# Fooling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation
## 3 Fouling in MD, OD and OMD
### Fouling in MD
#### Inorganic Fouling  
Inorganic fouling in MD is due to the precipitation and crystallization of salts present in the used aqueous feed solutions. In the case of desalination of seawater, crystallization fouling is attributed mainly to sodium chloride (NaCl), which is the predominant salt in the feed solution other than the divalent ions such as calcium or magnesium salts. It is worth noting that sodium chloride can precipitate as halite (i.e., cubic crystals of NaCl), while calcium sulfate precipitates following its hydration, in the form of anhydrite (CaSO\({}_{4}\)), hemihydrate (CaSO\({}_{4}\cdot\frac{1}{2}\)H\({}_{2}\)O) or gypsum (CaSO\({}_{4}\cdot\)2H\({}_{2}\)O) [65]. Calcium carbonate (CaCO\({}_{3}\)) precipitates as calcite (cubic crystals of CaCO\({}_{3}\)) and it is usually found in the form of rhombohedron, which is the most thermodynamically stable form of all other varieties such as aragonite (less stable than calcite) and vaterite (spherical crystals of CaCO\({}_{3}\)) [65]. It is to be noted that the concentration and temperature polarization may have a major influence on inorganic fouling in MD. For a salt whose solubility decreases with increasing temperature (e.g., CaCO\({}_{3}\), CaSO\({}_{4}\), BaSO\({}_{4}\)) the temperature polarization phenomenon induces the formation of these salt(s) crystals in the bulk feed and not on the membrane surface provided that the temperature at the membrane surface is lower. On the other hand, for salts whose solubility increases with increasing temperature (e.g., NaCl), the concentration and temperature polarization phenomena encourage the formation of crystals on the membrane surface, where the concentration is higher and temperature is lower and not in the bulk feed, where the temperature is higher and concentration is lower. Basically, the carbonic system is derived from the dissolution of carbon dioxide and carbonate minerals in water. Thus, carbonate is a weak acid-base system, which exists in aqueous solutions as dissolved carbon dioxide CO2 aq. carbonic acid (H2 CO3), bicarbonate (HCO3\({}^{-}\)), carbonate ions (CO3\({}^{2-}\)) and complexes of these ions such as CaHCO3\({}^{+}\),CaCO3 [65]. Dissolved carbon dioxide is hydrated in a few minutes according to the reaction (CaCO3 aq + H2O \(\Leftrightarrow\) CO2, H2O). The term H2 CO3 refers to the composite form, which is the sum of the activities of molecularly dissolved carbon dioxide CO2 aq, and the hydrated form CO2, H2O. In MD literature, research studies on inorganic fouling have focused on the treatment of aqueous solutions containing NaCl by VMD using hollow fiber membranes [66] or by DCMD using flat sheet membranes [67]. It was detected a limited permeate flux decline of about 30% to 35% at concentrations ranging from 15 to 300 g/L. When the salts crystallize on the membrane surface, these can cover the pores and reduce the effective area available for vapor-liquid interface reducing subsequently the permeate flux. On the other hand, another negative effect is the possible partial or total wetting of the membrane pores. He, et al. [34] did not observe any decrease of the permeate flux even for very high calcium sulfate and calcium carbonate aqueous solutions [35]. In the most unfavorable case, the decrease of the permeate flux was only 11% after 6 h of DCMD operation of a feed solution containing both calcium sulfate and calcium carbonate with the saturation index (SI) 1.21 and 49 at 75 \({}^{\circ}\)C, respectively. The treatment with HCl acid to reduce the pH value and limit the precipitation of carbonates was not necessary [35]. However, [PERSON], et al. [68] observed a high decline of the permeate flux due to the blockage of the membrane pores when wastewater was concentrated in salts up to 48.9 g/L (mainly NaCl in presence of other salts like Mg, K and Ca) [68].