# Fooling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation
## 5 Methodologies for Membrane Fouling Prevention and Reduction
### Membrane Modification  
As previously mentioned, another methodology to prevent membrane fouling is to design fouling resistant membranes. This can be achieved using specific material and membrane surface modification. For instance, hydrophobicity, roughness and charge of the membrane surface are strongly related to fouling due to several interactions or adsorption/desorption between the membrane and the foulant(s). Various surface modification methods such as photochemical land redox grafting, immobilization of nanoparticles, plasma treatment, physical coating with polymers and chemical reactions on the membrane surface have been adopted in order to reduce or inhibit membrane fouling. In general, for MD, OD and OMD, membrane modification should focus on the improvement of both hydrophobicity, membrane surface omniphoicity and _LEP_ characteristics to prevent pore wetting [24; 27; 28]. In order to prepare a superhydrophobic membrane, [PERSON], et al. [152] modified a microporous PVDF membrane by depositing TiO\({}_{2}\) nanoparticles on the membrane surface using a low temperature hydrothermal. Subsequently, the TiO\({}_{2}\) coated membrane was fluorosilanized by H,1H,2H,2H-perfluorododecyltrichlorosilane. The modified membrane showed a good hydrophobicity exhibiting a water contact angle up to 166\({}^{\circ}\) compared to new membrane, 125\({}^{\circ}\). Moreover, the _LEP_ was increased from 120 kPa to 190 kPa after membrane modification. Fouling mitigation was examined in a DCMD process using HA and NaCl as a feed solution. A 20 h fouling experiment with HA did not show any reduction of the permeate flux for virgin and modified membranes but when 3.8 mM CaCl\({}_{2}\) was added in the feed solution a significant permeate flux reduction was observed due to the formation of the complexes with HA. Nevertheless, the modified membrane was less prone to fouling than the unmodified one. [PERSON], et al. [153] coated microporous PTFE membranes with sodium alginate hydrogel for OD of oily feeds. The results showed that the transmembrane mass transfer coefficient decreased by less than 5% because of the membrane surface coating and the OD permeate flux, when using 0.2, 0.4 and 0.8 wt % orange oil water mixtures as feed over a period of 300 min, indicated that the coated membranes were resistant to wetting. However, the uncoated membrane was immediately wet by 0.2 wt % orange oil water feed solution. In another study, [PERSON] and [PERSON] [154] developed an effective method to modify hydrophobic PVDF membrane to enhance anti-oil fouling property for MD applications. The PVDF flat sheet membrane surface was successfully grafted by polyethylene glycol (PEG) and TiO\({}_{2}\) particles via plasma treatment. Compared to the modified PTFE membranes by [PERSON], et al. [153], the MD experiments showed that the modified membrane presented a stable water permeate flux over 24 h of operation without oil fouling nor wetting. [PERSON], et al. [155] fabricated a superhydrophobic membrane by spraying a mixture of polydimethylsiloxane (PDMS) and hydrophobic SiO\({}_{2}\) nanoparticles on PVDF flat sheet membranes surfaces. The water contact angle and _LEP_ increased from 107\({}^{\circ}\) and 210 kPa to 156\({}^{\circ}\) and 275 kPa, respectively. The results of the DCMD experiments, which lasted 180 h using 25 wt % NaCl as a feed solution, indicated for the modified membrane a rejection factor above 99.99% with a slight decline of the permeate flux. However, the permeate flux of the unmodified membrane was decreased considerably. In addition, the SEM image indicated good fouling resistance of the modified membrane. Comparatively, [PERSON], et al. [30] showed that the hydrophobic PDMS microspheres coated on an electrospun PVDF-HFP membrane improved anti-wetting (i.e., a significant increase of the water contact angle 155.4\({}^{\circ}\)) and antifouling properties of the membrane when treating dyes aqueous solutions.