# Fooling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation
###### Abstract  
Various membrane separation processes are being used for seawater desalination and treatment of wastewaters in order to deal with the worldwide water shortage problem. Different types of membranes of distinct morphologies, structures and physico-chemical characteristics are employed. Among the considered membrane technologies, membrane distillation (MD), osmotic distillation (OD) and osmotic membrane distillation (OMD) use porous and hydrophobic membranes for production of distilled water and/or concentration of wastewaters for recovery and recycling of valuable compounds. However, the efficiency of these technologies is hampered by fouling phenomena. This refers to the accumulation of organic/inorganic deposits including biological matter on the membrane surface and/or in the membrane pores. Fouling in MD, OD and OMD differs from that observed in electric and pressure-driven membrane processes such electrodialysis (ED), membrane capacitive deionization (MCD), reverse osmosis (RO), nanofiltration (NF), ultrafiltration (UF), microfiltration (MF), etc. Other than pore blockage, fouling in MD, OD and OMD increases the risk of membrane pores wetting and reduces therefore the quantity and quality of the produced water or the concentration efficiency of the process. This review deals with the observed fouling phenomena in MD, OD and OMD. It highlights different detected fouling types (organic fouling, inorganic fouling and biofouling), fouling characterization techniques as well as various methods of fouling reduction including pretreatment, membrane modification, membrane cleaning and antiscalants application. m 1 2017 7 334  
but also for the treatment of wastewaters (e.g., textile, radioactive, pharmaceutical, metallurgical, petrochemical, etc.) and concentration of heat-sensitive solutions such as fruit juices, liquid foods, natural colors and biological fluids since these processes operate at moderate temperatures and under atmospheric pressure. MD is a thermally-driven separation process, in which only vapor molecules are transported through a microporous hydrophobic membrane. The MD driving force is the transmembrane vapor pressure difference [1; 2; 3]. Various MD configurations have been considered to apply this driving force. In direct contact membrane distillation (DCMD) both the hot feed solution and the cold liquid permeate are maintained in direct contact with both sides of the membrane. This is the widely employed MD variant because of its simple design [1]. In this case volatile molecules evaporate at the hot liquid/vapor interface, cross the membrane pores in vapor phase and condense in the cold liquid/vapor interface inside the membrane module [2]. Liquid gap MD (LGMD) is another DCM variant in which a stagnant cold liquid, frequently distilled water, is maintained in the permeate side between the membrane and a cold surface [2]. The main disadvantage of DCMD and LGMD is the heat lost by conduction through the membrane [1]. If in LGMD the liquid is evacuated from the permeate side leaving a stagnant air gap between the membrane and the cold surface for the condensation of the volatile molecules, the configuration is termed air gap MD (AGMD) [2]. One of the advantages of this MD variant is the low heat transfer by conduction through the membrane from the feed to the permeate side [1]. However, in this case an additional resistance to mass transfer is built reducing the permeate flux. If in AGMD, a cold inert gas is circulated through the permeate side to carry out the produced vapor at the permeate membrane surface for condensation outside the membrane module in an external condenser(s), the configuration is called thermostatic sweeping gas MD (TSGMD) [2]. If the condensing surface is removed from the permeate side, the process is termed simply sweeping gas MD (SGMD). Therefore, TSGMD is a combination of AGMD and SGMD and it was proposed to reduce the increase of the gas temperature along the membrane module length [1].