# Fooling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation
## 5 Methodologies for Membrane Fouling Prevention and Reduction
### Pretreatment
Chemical pretreatment has been focused on bacteria, hardness scale and oxidizing agents for their inhibition, destruction or reduction. It involves several methods such as coagulation, flocculation, precipitation, softening, pH control, etc. [PERSON], et al. [35] applied acidification with HCl (pH = 4.1) in order to mitigate membrane scaling in DCMD. The results showed a quite stable permeate flux during the DCMD operating time (i.e., 7 h) proving that the acidification with HCl reduced the concentration of HCO\({}_{3}\)\({}^{-}\) or CO\({}^{2}\)\({}_{3}\)\({}^{-}\) in the feed aqueous solution due to the neutralization by H\({}^{+}\) following this reaction (H\({}^{+}\) + HCO\({}_{3}\)\({}^{-}\)\(\rightarrow\) H\({}_{2}\)O + CO\({}_{2}\)). [PERSON], et al. [85] employed coagulation/floculation and MF as pre-treatment processes for the treatment of OMW by DCMD. It was found that MF was the optimum pretreatment to be integrated to DCMD for OMW compared to coagulation/floculation. MF allowed a reduction of about 30% of the total solids, whereas coagulation/floculation permitted only 23% of the total solids. Thermal pretreatment was considered to remove most bicarbonate from water, which in turn reduced the amount of precipitate formed during MD process. Thermal softening pretreatment or boiling followed by filtration was used to reduce the concentration of HCO\({}_{3}\)\({}^{-}\) ion below the level of 0.6 mmol/L (i.e., lowered 2-3 times) by keeping water at the boiling [144]. [PERSON] [33] investigated the effect of boiling and filtration of saline wastewater containing proteins to limit the intensity of fouling in DCMD. Two pretreatments were investigated: (i) sedimentation followed by UF and the obtained UF permeate was used as a feed for DCMD process, so that the results showed a significant permeate flux decrease; (ii) wastewater was boiled for 30 min and after cooling at room temperature after 12 h it was filtered through a filter paper and the result showed that TOC concentration decreased from 2780 to 1120 mg TOC/dm\({}^{3}\) while the solution turbidity was reduced from 68.1 to 14.3 NTU. This proved that the thermal pretreatment followed by filtration permitted to avoid the rapid permeate flux decline and allowed to remove proteins foulants from the feed, therefore minimizing their precipitation on the membrane surface. [PERSON], et al. [145] studied the performance and the effect of coagulation pretreatment on the efficiency of MD process for desalination of recirculating cooling water (RCW). Coagulation, filtration, acidification and degasification units were used in this study. Poly-aluminum chloride (PACI) used as coagulant was effective to remove most of the natural organic matter (NOM), antiscalant additives total phosphorus (TP) substances and suspended substance (SS) in RCW. After the coagulation and sedimentation processes, the RCW was filtered through 5 \(\upmu\)m filter and then an acidification treatment was carried out using 0.2 mol/L HCl followed by degassing in order to reduce the CO\({}_{2}\) concentration. The results indicated that when coagulation pretreatment was not employed for desalination by MD process, a rapid decline of the MD permeate flux was observed. However, about 23% improvement of the permeate flux was obtained by using coagulation pretreatment after 30 days operation. The efficiency of any pretreatment depends on many parameters such as the nature of feed solutions, their foulant(s) properties and the characteristics of the membrane. All pretreatments represent an additional economic cost since either another membrane-based filtration system must be set up or a higher energy cost (i.e., boiling temperature, thermal softening, cooling recirculation, etc.) must be added. However, the importance of a pretreatment in MD cannot be overlooked because it acts as the first strategy plan for membrane fouling prevention. Independently of the selected pretreatment, the performance of MD and OD processes is always improved when a pretreatment was carried out.