# Fooling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation
## 3 Fouling in MD, OD and OMD
### Fouling in MD
#### Inorganic Fouling
The effect of the salts crystallization on pore wetting was detected by [PERSON] [33,69] who found a significant wetting of pores in the presence of CaCO3 crystals when treating water containing organic matter (TDS = 409-430 mg\(\cdot\)L\({}^{-1}\), TOC = 6.8-8.5 mg\(\cdot\)L\({}^{-1}\)). This phenomenon led to a decrease of the permeate flux and an increase of the salts concentration of the permeate. Moreover, the precipitation of calcium carbonate may lead to the degassing of CO2 that eventually is transported through the membrane pores to the permeate according to the following reactions: 2 HCO3\({}^{-}\Leftrightarrow\)H2O + CO2\(\uparrow\) + CO3\({}^{2-}\); CO3\({}^{2-}\) + Ca2\({}^{+}\Rightarrow\) CaCO3\(\downarrow\); 2 HCO3\({}^{-}\) + Ca2\({}^{\pm}\) + H2O + CO2\(\uparrow\) + CaCO3\(\downarrow\)[35]. When salts are precipitated, these can be removed by a simple circulation of water tangentially to the membrane [34]. [PERSON], et al. [35] cleaned the membranes by washing first with HCl (e.g., elimination of CaCO3 by reducing the pH) then with pure water (e.g., elimination of salts by dissolution) followed by membrane drying. However, the mixture of different salts can form a more compact agglomerate difficult to be detached from the membrane [35]. [PERSON], et al. [70] and [PERSON], et al. [71] showed the evolution of the permeate flux with the precipitation of a mixture of Na2 SO4 and NaCl salts. The first step is the gradual decline of the permeate flux until reaching the saturation point in which the reduction of is more significant; then a new regime corresponding to low permeate flux occurs when the membrane is almost completely covered by the salts [70,71]. [PERSON], et al. [41] investigated the effects of membrane scaling (i.e., salt deposition) on the properties of two commercial hydrophobic membranes (i.e., PTFE and PVDF) [41]. It was proved that hydrophobic PVDF and PTFE membranes were not immune to fouling by salt deposition but behaved differently against it [41]. All the used characterization techniques to detect salt deposition showed significant scaling (i.e., presence of NaCl, MgSiO3, MgSO and CaCO3) starting from the first week of seawater desalination. Figure 2 shows the cross sectional SEM images of the PTFE and PVDF membranes after the 4 th week of seawater exposure. It was found that the thickness of the salt layer deposited on the membrane surface was about 7 \(\upmu\)m for the PTFE membrane having a thickness of 50 \(\upmu\)m (Figure 2a) and about 4 \(\upmu\)m for the PVDF membrane having a thickness of 23 \(\upmu\)m. Similarly, the PVDF membrane with 200 \(\upmu\)m thickness showed a salt layer near 15 \(\upmu\)m thick (Figure 2b) while the PVDF membrane having 125 \(\upmu\)m thickness had about 10 \(\upmu\)m thick salt layer [41]. In order to further understand the effects of salt deposition on MD parameters, the mechanical strength, pore size distribution and gas permeability of the membranes were evaluated before and after inorganic fouling. It was proved that inorganic fouling not only altered the membrane's properties but also the MD performance (i.e., permeate flux and salt rejection) [41]. In order to prevent inorganic fouling, desalination can be carried out using a hybrid process, namely membrane crystallization, that combines both crystallization of salts and MD [70; 72; 73; 74; 75]. In this hybrid process crystallization of salts is carried out in a separate tank and the formed crystals are eliminated leading to a conversion rate very close to 100% [73].