Treatment of high salinity solutions: Application of air gap membrane distillation Abdullah Alkhudhiri a , Naif Darwish b , Nidal Hilal a, c, ⁎ a Centre Water Advanced Technologies and Environmental Research (CWATER), Multidisciplinary Nanotechnology Center, School of Engineering, Swansea University, Swansea SA2 8PP, UK b Department of Chemical Engineering, American University of Shatjah, Sharjah, UAE c Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emirates abstract article info Article history: Received 11 July 2011 Received in revised form 20 August 2011 Accepted 22 August 2011 Available online 16 September 2011 Keywords: Desalination Air Gap Membrane Distillation (AGMD) High concentration solution Air Gap Membrane Distillation, using a high concentration of NaCl, MgCl 2 , Na 2 CO 3 , and Na 2 SO 4 , is implemen- ted in this study. Permeate fluxes are measured for different feed concentrations and membrane pore sizes (0.2 and 0.45 μm). The flux declines as the concentration of salt increases, and increases as the pore size in- creases. The TF200 membrane showed excellent hydrophobicity compared to TF450. Moreover, the energy consumption was measured at different salt concentrations for the different membrane sizes, and was found to be independent of membrane pore size, salt type and salt concentration in the feed solution. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Water desalination can be achieved using several techniques, such as thermal and membrane processes. Membrane distillation (MD) combines the advantages of thermal and membrane technologies, as it is considered a thermally-driven separation process [1]. Vapour molecules only are able to pass through a porous hydrophobic mem- brane, so high purity water can be extracted from aqueous solution. This separation process is driven by the vapour pressure difference existing between the porous hydrophobic membrane surfaces. MD has many attractive features, such as low operating temperatures in comparison to those encountered in conventional processes; the so- lution (mainly saline water) is not necessarily heated to the boiling point [1]. Moreover, the hydrostatic pressure encountered in MD is lower than that used in pressure-driven membrane processes like re- verse osmosis (RO). Therefore, MD is expected to be a cost-effective process, which requires less demanding membrane characteristics too. In this respect, less expensive material can be involved, such as plastic, for example, thus alleviating corrosion problems [2]. In terms of permeate collection and driving force generation [3,4], MD technology can be classified into four categories. (1) Direct Con- tact Membrane Distillation (DCMD), where the hot and cold fluid is in direct contact with the membrane surface; (2) Air Gap membrane Distillation (AGMD), where a stagnant air layer is introduced be- tween the membrane and the condensation surface; (3) Sweeping Gas Membrane Distillation (SGMD), where an inert gas is used to sweep the vapour at the permeate membrane side to condense out- side the membrane module; and (4) Vacuum Membrane Distillation VMD, where vacuum is created in the permeate membrane side using a vacuum pump. In SGMD and VMD, the condensation takes place outside the membrane module. It is worthwhile stating that AGMD and SGMD can be combined in a process called thermostatic sweeping gas membrane distillation (TSGMD) [4,5]. There are many applications of membrane distillation processes. For example, DCMD is widely employed in desalination processes, concentration of aqueous solutions in food industries [6–10], acids manufacturing [11] and heavy metal removal [12]. Moreover, AGMD is suitable for desalination [13,14] and removing volatile compounds from aqueous solutions [15–17]. Furthermore, SGMD and VMD are use- ful for removing volatile compounds from aqueous solution [14,18–20]. There have been several studies to investigate the influence of high salt concentration on the permeate flux. The effect of high salt concentration, such as in NaCl solutions, using DCMD, was reported by Martinez [21], who attributed the reduction of the permeate flux to the decrease in water activity. Also, Yun [22] found that for a highly concentrated NaCl solution there is variation in the permeate flux with time, and that it is difficult to calculate the permeate flux using existing models. It is postulated that the boundary layer solution at the membrane surface reaches saturation, so its properties become different from those of the bulk solution. Accordingly, Gekas and Hall- strom [23] suggested introducing a “Schmidt number” correction fac- tor when a high concentration polarization occurs between the bulk and the boundary layer. Safavi and Mohammadi [24], who used VMD to treat highly saline water, found that the VMD performance improves with decreasing feed concentration, and that salt rejection is not affected by feed concentration. Desalination 287 (2012) 55–60 ⁎ Corresponding author at: Centre Water Advanced Technologies and Environmental Research (CWATER), Multidisciplinary Nanotechnology Center, School of Engineering, Swansea University, Swansea SA2 8PP, UK. Tel: + 44 1792 606644. E-mail address: n.hilal@swansea.ac.uk (N. Hilal). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.08.056 Contents lists available at SciVerse ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal