# Fooling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation
## 6 Conclusions and Future Perspectives  
Membrane fouling is a common problem and complex phenomena in all processes used for seawater desalination and waste water treatment applications. For MD, OD and OMD, fouling leads to membrane pore wetting and blocking. Compared to other membrane separation processes such as the pressure-driven membrane processes (MF, UF, RO, etc. ), very few research 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 and poorly understood. 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. In MD, OD and OMD, fouling is 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 the fee aqueous solutions. As it is well known, MD, OD and OMD membrane technologies suffer from the temperature and concentration polarization phenomena and various strategies have been adopted in order to reduce their effects including the increase of the flow rate of both the feed and permeate solutions, turbulent promoters, etc. These polarization phenomena can have a major influence on inorganic fouling. For salts like CaCO\({}_{3}\) and CaCO\({}_{4}\) whose solubility in water decreases with increasing temperature, the temperature polarization phenomenon encourages the crystals formation of these salts in the bulk feed solution. However, for salts like NaCl whose solubility in water increases with increasing temperature, the concentration and temperature polarization phenomena encourage the crystals formation of these salts on the membrane surface where the temperature is lower and the concentration is higher. Greater hydrophobicity and lower pore size tend to increase organic fouling effects. This type of fouling also depends on the nature of the organic matter, the MD operating conditions (temperature, transmembrane pressure, flow rate) and the characteristics of the feed solution (pH, ionic strength). Biofouling refers to the growth of bacteria or micro-organisms on the membrane surface and to biological particles that may be trapped at both the membrane surface and/or pores forming a biofilm. This type of fouling is the least studied in MD, OD and OMD compared to the other types of fouling. Fouling investigation in OD has received less attention compared to MD and fouling in OMD may take place following the same mechanisms as those occurred in DCMD and OD fouling, provided that OMD combines both MD and OD. Many research studies using different membrane characterization techniques still need to be done in order to understand the formation mechanisms of the different fouling types. Some characterization techniques reported in the present review paper (TEM, UTDR, EEM and FFF) are yet to be applied to analyze fouled membranes used in MD, OD and OMD processes. No analytical technique can be used on its own to characterize the fouling deposit on the membrane surface. A combination of different techniques seems to be more appropriate. Feed pretreatment, membrane modification, use of antiscalants and cleaning strategies of membrane surfaces are the most used methods to diminish or prevent foulant(s) deposition onto the membrane surface and in its pores during MD, OD or OMD applications. Up to now, pretreatment has been the appropriate method to prevent and minimize fouling in MD, OD and OMD processes. Antiscalants inhibit the inorganic fouling (i.e., scaling) and minimize the potential for forming scale layer on the membrane surface. Condensed polyphosphates, organophosphonates and polyelectrolytes are the common antiscalants used in desalination [140]. Attention should be paid to some antiscalants that are nutrients for microbes, algae and bacteria increasing therefore the risk of microbiological contamination.