# Fooling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation
## 3 Fouling in MD, OD and OMD
### Fouling in OD  
Similar to DCMD, OD operates under atmospheric pressure using porous and hydrophobic membranes but under the same temperature at both the feed and permeate aqueous solutions. Fouling investigation in OD has received less attention compared to MD as it can be drawn from the number of published papers summarized in Table 3.  
Figure 3: SEM micrograph of the micro-organisms (nonfermenting gram-negative rods) in the deposit collected from the distillate tank walls. Reproduced with permission from [69], Copyright Elsevier, 2005. As stated previously, polarization phenomena exert a major influence on fouling and scaling. According to [PERSON], et al. [87], who quantified the effects of the concentration and temperature polarization in OD using glucose solutions and PVDF hollow fiber membranes, found that the polarization phenomena contributed up to 18% of the permeate flux reduction. In OD, provided that both sides of the membranes are brought in contact with the feed aqueous solution to be treated and the osmotic solution in the permeate side, polarization effects are more significant than in DCMD [87]. In a similar study, [PERSON] and [PERSON] [84] used PP membranes to concentrate an apple juice solution and observed a permeate flux decline from 2.25 to 0.9 kg/m\({}^{2}\)h. This was explained by the grip of some substances (e.g., fat and wax) to the membrane surface. Cleaning using filtered water and NaOH (0.1-1%) aqueous solution in the feed side of the OD system was carried out to recover the initial permeate flux. [PERSON] and [PERSON] [82] used Gore-tex PTFE membrane and Gelman 11104/2 TPR (a cross linked acrylic fluoroethane copolymer) to concentrate tomato pure by OD and to study membrane fouling due to the adhesion of fatty components, tomato pigments, lycopene and beta caroten to the membrane surface. Tomatoes are composed of 95% water, 3-4% carbohydrate, 0.51% protein and 0.1-0.3% fat [88,89]. It was found that the permeate flux decreased from 1.25 kg/m\({}^{2}\)h when the feed solution contained 6% of Tomato to 0.7 kg/m\({}^{2}\)h when the feed contained 21.5% of tomato. The repetitive fouling and cleaning of the membrane with either water, P3 Ultrasil 56 or 1% NaOH resulted in membrane pore wetting allowing salt leakage into the feed after only 2 to 3 cleaning runs [82]. The authors claimed that Gelman 11104/2 TPR membrane was more suitable than Gore-tex PTFE membrane for the concentration of tomato purece by OD [82]. [PERSON], et al. [81] evaluated PTFE membranes fouling in OD by comparing the measured water permeate flux before and after crude olive mill waste water (OMW) treatment. This was expressed by means of the permeate flux reduction rate (\(FR\)) defined as [81]:  
\[FR(\%)=\left(1-\frac{D_{WFo}}{D_{Wfb}}\right)\times 100 \tag{2}\]  
where \(D_{WFo}\) and \(D_{Wfb}\) are respectively the water permeate flux after and before OD of OMW treatment under the same operating conditions. It was found that \(FR\) depended on the membrane pore size (i.e., 2.94, 4.02 and 4.14% for the membranes TF200 (0.2 um pore size), TF450 (0.45 um pore size) and TF1000 (1 um pore size), respectively). However, in all cases all the used PTFE membranes showed a high fouling resistance and \(FR\) did not exceed 5%. The decrease of the permeate flux with time was more obvious for the membrane TF1000 that exhibited the highest permeate flux (i.e., the permeate flux decreased by 15%, 13% and 26% for the membranes TF200, TF450 and TF1000, respectively; after 280 min of OMW treatment by OD) [81]. [PERSON], et al. [83] applied OD for the dehydration of red grape juice of different concentrations (5-20 \({}^{\circ}\)Brix) at 35 \({}^{\circ}\)C using three PTFE membranes with different pore sizes (0.2, 0.45 and 1.2 um) and calcium chloride CaCl\({}_{2}\) solution (50 wt %) as stripping solutions.