Desalination using membrane distillation: Flux enhancement by feed water deaeration on spiral-wound modules

Towards sustainable circular brine reclamation using seawater reverse osmosis, membrane distillation and forward osmosis hybrids: An experimental investigation

Desalination and wastewater treatment technologies require an effective solution for brine management to ensure environmental sustainability, which is closely linked with efficient process operations, reduction of chemical dosages, and valorization of brines. Within the scope of desalination brine reclamation, a circular system consisting of seawater reverse osmosis (SWRO), membrane distillation (MD), and forward osmosis (FO) three-process hybrid is investigated. The proposed design increases water recovery from SWRO brine (by MD) and dilutes concentrated brine to seawater level (by FO) for SWRO feed. It ultimately reduces SWRO process brine disposal and improves crystallization efficiency for a zero-liquid discharge application. The operating range of the hybrid system is indicated by a seawater volumetric concentration factor (VCF) ranging from 1.0 to 2.2, which covers practical and sustainable operation in full-scale applications. Within the proposed VCF range, different operating conditions of the MD and FO processes were evaluated in series with concentrated seawater as well as real SWRO brine from a full-scale desalination plant. Water quality and membrane surface were analyzed before and after experiments to assess the impact of the SWRO brine. Despite their low concentration (0.13 mg/L as phosphorous), antiscalants present in SWRO brine alleviated the flux decline in MD operations by 68.3% compared to operations using seawater concentrate, while no significant influence was observed on the FO process. A full spectrum of water quality analysis of real SWRO brine and Red Sea water is made available for future SWRO brine reclamation studies. The operating conditions and experimental results have shown the potential of the SWRO-MD-FO hybrid system for a circular brine reclamation.

Distributed solar desalination by membrane distillation: current status and future perspectives

Membrane distillation (MD) has been proven promising in solar-driven desalination. Moreover, its unique characteristics such as simple process, module compactness, high salt rejection rate, etc. allow for a small-scale device in a distributed system. Both theoretical and experimental researches on the coupling between solar collectors and MD aiming at compact and autonomous desalination system have been devoted to enhance freshwater productivity and energy efficiency. In this paper, certain critical gaps are summarized upon a panoramic review of the current status, including limited production and energy performance compared with commercial-scale desalination, unclear relation between solar collecting area and membrane area, and few discussions on efficient condensation, etc. To tackle these challenges, perspectives on the essential future research directions are proposed. Solar direct heating and solar concentration constitute the possible resolution to enhance solar energy utilization for higher water production, which also raise the question of optimizing solar/MD areas. Meanwhile, module stacking, module internal heat recovery and external evaporation heat recovery are deemed prospective in further reducing MD energy consumption. Subsequently, an enhanced vapor condensation needs more exploration. Those aspects and a potential combination among them are the main tasks in the near future, together with more field tests on small distributed solar-driven MD systems.

PVDF Composite Membranes with Hydrophobically-Capped CuONPs for Direct-Contact Membrane Distillation

Water scarcity is an imminent problem that humanity is beginning to attempt to solve. Among the several technologies that have been developed to mitigate water scarcity, membrane distillation is of particular note. In the present work, CuO nanoparticles capped with 1-octanethiol (CuONPs@CH) or 1H,1H,2H,2H-perfluorodecanethiol (CuONPs@CF) are prepared. The nanoparticles are characterized by FT-IR and TGA methods. Two weight losses are observed in both cases, with the decomposition of the organic fragments beginning at 158 °C and 230 °C for CuONPs@CF and CuONPs@CH, respectively. Flat sheet PVDF composite membranes containing nanoparticles are prepared by the casting solution method using nanoparticle concentrations that ranged between 2-20% with a non-woven polyester fabric as support. The obtained membranes showed a thickness of 240 ± 40 μm. According to water contact angle (87° for CuONPs@CH and 95° for CuONPs@CF, both at 10% w.t) and roughness (12 pixel for CuONPs@CH and 14 pixels for CuONPs@CF, both at 10% w.t) determinations, the hydrophobicity of membranes changed due to a decrease in surface energy, while, for naked CuONPs, the roughness factor represents the main role. Membranes prepared with capped nanoparticles showed similar porosity (60-64%). SEM micrographs show asymmetric porous membranes with a 200-nm surface pore diameter. The largest finger-like pores in the membranes prepared with CuONPs, CuONPs@CH and CuONPs@CF had values of 63 ± 10 μm, 32 ± 8 μm, and 45 ± 10 μm, respectively. These membranes were submitted to a direct contact membrane distillation module and flux values of 1.8, 2.7, and 3.9 kg(m2·h)-1 at ΔT = 30 °C were obtained for the CuONPs, CuONPs@CH, and CuONPs@CF, respectively. The membranes showed 100% salt rejection during the testing time (240 min).

Advances in seawater membrane distillation (SWMD) towards stand-alone zero liquid discharge (ZLD) desalination

Seawater membrane distillation (SWMD) is a promising separation technology due to its ability to operate as a stand-alone desalination unit operation. This paper reviews approaches to improve laboratory-to-pilot-scale MD performance, which comprise operational strategies, module design, and specifically tailored membranes. A detailed comparison of SWMD and sea water reverse osmosis is presented to further analyze the critical shortcomings of SWMD. The unique features of SWMD, namely the ability to operate with extremely high salt rejection and at extreme feed concentration, highlight the SWMD potential to be operated under zero liquid discharge (ZLD) conditions, which results in the production of high-purity water and simultaneous salt recovery, as well as the elimination of the brine disposal cost. However, technical challenges, such as thermal energy requirements, inefficient heat transfer and integration, low water recovery factors, and lack of studies on real-case valuable-salt recovery, are impeding the commercialization of ZLD SWMD. This review highlights the possibility of applying selected strategies to push forward ZLD SWMD commercialization. Suggestions are projected to include intermittent removal of valuable salts, in-depth study on the robustness of novel membranes, module and configuration, utilization of a low-cost heat exchanger, and capital cost reduction in a renewable-energy-integrated SWMD plant.

Emerging forward osmosis and membrane distillation for liquid food concentration: A review

As emerging membrane technologies, forward osmosis (FO) and membrane distillation (MD), which work with novel driving forces, show great potential for liquid food concentration, owing to their low fouling propensity and great driving force. In the last decades, they have attracted the attention of food industry scientists in global scope. However, discussions of the FO and MD in liquid food concentration advancement, membrane fouling, and economic assessment have been scant. This review aims to provide an up-to-date knowledge about liquid food concentration by FO and MD. First, we introduce the principle and applications of FO and MD in liquid food concentration, and highlight the effect of process on liquid food composition, membrane fouling mechanism, and strategies for fouling mitigation. Besides, economic assessment of FO and MD processes is reviewed. Moreover, the challenges as well as future prospects of FO and MD applied in liquid food concentration are proposed and discussed. Comparing with conventional membrane-based or thermal-based technologies, FO and MD show outstanding advantages in high concentration rate, good concentrate quality, low fouling propensity, and low cost. Future efforts for liquid food concentration by FO and MD include (1) development of novel FO draw solution (DS); (2) understanding the effects of liquid food complex compositions on membrane fouling in FO and MD concentration process; and (3) fabrication of novel membranes and innovation of membrane module and process configuration for liquid food processing.

Enhancement of brackish water desalination using hybrid membrane distillation and reverse osmosis systems

Desalination of geothermal brackish water by membrane distillation (MD) provides a low recovery rate, but integrating MD with reverse osmosis (RO) can maximize the production rate. In this study, different design configurations of a hybrid system involving brine recycling and cascading are studied via simulations, and the performance improvement due to the process integration is substantiated via the increased recovery rate and reduced specific energy consumption. Brine recycling is also found to improve the recovery rate considerably to 40% at an energy cost of 0.9 $/m3. However, this achievement is only valid when the final brine is recycled to the RO feed: when the final brine is recycled to the MD feed, the overall performance degrades because the recycled brine cools the feed and causes a serious reduction in the driving force and the consequent production rate. Configuring the hybrid system in multiple stages connected in series increases the recovery rate to 90% and reduces the specific energy consumption to 0.9 MJ/kg. Although the specific energy cost increases dramatically because external inter-stage heating is implemented, using a free energy source (such as a geothermal or waste-energy source) for inter-stage heating could provide the optimum configuration.

Energy Efficiency and Performance Limiting Effects in Thermo-Osmotic Energy Conversion from Low-Grade Heat

Low-grade heat energy from sources below 100 °C is available in massive quantities around the world, but cannot be converted to electricity effectively using existing technologies due to variability in the heat output and the small temperature difference between the source and environment. The recently developed thermo-osmotic energy conversion (TOEC) process has the potential to harvest energy from low-grade heat sources by using a temperature difference to create a pressurized liquid flux across a membrane, which can be converted to mechanical work via a turbine. In this study, we perform the first analysis of energy efficiency and the expected performance of the TOEC technology, focusing on systems utilizing hydrophobic porous vapor-gap membranes and water as a working fluid. We begin by developing a framework to analyze realistic mass and heat transport in the process, probing the impact of various membrane parameters and system operating conditions. Our analysis reveals that an optimized system can achieve heat-to-electricity energy conversion efficiencies up to 4.1% (34% of the Carnot efficiency) with hot and cold working temperatures of 60 and 20 °C, respectively, and an operating pressure of 5 MPa (50 bar). Lower energy efficiencies, however, will occur in systems operating with high power densities (>5 W/m²) and with finite-sized heat exchangers. We identify that the most important membrane properties for achieving high performance are an asymmetric pore structure, high pressure resistance, a high porosity, and a thickness of 30 to 100 μm. We also quantify the benefits in performance from utilizing deaerated water streams, strong hydrodynamic mixing in the membrane module, and high heat exchanger efficiencies. Overall, our study demonstrates the promise of full-scale TOEC systems to extract energy from low-grade heat and identifies key factors for performance optimization moving forward.