeter scale. We demonstrated that a moving scale layer caused these interesting phenomena. The initial high vapor flux induced high concentration and temperature polarizations, which led to crystallization at the gas-liquid interface and the formation of an initial scale layer. On the one hand, this scale layer bridged the water into the hydrophobic pores; on the other hand, it blocked the membrane pores and reduced the vapor flux. In this way, the decreased vapor flux mitigated the concentration/temperature polarizations, and consequently led to the dissolution of the feed-facing side of the scale layer. This dissolution prevented the membrane pores from being completely blocked, facilitating the transportation and crystallization of salts at the distillate-facing side of the scale layer (i.e., the gas-liquid interface), thus the proceeding of the wetting layer.

Highly efficient removal of total nitrogen and dissolved organic compound in waste reverse osmosis concentrate mediated by chlorine radical on 3D Co3O4 nanowires anode

Reverse osmosis concentrate (ROC) from wastewater reclamation has posed significant disposal challenges due to its highly concentrated NH₃-N, chloride ion and bio-refractory organics, and developing technologies for their removal are essential. Herein, we developed an efficient electrochemical system to remove total nitrogen and dissolved organic compound (DOC) simultaneously mediated by chlorine radical (Cl•), which is generated by activation of chloride ion existing in ROC on an inexpensive, three-dimensional Co₃O₄ nanowires. Results showed that the total nitrogen and total organic carbon removal were 98.2% and 56.9% in 60 min for synthetic ROC with 56 mg/L of NH₃-N and 20 mg/L of DOC. The utilization of Co₃O₄ nanowires enhanced NH₃-N degradation by 2.58 times compared with Co₃O₄ nanoplates, which were 1.69 and 17.5 times these of RuO₂ and Pt. We found that structural Co³⁺/Co²⁺ acts as cyclic catalysis to produce Cl• via single-electron transfer, which convert NH₃-N to N₂ and lead to faster DOC degradation. This architecture provides abundant catalytic sites and sufficient accessibility of reactants. Small amount of nitrate generated by oxidation of NH₃-N was further reduced to N₂ on Pd-Cu/NF cathode. These findings provide new insights for utilization of waste Cl^- and development of novel electrochemical system for ROC disposal.

Membrane distillation for achieving high water recovery for potable water reuse

Achieving high water recovery using reverse osmosis membranes is challenging during water recycling because the increased concentrations of organics and inorganics in wastewater can cause rapid membrane fouling, necessitating frequent cleaning using chemical agents. This study evaluated the potential of membrane distillation to purify reverse osmosis-concentrated wastewater and achieve 98% overall water recovery for potable water reuse. The results indicate that membrane fouling during membrane distillation treatment was low (4% reduction in permeability) until 98% water recovery. In contrast, membrane fouling during reverse osmosis treatments was high (73% reduction in permeability) before reaching 90% water recovery. Furthermore, membrane distillation showed superior performance in removing dissolved ions (99.9%) from wastewater as compared with reverse osmosis (98.9%). However, although membrane distillation removed most trace organic chemicals tested in this study, a negligible rejection (11%) was observed for N-nitrosodimethylamine, a disinfection byproduct regulated in potable water reuse. In contrast, RO treatment exhibited a high removal of N-nitrosodimethylamine (70%). Post-treatment (e.g., advanced oxidation) after reverse osmosis and membrane distillation may be needed to comply with the N-nitrosodimethylamine regulations. Overall, the membrane distillation process had the capacity to purify reverse osmosis concentrate with insignificant membrane fouling.

Membrane Distillation: Pre-Treatment Effects on Fouling Dynamics

In the research reported in this paper, membrane distillation was employed to recover water from a concentrated saline petrochemical effluent. According to the results, the use of membrane distillation is technically feasible when pre-treatments are employed to mitigate fouling. A mathematical model was used to evaluate the fouling mechanism, showing that the deposition of particulate and precipitated material occurred in all tests; however, the fouling dynamic depends on the pre-treatment employed (filtration, or filtration associated with a pH adjustment). The deposit layer formed by particles is not cohesive, allowing its entrainment to the bulk flow. The precipitate fouling showed a minimal tendency to entrainment. Also, precipitate fouling served as a coupling agent among adjacent particles, increasing the fouling layer cohesion.

Integration of membrane distillation as volume reduction technology for in-land desalination brines management: Pre-treatments and scaling limitations

Management of in-land reverse osmosis (RO) desalination brines generated from surface brackish waters is a current challenge. Among the different near-Zero and Zero Liquid Discharge (ZLD) alternatives, Membrane Distillation (MD), in which the transport of water is thermally driven, appears as an attractive technology if a residual heat source is available. The aim of this study was to identify the limits of Direct Contact MD (DCMD) pre-treatments such as acidification and aeration, or the combination of both to quantify the scaling reduction potential when treating a RO brine from surface brackish water. Experimental data were used to evaluate the effectiveness of DCMD to achieve the highest concentration factors, depending on the chosen pre-treatment. Additionally, an economic analysis of the operational cost, taking as case study a site where the current management of the brine is the discharge to the sea, was also carried out. Results showed that pre-treatments enhanced MD performance by increasing the concentration factor achieved and highest volume reductions (about 3 times) were reached with the combination of acidification and aeration pre-treatments. Both processes reduced the precipitation potential of CaCO₃(s) by reducing the total inorganic carbon (>90%); however, CaSO₄·2H₂O(s) precipitated. Results also indicated that even if a waste heat source is available, brine disposal into the sea is the cheapest option, while ZLD alternatives were not attractive in the current regulatory framework since their cost was higher than the discharge to the sea. Other options related to the Minimal Liquid Discharge may be more economically attractive.

Organic Fouling Impact in a Direct Contact Membrane Distillation System Treating Wastewater: Experimental Observations and Modeling Approach

This study aims to investigate the effect of operational conditions on organic fouling occurring in a direct contact membrane distillation (DCMD) system used to treat wastewater. A mixed solution of sodium alginate (SA) and bovine serum albumin (BSA) was used as a feed solution to simulate polysaccharides and proteins, respectively, assumed as the main organic foulants. The permeate flux was observed at two feed temperatures 35 and 50 °C, as well as three feed solution pH 4, 6, and 8. Higher permeate flux was observed for higher feed temperature, which allows higher vapor pressure. At higher pH, a smaller particle size was detected with lower permeate flux. A mathematical model based on mass balance was developed to simulate permeate flux with time by assuming (i) the cake formation controlled by attachment and detachment of foulant materials and (ii) the increase in specific cake resistance, the function of the cake porosity, as the main mechanisms controlling membrane fouling to investigate the fouling mechanism responsible of permeate flux decline. The model fitted well with the experimental data with R² superior to 0.9. High specific cake resistance fostered by small particle size would be responsible for the low permeate flux observed at pH 8.

Feasibility of membrane distillation process for potable water reuse: A barrier for dissolved organic matters and pharmaceuticals

In this study, the feasibility of the membrane distillation (MD) process as a wastewater reclamation system for portable reuse was investigated. The flux was stably maintained at about 20 L/m²h (LMH) at ΔT 30 °C, compared to higher flux at ΔT 50 °C, which showed a rapid decrease in the flux due to severe fouling. MD produced excellent quality of potable water satisfied the drinking water standards of Korea from effluent of sewage treatment plant (ESTP). The fractions of the hydrophobic OC (HOC) and chromatographic DOC (CDOC) from LC-OCD analysis was firstly suggested to understand different organic transport during the MD process. The transport of organic matters across the MD membrane mitigated at low operation temperature and the transported organics in all the tested waters were mostly volatile low molecular weight organics, aromatic amino acids. All of thirteen selected pharmaceuticals were completely removed by MD, regardless of their properties. In order to retard the membrane fouling of the MD process, coagulation and filtration pre-treatments were applied. The pre-treatment process coupled MD process could successfully remove impurities including NH₄-N without severe membrane fouling. Moreover, coagulation pretreatment reduced transport of ammonia due to decrease in pH.

Graphene coupled TiO2 photocatalysts for environmental applications: A review

Nanostructured photocatalysts have always offered opportunities to solve issues concerned with the environmental challenges caused by rapid urbanization and industrialization. These materials, due to their tunable physicochemical characteristics, are capable of providing a clean and sustainable ecosystem to humanity. One of the current thriving research focuses of visible-light-driven photocatalysts is on the nanocomposites of titanium dioxide (TiO₂) with carbon nanostructures, especially graphene. Coupling TiO₂ with graphene has proven more active by photocatalysis than TiO₂ alone. It is generally considered that graphene sheets act as an electron acceptor facilitating the transfer and separation of photogenerated electrons during TiO₂ excitation, thereby reducing electron-hole recombination. This study briefly reviews the fundamental mechanism and interfacial charge-transfer dynamics in TiO₂/graphene nanocomposites. Design strategies of various graphene-based hybrids are highlighted along with some specialized synthetic routes adopted to attain preferred properties. Importantly, the enhancing interfacial charge transfer of photogenerated e¯_CB through the graphene layers by morphology orientation of TiO₂, predominated exposure of their high energy crystal facets, defect engineering, enhancing catalytic sites in graphene, constructing dedicated architectures, tuning the nanomaterial dimensionality at the interface, and employing the synergism adopted through various modifications, are systematically compiled. Portraying the significance of these photocatalytic hybrids in environmental remediation, important applications including air and water purification, self-cleaning surfaces, H₂ production, and CO₂ reduction to desired fuels, are addressed.

Frontiers of Membrane Desalination Processes for Brackish Water Treatment: A Review

Climate change, population growth, and increased industrial activities are exacerbating freshwater scarcity and leading to increased interest in desalination of saline water. Brackish water is an attractive alternative to freshwater due to its low salinity and widespread availability in many water-scarce areas. However, partial or total desalination of brackish water is essential to reach the water quality requirements for a variety of applications. Selection of appropriate technology requires knowledge and understanding of the operational principles, capabilities, and limitations of the available desalination processes. Proper combination of feedwater technology improves the energy efficiency of desalination. In this article, we focus on pressure-driven and electro-driven membrane desalination processes. We review the principles, as well as challenges and recent improvements for reverse osmosis (RO), nanofiltration (NF), electrodialysis (ED), and membrane capacitive deionization (MCDI). RO is the dominant membrane process for large-scale desalination of brackish water with higher salinity, while ED and MCDI are energy-efficient for lower salinity ranges. Selective removal of multivalent components makes NF an excellent option for water softening. Brackish water desalination with membrane processes faces a series of challenges. Membrane fouling and scaling are the common issues associated with these processes, resulting in a reduction in their water recovery and energy efficiency. To overcome such adverse effects, many efforts have been dedicated toward development of pre-treatment steps, surface modification of membranes, use of anti-scalant, and modification of operational conditions. However, the effectiveness of these approaches depends on the fouling propensity of the feed water. In addition to the fouling and scaling, each process may face other challenges depending on their state of development and maturity. This review provides recent advances in the material, architecture, and operation of these processes that can assist in the selection and design of technologies for particular applications. The active research directions to improve the performance of these processes are also identified. The review shows that technologies that are tunable and particularly efficient for partial desalination such as ED and MCDI are increasingly competitive with traditional RO processes. Development of cost-effective ion exchange membranes with high chemical and mechanical stability can further improve the economy of desalination with electro-membrane processes and advance their future applications.

Operation of Three-Stage Process of Lithium Recovery from Geothermal Brine: Simulation

Lithium-rich geothermal waters are considered as an alternative source, and further concentration of lithium is required for its effective recovery. In this work, we have simulated a three-stage lithium recovery process including the brine softening by precipitation Ca²⁺/Mg²⁺ cations with sodium carbonate (calculated in PHREEQC), followed by an integrated system consisting of membrane distillation unit (water evaporation), crystallizer (NaCl precipitation), and membrane extraction (Li⁺ recovery), which was simulated in Simulink/MATLAB. It was shown that the deterioration of membrane performance in time due to scaling/fouling plays a critical role in the performance of the system resulting in the dramatic increase of the replaced membrane modules by a factor of 5. Low cost membranes are required. The process simulation based on the experimental and literature data on the high salinity solutions with the membrane distillation revealed that the specific productivity can be achieved in the range of 9.9–880 g (Li⁺) per square meter of membranes in the module used before its replacement. The increase of energy efficiency is needed. The mass-flow-rate of saline solution circulated to the crystallizer was set at its almost minimum value as 6.5 kg/min to enable its successful operation at the given parameters of the membrane distillation unit. In other words, the operation of the integrated system having 140 kg of saline solution in the loop and a membrane module of 2.5 m² for concentration of lithium presence from 0.11 up to 2.3 g/kg would be associated with the circulation of about of 259 tons of saline solution per month between the distillation unit (60 °C) and the crystallizer (15 °C) to yield of up to 1.4 kg of lithium ions. The comprehensive summary and discussion are presented in the conclusions section.

A Critical Review of Membrane Wettability in Membrane Distillation from the Perspective of Interfacial Interactions

Hydrophobic membranes used in membrane distillation (MD) systems are often subject to wetting during long-term operation. Thus, it is of great importance to fully understand factors that influence the wettability of hydrophobic membranes and their impact on the overall separation efficiency that can be achieved in MD systems. This Critical Review summarizes both fundamental and applied aspects of membrane wetting with particular emphasis on interfacial interaction between the membrane and solutes in the feed solution. First, the theoretical background of surface wetting, including the relationship between wettability and interfacial interaction, definition and measurement of contact angle, surface tension, surface free energy, adhesion force, and liquid entry pressure, is described. Second, the nature of wettability, membrane wetting mechanisms, influence of membrane properties, feed characteristics and operating conditions on membrane wetting, and evolution of membrane wetting are reviewed in the context of an MD process. Third, specific membrane features that increase resistance to wetting (e.g., superhydrophobic, omniphobic, and Janus membranes) are discussed briefly followed by the comparison of various cleaning approaches to restore membrane hydrophobicity. Finally, challenges with the prevention of membrane wetting are summarized, and future work is proposed to improve the use of MD technology in a variety of applications.

The use of carbon nanomaterials in membrane distillation membranes: a review

Membrane distillation (MD) is a thermal-based separation technique with the potential to treat a wide range of water types for various applications and industries. Certain challenges remain however, which prevent it from becoming commercially widespread including moderate permeate flux, decline in separation performance over time due to pore wetting and high thermal energy requirements. Nevertheless, its attractive characteristics such as high rejection (ca. 100%) of nonvolatile species, its ability to treat highly saline solutions under low operating pressures (typically atmospheric) as well as its ability to operate at low temperatures, enabling waste-heat integration, continue to drive research interests globally. Of particular interest is the class of carbon-based nanomaterials which includes graphene and carbon nanotubes, whose wide range of properties have been exploited in an attempt to overcome the technical challenges that MD faces. These low dimensional materials exhibit properties such as high specific surface area, high strength, tuneable hydrophobicity, enhanced vapour transport, high thermal and electrical conductivity and others. Their use in MD has resulted in improved membrane performance characteristics like increased permeability and reduced fouling propensity. They have also enabled novel membrane capabilities such as in-situ fouling detection and localised heat generation. In this review we provide a brief introduction to MD and describe key membrane characteristics and fabrication methods. We then give an account of the various uses of carbon nanomaterials for MD applications, focussing on polymeric membrane systems. Future research directions based on the findings are also suggested.

Multifunctional nanocoated membranes for high-rate electrothermal desalination of hypersaline waters

Surface heating membrane distillation overcomes several limitations inherent in conventional membrane distillation technology. Here we report a successful effort to grow in situ a hexagonal boron nitride (hBN) nanocoating on a stainless-steel wire cloth (hBN-SSWC), and its application as a scalable electrothermal heating material in surface heating membrane distillation. The novel hBN-SSWC provides superior vapour permeability, thermal conductivity, electrical insulation and anticorrosion properties, all of which are critical for the long-term surface heating membrane distillation performance, particularly with hypersaline solutions. By simply attaching hBN-SSWC to a commercial membrane and providing power with an a.c. supply at household frequency, we demonstrate that hBN-SSWC is able to support an ultrahigh power intensity (50 kW m^-2) to desalinate hypersaline solutions with exceptionally high water flux (and throughput), single-pass water recovery and heat utilization efficiency while maintaining excellent material stability. We also demonstrate the exceptional performance of hBN-SSWC in a scalable and compact spiral-wound electrothermal membrane distillation module.

Selective copper recovery by membrane distillation and adsorption system from synthetic acid mine drainage

Acid mine drainage (AMD) which involves high sulfur and heavy metals concentrations and furthermore are acidic in character, has been a major environmental and economic issue due to the associated toxicity and treatment costs. A large quantity of AMD in nature has a variety of resources including water and heavy metals such as Cu, Al, Fe and Ni. In this study, the valuable resource of Cu was selectively recovered from model AMD solution through membrane distillation and adsorption systems. Direct contact membrane distillation (DCMD) system enabled to concentrate the Cu concentration in AMD by more than 2.5 times while recovering 80% of high-quality water for reuse purposes. For adsorption, mesoporous silica material was used after multi-modification with Mn and amine grafting to enhance the adsorption capacity as well as selectivity for Cu. Under acidic conditions, heavy metals cannot be adsorbed on amine grafted SBA-15. Therefore, the pH of synthetic AMD (pH = 2.2) had to be adjusted to the 5.0-5.2 range, in order to enable adsorption of Cu on modified SBA-15 (this is to prevent protonation of amine groups grafted on prepared SBA-15). Moreover, an increase in pH helped to precipitate more than 99% of Fe and Al (predominant metals in AMD). Cu adsorption on modified SBA-15 was 24.53 mg/g for KOH-treated AMD. However, Cu adsorption on modified SBA-15 decreased by 26% (18.11 mg/g) for NaOH-treated AMD. Cu adsorption with modified SBA-15 significantly improved to 55.75 mg/g when the Cu concentration was concentrated by DCMD.

Increasing net water recovery of reverse osmosis with membrane distillation using natural thermal differentials between brine and co-located water sources: Impacts at large reclamation facilities

Maximizing water recovery and minimizing the volume of RO concentrate (i.e., brine) produced is a growing challenge, especially for inland communities that lack ocean disposal options. In such regions, transitioning towards zero liquid discharge (ZLD) can avoid detrimental impacts associated with salt disposal via regional sewer discharge or deep-well injection. On-site ZLD energy requirements are proportional to the RO brine flowrate. Thus, system-level strategies that reduce RO brine flows will lower ZLD costs while simultaneously increasing the overall water recovery for beneficial reuse in reclamation facilities. We investigated a membrane distillation (MD) system operating using co-located, cooler source water to treat warmer wastewater RO brine. Using experimentally-quantified MD fluxes based on observed monthly water temperatures of co-located water and RO brine at a facility in central Arizona, and based on the previously reported performance of large-scale MD systems, energy consumption and operating cost were estimated to evaluate the potential capabilities of MD to treat RO brine at full scale facilities. When the RO unit was combined with MD brine treatment, net water recovery at the full-scale facility can increase from 85% to up to 91% while brine flow can be reduced by 26%. A 25% lower thermal energy was required to achieve RO net water recovery of 95% when using co-located water, compared against conventional MD without using co-located water. Overall, this work demonstrates the potential to use local thermal gradients to reduce RO brine volumes, thereby reducing ZLD costs.

Trace Organic Compound Removal from Wastewater Reverse-Osmosis Concentrate by Advanced Oxidation Processes with UV/O3/H2O2

Advanced technologies, such as reverse osmosis (RO), allow the reuse of treated wastewater for direct or indirect potable use. However, even highly efficient RO systems produce ~10-15% highly contaminated concentrate as a byproduct. This wastewater RO concentrate (WWROC) is very rich in metal ions, nutrients, and hard-to-degrade trace organic compounds (TOrCs), such as pharmaceuticals, plasticizers, flame retardants, and detergents, which must be treated before disposal. WWROC could be up to 10 times more concentrated than secondary effluent. We examined the efficiency of several advanced oxidation processes (AOPs) on TOrC removal from a two-stage WWROC matrix in a pilot wastewater-treatment facility. WWROC ozonation or UV irradiation, with H₂O₂ addition, demonstrated efficient removal of TOrCs, varying between 21% and over 99% degradation, and indicating that radical oxidation (by HO·) is the dominant mechanism. However, AOPs are not sufficient to fully treat the WWROC, and thus, additional procedures are required to decrease metal ion and nutrient concentrations. Further biological treatment post-AOP is also highly important, to eliminate the degradable organic molecules obtained from the AOP.

Volume reduction and water reclamation of reverse osmosis concentrate from coal chemical industry by forward osmosis with an osmotic backwash strategy

Coal chemical industry (CCI) generally utilizes reverse osmosis (RO) for water reclamation, which generates a highly concentrated stream containing refractory organic substances and high-concentration total dissolved solids (TDS). To address this issue, the present work focuses on volume reduction of RO concentrate (ROC) produced from CCI by forward osmosis (FO). We investigated the effects of membrane orientation and draw solution (DS) concentration on FO performance. Foulant removal was tested by using chemical cleaning, physical cleaning and osmotic backwash (OB). AL-FS (active layer facing feed solution) mode outcompeted AL-DS (active layer facing draw solution) mode, achieving a flux of 26.4 LMH, 92.5% water reclamation and energy consumption of 0.050 kWh·m^-3 with 4 M NaCl as DS. The FO process was able to reject >98% SO₄ ^2-, Mg²⁺and Ca²⁺, 92-98% Si and 33-55% total organic carbon (TOC). Ten-cycle (10 × 20 h) accelerated fouling test demonstrated approximately 30% flux decline in association with Si-containing foulants, which could be removed almost completely through OB with 97.1% flux recovery. This study provides a proof-of-concept demonstration of FO for volume reduction and water reclamation of ROC produced from CCI, making the treatment of ROC more efficient and more energy effective.

Direct membrane filtration for wastewater treatment and resource recovery: A review

Direct membrane filtration has shown great potential in wastewater treatment and resource recovery in terms of its superior treated water quality, efficient nutrient recovery, and sustainable operation, especially under some scenarios where biological treatment is not feasible. This paper aims to give a comprehensive review of the state-of-the-art of direct membrane filtration processes (including pressure-driven, osmotic-driven, thermal-driven, and electrical-driven) in treating different types of wastewater for water reclamation and resource recovery. The factors influencing membrane performance and treatment efficiency in these direct membrane filtration processes are well illustrated, in which membrane fouling was identified as the main challenge. The strategies for improving direct membrane filtration performance, such as physical and chemical cleaning techniques and pretreatment of feed water, are highlighted. Towards scaling-up and long-term operation of direct membrane filtration for effective wastewater reclamation and resource recovery, the challenges are emphasized and the prospects are discussed.

A Review on the Mechanism, Impacts and Control Methods of Membrane Fouling in MBR System

Compared with the traditional activated sludge process, a membrane bioreactor (MBR) has many advantages, such as good effluent quality, small floor space, low residual sludge yield and easy automatic control. It has a promising prospect in wastewater treatment and reuse. However, membrane fouling is the biggest obstacle to the wide application of MBR. This paper aims at summarizing the new research progress of membrane fouling mechanism, control, prediction and detection in the MBR systems. Classification, mechanism, influencing factors and control of membrane fouling, membrane life prediction and online monitoring of membrane fouling are discussed. The research trends of relevant research areas in MBR membrane fouling are prospected.

Recovery of sodium sulfate from seawater brine using fractional submerged membrane distillation crystallizer

Seawater reverse osmosis (SWRO) brine contain many valuable resources. In this study, fractional-submerged membrane distillation crystallizer (F-SMDC) was used to recover sodium sulfate (Na₂SO₄) from SWRO brine. The concentration/temperature gradient (CG/TG) in the reactor enhanced water recovery utilizing MD and Na₂SO₄ crystallization via a crystallizer. Crystals were not obtained at the bottom section of the F-SMDC due to: firstly, calcium sulfate crystallization occurring on the membrane surface; and secondly, low temperature-sensitivity solubility component such as NaCl exerting a negative influence. In order to obtain supersaturation, a sulfate-rich scenario was created in the reactor through the addition of the following three components: Na₂SO₄, MgSO₄ and (NH₄)₂SO₄. When Na₂SO₄ and MgSO₄ were added, a larger concentration was observed at the top section, resulting in a low concentration gradient (CG) ratio, i.e. around 1.7. Conversely, the addition of (NH₄)₂SO₄ achieved faster Na₂SO₄ crystallization (VCF 1.42) at the bottom section with a greater CG ratio of more than 2.0. Total water recovery ratio of 72% and 223.73 g Na₂SO₄ crystals were successfully extracted from simulated SWRO brine using laboratory scale F-SMDC.

A review of membrane development in membrane distillation for emulsified industrial or shale gas wastewater treatments with feed containing hybrid impurities

Investigations on membrane materials for membrane distillation (MD) and its applications have been ongoing since the 1990s. However, a lack of materials that produce robustly stable and up-to-the-mark membranes for MD for different industrial applications remains an ongoing problem. This paper provides an overview of materials developed for MD applications. Although key aspects of published articles reviewed in this paper pertain to MD membranes synthesized for desalination, future MD can also be applied to organic wastewater containing surfactants with inorganic compounds, either with the help of hybrid treatment processes or with customized membrane materials. Many industrial discharges produce effluents at a very high temperature, which is an available driving force for MD. However, there remains a lack of cost-effective membrane materials. Amphiphobic and omniphobic membranes have recently been developed for treating emulsified and shale gas produced water, but the problem of organic fouling and pore wetting remains a major challenge, especially when NaCl and other inorganic impurities are present, which further deteriorate separation performance. Therefore, further advancements in materials are required for the treatment of emulsified industrial wastewater containing surfactants, salts, and for oil or shale gas wastewater for its commercialized reuse. Integrated MD systems, however, may represent a major change in shale gas wastewater and emulsified wastewater that are difficult to treat.