# Fooling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation
## 3 Fouling in MD, OD and OMD
### Fouling in MD
#### 3.1.1 Organic Fouling  
Organic fouling results from the deposition of natural organic matter (NOM) on the membrane surface like carboxylic acid, humic acid (HA), alginate acid (AA), proteins, polysaccharides, etc. The principal constituents of NOM are the humic substances, which are found in surface water, ground water and seawater, followed by carbohydrates (including polysaccharides), protein and a variety of acidic and low molecular weight (LMW) species [40; 60; 61]. This fouling is dependent on several factors including the surface characteristics of the membrane. For instance, greater hydrophobicity and lower pore size tends to increase fouling effects [31]. However, 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). Figure 1 shows the adsorption-desorption mechanism for HA migration through a membrane pore. The process involves the adsorption of HA onto membrane surface, hydrogen bonding between water and HA and weakening of hydrogen bond as water vapor moves through the membrane, and re-adsorption of HA onto the membrane as well as pore wetting. [PERSON] and [PERSON] [62] investigated the effect of NOM properties on NF membrane fouling by fractionating NOM into hydrophilic and hydrophobic components. Fouling tests revealed that  
Figure 1: Illustration of the adsorption–desorption mechanism for humic acid (HA) migration through a membrane pore. The process involves (**a**) adsorption of HA onto membrane surface; (**b**) hydrogen bonding between water and HA; (**c,d**) weakening of hydrogen bond as water vapor moves through the membrane; (**e**) re- adsorption of HA onto the membrane and (**f**) the pore wetting phenomenon. the hydrophobic fraction of NOM was mostly responsible for permeate flux decline, whereas the hydrophilic fraction caused a lesser fouling. [PERSON], et al. [11; 31] performed DCMD treatment by HA solutions (10 to 50 mg\(\cdot\)L\({}^{-1}\)) and the obtained results showed a permeate flux reduction less than 8% after 30 h of operation for the commercial PTFE and PVDF membranes. Other studies used HA solutions with concentrations ranging from 20 to 100 mg\(\cdot\)L\({}^{-1}\) and observed a very limited fouling with a permeate flux decline less than 6% [42]. The reversibility or cleaning of organic fouling was also investigated but the drawn conclusions varied because of the possible variety of organic fouling such as organic or organometallic complex (i.e., compound containing a metal atom bonded to an organic group). [PERSON], et al. [40] studied organic fouling behavior in DCMD using synthetic model solutions of HA, AA and bovine serum albumin (BSA). BSA and HA showed a dominant fouling with a permeate flux decline of 50%, whereas AA showed minimal fouling due to its hydrophilic nature [40]. When comparing BSA and HA feed solutions, higher deposits were detected for BSA feed solution (i.e., 35.2% higher carbon mass); and the analysis of fouled membrane proved the penetration of organic compounds through the membrane pores when using HA feed solution [40]. It was demonstrated that the aggregation of the HA and the increase of membrane fouling is favored with the addition of multivalent cations and the increase of electrolyte (NaCl) concentration in the feed aqueous solution. Multivalent cations including Calcium (Ca\({}^{2+}\)) are known to form complexes with NOM and interact specifically with humic carboxyl functional groups reducing the humic charge and the electrostatic repulsion forces between humic macromolecules resulting in aggregates in NF or MF processes [60; 63]. Some studies showed the irreversibility of organic fouling in DCMD [11; 31].