Alternative heating techniques in membrane distillation: A review

Effect of coagulation pretreatment on membrane distillation process for desalination of recirculating cooling water

State-of-the-art methods for overcoming temperature polarization in membrane distillation process: A review

Fouling and wetting in the membrane distillation driven wastewater reclamation process – A review

Polyphosphates used for membrane scaling inhibition during water desalination by membrane distillation

The demand to fabricate membranes much cheaper than usual expensive methods creates an opportunity to find low‐cost and more available modifying agents for synthesising an appropriate membrane for membrane distillation (MD) process. In this work, reactive ethylene terpolymer was applied to modify a commercial polyvinylidene fluoride (PVDF) membrane for using in the desalination process. Elvaloy4170 with a hydrophobic structure containing three different functional groups was coated (different contents of 0.5, 1, 1.5 and 2 wt%) on the top surface of the microporous commercial PVDF membrane and the resultant membranes were analysed under air‐gap MD process. The fabricated membrane structures were determined by scanning electron microscopy and atomic force microscopy to investigate their elemental and topographical properties. After experiments, the membrane with 1.5 wt% of Elvaloy4170 showed water vapour flux of 1.93 kg/m2 h and salt rejection of >99.7 which confirmed its suitability to be used in the desalination process. Moreover, to assess the anti‐fouling properties of the membranes, real seawater was used as feed solution, and as a result the membrane with 1.5 wt% of Elvaloy4170 presented flux recovery of 87% indicating its enhanced anti‐fouling properties compared with the commercial PVDF membrane (flux recovery of 71%).

Membrane processes with outstanding advantages have been developed over the past three decades and could be used for oily wastewater treatment applications, effectively. Nowadays, forward osmosis (FO) and membrane distillation (MD) processes, as two essential membrane processes have received considerable attention, for oily wastewater treatment applications. The excellence of MD and FO processes compared to other membrane processes is their cost-effectiveness and capability to remove smaller oil drops with lower energy consumption. Membrane fouling as a challenge with decreasing separation efficiency has hindered the commercialization of FO and MD processes. Therefore, developing new FO and MD membranes with lower fouling tendency is very important for water treatment applications especially for oily wastewater treatment. In this chapter, after bibliographic analysis, the conventional methods developed for treating various types of oily wastewaters are reviewed, in brief. Then, various membrane processes with particular emphasis on their challenges and advances in oily wastewater treatment applications are presented. Afterward, focusing on FO and MD processes, the current materials used to fabricate proper membranes for FO and MD processes are discussed.

Membrane distillation (MD) processes for water desalination applications. Can dense selfstanding membranes compete with microporous hydrophobic materials?

The growing scarcity of fresh water is driving the implementation of wastewater treatment and water reuse on an increasingly large scale. Various methods have been developed and used for water reuse from wastewater; however, the membrane distillation (MD) process, as a promising separation technology, has recently gained more attention. The MD process is a non-isothermal membrane-based separation used in various applications, especially for desalination and water/wastewater treatment. Compared with other separation processes, the MD process possesses several unique characteristics such as total (100%) rejection, intensive to feed concentration, mild operating conditions as well as stable performance at high contaminant concentrations. Due to the high fresh water demand in recent years, extensive researches have been devoted to the MD process in areas of water/wastewater treatment. The present paper offers a comprehensive MD state of the art review covering the MD applications for wastewater treatment and water reuse.

Membrane distillation (MD) can be used in drinking water treatment, such as seawater desalination, ultra-pure water production, chemical substances concentration, removal or recovery of volatile solutes in an aqueous solution, concentration of fruit juice or liquid food, and wastewater treatment. However, there is still much work to do to determine appropriate industrial implementation. MD processes refer to thermally driven transport of vapor through non-wetted porous hydrophobic membranes, which use the vapor pressure difference between the two sides of the membrane pores as the driving force. Recently, computational fluid dynamics (CFD) simulation has been widely used in MD process analysis, such as MD mechanism and characteristics analysis, membrane module development, preparing novel membranes, etc. A series of related research results have been achieved, including the solutions of temperature/concentration polarization and permeate flux enhancement. In this article, the research of CFD applications in MD progress is reviewed, including the applications of CFD in the mechanism and characteristics analysis of different MD structures, in the design and optimization of membrane modules, and in the preparation and characteristics analysis of novel membranes. The physical phenomena and geometric structures have been greatly simplified in most CFD simulations of MD processes, so there still is much work to do in this field in the future. A great deal of attention has been paid to the hydrodynamics and heat transfer in the channels of MD modules, as well as the optimization of these modules. However, the study of momentum transfer, heat, and mass transfer mechanisms in membrane pores is rarely involved. These projects should be combined with mass transfer, heat transfer and momentum transfer for more comprehensive and in-depth research. In most CFD simulations of MD processes, some physical phenomena, such as surface diffusion, which occur on the membrane surface and have an important guiding significance for the preparation of novel membranes to be further studied, are also ignored. As a result, although CFD simulation has been widely used in MD process modeling already, there are still some problems remaining, which should be studied in the future. It can be predicted that more complex mechanisms, such as permeable wall conditions, fouling dynamics, and multiple ionic component diffusion, will be included in the CFD modeling of MD processes. Furthermore, users’ developed routines for MD processes will also be incorporated into the existing commercial or open source CFD software packages.

Mitigating near-surface polarizations in membrane distillation via membrane surface decoration

Membrane distillation (MD) has been known as a promising water treatment process for many years. However, despite its advantages, MD has never been able to compete with other processes for industrial water treatment and supply. Instead, it has been orientated towards several unique strategic water treatment applications. This review aims to uncover the opportunities and technical challenges pertinent to the MD process and the current status of its strategic water treatment applications most notably including decentralised small-scale desalination for fresh water provision in remote areas, hybridisation with forward osmosis (FO) for treatment of challenging polluted waters, regeneration of liquid desiccant solutions for air conditioning, and treatment of acid effluents for beneficial reuse. Pilot and small-scale MD systems have been demonstrated for decentralised desalination using various renewable energy sources to supply fresh water in remote, rural areas and on ships where other desalination processes are inefficient or unfeasible. For this strategic desalination application, MD is technically viable, but more works on configuration modification and process optimisation are required to reduce the process energy consumption and water production costs. For the three other strategic applications, the technical viability of the MD process has been proved by extensive lab-scale researches, but its economic feasibility is still questionable due to the lack of large-scale evaluation and the uncertain costs of MD systems. The orientation of MD towards strategic water treatment applications is clear. However, huge efforts are required to facilitate these applications at commercial and full scale.

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

Membrane Distillation (MD) is a membrane-based, temperature-driven water reclamation process. While research emphasis has been largely on membrane design, upscaling of MD has prompted advancements in energy-efficient module design and configurations. Apart from the four conventional configurations, researchers have come up with novel MD membrane module designs and configurations to improve thermal efficiency. While membrane design has been the focus of many studies, development of appropriate system configurations for optimal energy efficiency for each application has received considerable attention, and is a critical aspect in advancing MD configurations. This review assesses advancements in modified and novel MD configurations design with emphasis on the effects of upscaling and pilot scale studies. Improved MD configurations discussed in this review are the material gap MD, conductive gap MD, permeate gap MD, vacuum-enhanced AGMD/DCMD, submerged MD, flashed-feed MD, dead-end MD, and vacuum-enhanced multi-effect MD. All of these modified MD configurations are designed either to reduce the heat loss by mitigating the temperature polarization or to improve the mass transfer and permeate flux. Vacuum-enhanced MD processes and MD process with non-contact feed solution show promise at the lab-scale and must be further investigated. Hollow fiber membrane-based pilot scale modules have not yet been sufficiently explored. In addition, comparison of various configurations is prevented by a lack of standardized testing conditions. We also reflect on recent pilot scale studies, ongoing hurdles in commercialization, and niche applications of the MD process.

Biofouling in membrane distillation (MD) has several repercussions, including reduced efficiency of the MD process and limiting membrane life. Additionally, the evaluation of MD biofouling using treated effluents from wastewater treatment plants remains an unexplored area. Thus, biofouling formation and development in a long term MD process (15 days) using treated effluent from a wastewater treatment plant was explored in this study. The results revealed that flux decline occurred in four phases: i) initial decline (0–1 d), ii) gradual decline (1–5 d), iii) progressive decline (5–10 d), and iv) rapid decline (10–15 d). Liquid Chromatography-Organic Carbon Detection (LC-OCD) analysis demonstrated that the treated effluent contained humic-like substances, which deposited on the membrane surface in phase 1. Whereas biopolymers development on the membrane surface in phase 2 and 3 was linked to biofouling. Microbial community analysis revealed that the initial colonisers were predominantly thermophilic bacteria, which were different from the microbial community of the treated effluent. The biofilm-forming bacteria included Schlegelella, Meiothermus, and Vulcaniibacterium. These microorganisms proliferate and release excessive extracellular polymeric substances (EPS), leading to the development of mature biofilm on membrane surface. This helped in the deposition of organics and inorganics from the bulk feed, which led to microbial community succession in phase 4 with the emergence of the Kallotenue genus. The results suggested that organic substances and microbial communities on membrane surface at different stages in a long-term MD process had a significant influence on MD performance for high-quality wastewater reuse.

Fertilizer-driven FO and MD integrated process for shale gas produced water treatment: Draw solution evaluation and PAC enhancement