# Fooling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation
## 3 Fouling in MD, OD and OMD
### Fouling in MD  
Compared to fouling in other membrane separation processes such as the pressure-driven membrane processes (MF, NF, RO, etc.) fouling in MD is still relatively less studied and poorly understood [33; 34; 35]. In MD, fouling can be divided into three types: organic fouling, inorganic fouling and biological fouling. In general, the foulants interact with each other and/or with the membrane surface to form deposits. This results in permeate flux decline by two phenomena, a partial or total blockage of the pores, which decreases the available evaporation area; or the formation of a fouling layer on the membrane surface leading to the appearance of a new resistance to mass transfer. As a consequence, the membrane becomes more prone to wetting, especially for long term MD operations. It is worth noting that most of the published studies on fouling phenomena in MD are for DCMD configuration. Table 1 summarizes the published papers on fouling in DCMD when using different membrane materials (polytetrafluoroethylene, PTFE, polyvinylidene fluoride, PVDF, and polypropylene, PP, polymers) and membrane type (flat-sheet, hollow fiber or capillary membranes). Very few studies have been published on fouling in other MD configurations. Table 2 listed the published papers on fouling in AGMD and VMD configurations. In order to compare the effects of the foulants, membranes, MD configurations, etc. the normalized flux decline, \(FD_{n}\), defined in the following equation was used:  
\[FD_{n}(\%)=\left(1-\frac{J_{f}}{J_{0}}\right)\times 100 \tag{1}\]  
where \(J_{0}\) and \(J_{f}\) correspond to the initial and final permeate fluxes, respectively. As it can be seen in Tables 1 and 2, the \(FD_{n}\) values varied between 0% and 95%. These values are strongly dependent on the foulant nature. In general, organic foulants induced greater normalized flux decline than the inorganic foulants. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Menance Material** & \(d_{p}\) & \(\epsilon\) (C) & \(f\) (sum) & **Foolment** & \( \begin{array}{c}T/T_{p}\\ eC\end{array} \) & \(\
u\) (ms\({}^{-1}\)) & \( \begin{array}{c}I_{p}\\ (\mathbf{log}-\mathbf{2}^{-1}\mathbf{3}-\mathbf{1})\end{array} \) & \(PD_{\mathbf{e}}\) (s) & **Observations** & **Ref. ** \\ \hline \multirow{4}{*}{0.1} & \multirow{4}{*}{-} & \multirow{4}{*}{30} & \multirow{4}{*}{\ \begin{tabular}{c} Organic fouling; \\ sludge and brown spots \\ \end{tabular}  } & \multirow{4}{*}{40-70/10} & \multirow{4}{*}{0.005-0.014} & \multirow{4}{*}{1.41-9.22} & \multirow{4}{*}{70\% at \(12\)g/L} & \multirow{4}{*}{
\begin{tabular}{c} The feed was from a thermophilic anaerobic \\ membrane bioreactor. After cleaning using \\ 50\% at \(6\)g/L \\ membrane coolant tower 50\% at \(6\)g/L \\ membrane coolant tower 50\% at \(6\)g/L \\  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \multirow{2}{*}{**MD**} & \multirow{2}{*}{**Membrane** **Material**} & \multirow{2}{*}{**\(d_{p}\)**} & \multirow{2}{*}{**\(\sigma_{p}\)**} & \multirow{2}{*}{**\(\sigma_{p}\)**} & \multirow{2}{*}{**\(\sigma_{p}\)**} & \multirow{2}{*}{**Foulent**} & \multirow{2}{*}{**\(\tau/T_{p}\)** **C**O**} & \multirow{2}{*}{**\(\tau/T_{p}\)** **C**O**} & \multirow{2}{*}{**\(\tau/T_{p}\)** **C**O**} & \multirow{2}{*}{**\(\tau/T_{p}\)** **C**O**} & \multirow{2}{*}{**\(\tau/T_{p}\)** **C**O**} & \multirow{2}{*}{**\(\tau/T_{p}\)** **C**O**} & \multirow{2}{*}{**\(\tau/T_{p}\)** **C**O**} & \multirow{2}{*}{**Observations**} & \multirow{2}{*}{**Ref. **} \\ \multicolumn{1}{c}{**Config. **} & \multirow{2}{*}{**and Type**} & \multirow{2}{*}{**(mm)**} & & & & & & & & & & \\ \hline \multirow{11}{*}{AGMD} & \multirow{4}{*}{**0.2**} & \multirow{4}{*}{1} & \multirow{4}{*}{0.85} & \multirow{4}{*}{150} & \multirow{4}{*}{Sults deposition} & \multirow{2}{*}{77/12} & \(\epsilon\cdot\) 5/8/8 & \multirow{2}{*}{119} & \multirow{2}{*}{-} & \multirow{2}{*}{Feed. tap water.}