Membrane distillation is a process that utilizes differences in vapor pressure to permeate water through a macro-porous membrane and reject other non-volatile constituents present in the influent water. This review considers the fundamental heat and mass transfer processes in membrane distillation, recent advances in membrane technology, module configurations, and the applications and economics of membrane distillation, and identifies areas that may lead to technological improvements in membrane distillation as well as the application characteristics required for commercial deployment. Keywordsa function of temperature, vapor pressure, and of the gas molecular mass K 0 membrane characteristic defined by Equation (9) Kn Knudsen number K(T) a function of temperature and molecular weight of the gas l mean free path of the molecules l m distance between parallel spacer fibres (m) LEP Limit Entry Pressure (kPa) M molecular mass (g/mol) M w molecular weights of water (g/mol) M a molecular weights of air (g/mol) n number of CNTs per unit cross section in bucky-paper P pressure in the air gap (kPa) half time to reach the maximum intensity-laser flash technique (s) t proportion of conductive heat (balance due to evaporative heat) loss through the membrane T mean temperature in the pores (K)

Membrane distillation (MD) is a rapidly emerging water treatment technology; however, membrane pore wetting is a primary barrier to widespread industrial use of MD. The primary causes of membrane wetting are exceedance of liquid entry pressure and membrane fouling. Developments in membrane design and the use of pretreatment have provided significant advancement toward wetting prevention in membrane distillation, but further progress is needed. In this study, a broad review is carried out on wetting incidence in membrane distillation processes. Based on this perspective, the study describes the wetting mechanisms, wetting causes, and wetting detection methods, as well as hydrophobicity measurements of MD membranes. This review discusses current understanding and areas for future investigation on the influence of operating conditions, MD configuration, and membrane non-wettability characteristics on wetting phenomena. Additionally, the review highlights mathematical wetting models and several approaches to wetting control, such as membrane fabrication and modification, as well as techniques for membrane restoration in MD. The literature shows that inorganic scaling and organic fouling are the main causes of membrane wetting. The regeneration of wetting MD membranes is found to be challenging and the obtained results are usually not favorable. Several pretreatment processes are found to inhibit membrane wetting by removing the wetting agents from the feed solution. Various advanced membrane designs are considered to bring membrane surface non-wettability to the states of superhydrophobicity and superomniphobicity; however, these methods commonly demand complex fabrication processes or high-specialized equipment. Recharging air in the feed to maintain protective air layers on the membrane surface has proven to be very effective to prevent wetting, but such techniques are immature and in need of significant research on design, optimization, and pilot-scale studies.

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In this paper, the performance of various membranes were assessed in direct contact membrane distillation (DCMD) under different feed velocities and inlet temperatures. The membranes studied included a polyvinylidenefluoride (PVDF) microfiltration membrane with a non-woven support layer, a polytetrafluoroethylene (PTFE) microfiltration membrane with a non-woven support layer, and three MD membranes made from PTFE of different pore size and all with a structured scrim support layer. The results showed that distillation using PTFE membranes produced much higher flux than that of the PVDF microfiltration membrane at the same operational conditions, and the support layer affected not only the flux, but also the energy efficiency (0.51 -0.24). The results also show that increasing the velocity of the feed and its inlet temperature increased the flux, but the rate of flux increase diminishes at high velocities. The mass transfer coefficient improved for thinner support and active layer membranes, leading to fluxes as high as 46 L.m -2 h -1 at 80˚C. The heat transfer characteristics were found to be superior for the open scrim backed membranes compared to the non-woven support membranes, resulting in significantly greater thermal efficiency for the scrim backed membranes.

Carbon nanotubes (CNTs) are nanoscale cylinders of graphene with exceptional properties such as high mechanical strength, high aspect ratio and large specific surface area. To exploit these properties for membranes, macroscopic structures need to be designed with controlled porosity and pore size. This manuscript reviews recent progress on two such structures: (i) CNT Bucky-papers, a non-woven, paper like structure of randomly entangled CNTs, and (ii) isoporous CNT membranes, where the hollow CNT interior acts as a membrane pore. The construction of these two types of membranes will be discussed, characterization and permeance results compared, and some promising applications presented.

A major challenge in silica membranes for gas separation is to maintain a robust pore structure in the presence of steam. In this work, use of a carbonized template is proposed to reduce damage to the pore structure by inhibiting silica migration along the membrane pore surface. This departs from the conventional wisdom of creating hydrophobic surfaces to achieve hydrostability. The carbonized‐template molecular sieve silica (CTMSS) membranes can then be applied to clean‐energy systems such as hydrogen separation and carbon dioxide sequestration, and membrane reactors where steam is present.

The economics membrane distillation (MD) and common seawater desalination methods including multi effect distillation (MED), multistage flash (MSF) and reverse osmosis (RO) are compared. MD also has the opportunity to enhance RO recovery, demonstrated experimentally on RO concentrate from groundwater. MD concentrated RO brine to 361,000 mg/L total dissolved solids, an order of magnitude more saline than typical seawater, validating this potential. On a reference 30,000 m 3 /day plant, MD has similar economics with other thermal desalination techniques, but RO is more cost effective. With the inclusion of a carbon tax of $23 per tonne carbon in Australia, RO remained the economically favourable process. However, when heat comes at a cost equivalent of 10% of the value of the steam needed for MD and MED, under a carbon tax regime, the cost of MD reduces to $0.66/m 3 which is cheaper than RO and MED. The favour to MD was due to lower material cost. On low thermally, high electrically efficient installations MD can desalinate water from low temperature (<50°C) heat sources at a cost of $0.57/m 3 . Our assessment has found that generally, MD opportunities occur when heat is available at low cost, while extended recovery of RO brine is also viable.

Self-supporting carbon nanotube (CNT) Bucky-Papers have unique structural and surface properties which can be utilised in many applications. In this work we characterised pure selfsupporting CNT membranes, where CNTs were held together only by Van der Waals forces, and evaluated their potential and performance in direct contact membrane distillation. The membranes were found to be highly hydrophobic (contact angle of 113°), highly porous (90%), and to exhibit a thermal conductivity of 2.7 kW/m 2 •h. We demonstrate, as a proof of concept, that self-supporting CNT Bucky-Paper membranes can be used for desalination in a direct contact membrane distillation setup with 99% salt rejection and a flux rate of ~12 kg/m 2 *h at a water vapour partial pressure difference of 22.7kPa. Ageing of the membranes by delamination, factor limiting their performance, is also reported but work is currently done to address this issue by investigating composite material structures.