# Fooling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation
###### Abstract
Another way to establish the necessary driving force in MD is by means of a vacuum pump connected to the permeate side of the membrane module. This configuration is called vacuum MD (VMD) [2]. In this case the applied vacuum pressure must be lower than the saturation pressure of the volatile molecules to be separated from the feed solution and the condensation takes place outside the membrane module. It is necessary to point out that MD was applied principally in desalination for the production of high purity water. Other fields of applications have also been considered at laboratory scale such as the treatment of textile wastewater, olive mill wastewater, humic acid (HA) and radioactive aqueous solutions, etc. [2]. Contrary to DCMD, osmotic distillation (OD) is an isothermal technology used to remove water from aqueous solutions [4] (i.e., concentration of wastewaters and recovery of valuable components) [5] using a porous and hydrophobic membrane that separates the feed solution to be treated and an osmotic solution (i.e., a draw solution having high osmotic pressure and low water chemical potential) [6]. Generally NaCl, CaCl\({}_{2}\), MgCl\({}_{2}\), MgSO\({}_{4}\), K\({}_{2}\)HPO\({}_{4}\) KH\({}_{2}\)PO\({}_{4}\) and some organic liquids like glycerol or polyglycols were considered to prepared the osmotic solution [7]. One of the advantages of the OD process is the less energy required compared to MD [8]. It was generally applied for concentrating liquid foods, such as milk, fruit and vegetable juices [5]. OMD is a combination of DCMD and OD being the driving force both the transmembrane temperature and concentration or which is the same water chemical potential. It is a non-isothermal process in which the membrane is brought into contact with the hot feed aqueous solution to be treated and a cold osmotic solution [9]. OMD is also able to concentrate liquid foods (i.e., fruit and vegetable juices), sucrose aqueous solutions [10]. The three separation processes MD, OD and OMD require the use of porous and hydrophobic membranes and, like other membrane technologies, they also suffer from different fouling phenomena that reduce not only the permeability and separation performance of the membrane but its lifetime as a consequence. The foulants (e.g., natural organic matter, NOM [11]; inorganic and biological solutes or microbial contaminants) contribute to the permeate flux decline, modify the membrane surface properties and change the product water quality. Analysis of fouling process and identification of foulants by means of characterization techniques are important to determine suitable treatment methods for fouling control. It is worth noting that very few studies have been published so far on fouling mechanisms in MD, OD and OMD and investigations on the kinetics behind fouling phenomena and fouling mitigation remain very scarce [12; 13]. Two review papers have been published on fouling and scaling in MD but not on OD and OMD [14; 15]. Moreover, when fouling is studied, the considered characterization techniques focused only on the average physico-chemical properties of the surface deposits but not on the underlying deposit layers [12]. One of the causes of porous and hydrophobic membrane fouling is pore blockage due to the deposit of foulant(s) such as organics, inorganic or minerals, colloids, microbial contaminants and particles not only on the membrane surface but also inside the membrane pores affecting the hydrophobic character of the membrane and its wettability [16; 17]. Fouling in MD, OD and OMD is a complex phenomenon influenced by various parameters such as the membrane characteristics, especially the pore size and the material of the membrane surface, operation conditions and nature of feed aqueous solutions.