# Fooling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation
## 5 Methodologies for Membrane Fouling Prevention and Reduction
### Pretreatment  
Appropriate and effective pretreatment is one of the essential keys to improve MD, OD or OMD processes and minimize their fouling and scaling problems. To date, pretreatment has been claimed to be the effective method to prevent and control fouling in the MD process [15]. The degree of pretreatment depends on the type of membrane material, nature of the feed solution, water recovery level and frequency of membrane cleaning [137; 138]. Pretreatment can change the properties (chemical and/or biological) of the feed solution leading to less fouling formation and improving the permeate flux, separation factor and life-time of the membrane. Pretreatment techniques and technologies can be categorized in three types: (i) mechanical; (ii) chemical; (iii) thermal and (iv) combination of mechanical and chemical or mechanical and thermal procedures. Table 4 summarizes some pretreatments considered before carrying out MD and OD processes. Mechanical pretreatment involves physical techniques such as membrane-based filtration (NF, UF or MF) to reduce suspended solids, colloids, microorganism and organic matter present in feed aqueous solutions subjected to MD, OD or OMD treatment. It is important to reduce or eliminate these species previous MD, OD and OMD process in order to minimize membrane fouling and prevent damage of the membrane. NF and RO permeates after treatment of surface water were used as feed for MD process, the results showed that the fouling problem was reduced in the MD process [139]. In spite of NF permeate, which still contained a significant amount of carbonate that can form a fouling layer on the membrane surface, small amounts of HCI (pH = 5) added to the NF permeate permitted the elimination of this phenomenon in the MD process. When the RO permeate was employed as feed for the MD process, the problem of fouling was not detected. [PERSON], et al. [140] studied the effect of UF as a pretreatment on the subsequent concentration of grape juice by OD. UF using membranes with pore diameters of 0.1 \(\upmu\)m or less showed an important enhancement of the permeate flux compared to that observed without using UF pretreatment. Furthermore, it was found that the content of fermentable sugars, considered as glucose and fructose, of whole juice was decreased and the removal of proteins with UF resulted in an enhancement of the juice surface tension and the subsequent reduction of the risk of membrane pore wetting. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Pretreatment** & **MDOD Set Up** & **Feed Solution** & **Species Addressed** & **Observation** & **Ref.** \\ \hline  & & & & - & UF reduced the suit density index from 50 to 2 \\  & UE, NF, RO & DCMD & Surface water (black) & Organic compounds, suspended solids, colloids & - & NF removed dissolved organic carbon and the \\  & UE, NF, RO & DCMD & Surface water (black) & Organic compounds, suspended solids, colloids & - & Injection of hardness between 60\% and 87\%. \\  & & & & & - The rejection of TDs obtained in the RO system was at a level of 97\%. \\ \hline  & & Hot brine & & & Hydrophobicity of the outside membrane surface was \\  & & City water & Organic compounds, reduced. & reduced. & \\  & & & & & very limited flux reduction at salts concentration up to 19.5\% from western. \\  & & & & & 19.5\% from western. \\ \hline  & & & & & 19.5\% from western. \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\ \hline  & & & & & \\  & & & & & \\  & & & & & \\ \hline  & & & & & \\  & & & & & \\ \hline  & & & & & \\ \end{tabular}
\end{table} 
Table 4: Pretreatments considered in MD and OD processes. Recently, [PERSON], et al. [142] used filtration (i.e., 10 \(\upmu\)m pore size) followed by degassing as pretreatments to LGMD pilot plant and obtained high quality distillate. It was reported that the effect of degassing on the MD process was not conclusive, even though all the results showed that degassing had a positive effect on the permeate flux.