# Fooling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation
## 3 Fouling in MD, OD and OMD
### Fouling in MD
#### 3.1.3 Biofouling  
Biofouling or biological fouling refers to the growth of bacteria or micro-organisms on the membrane surface (see Figure 3) and biological particles or colloidal species that may be trapped at both the membrane surface and/or pores forming a biofilm [69]. This type of fouling has been less studied in MD compared to other membrane processes such as MF.  
It is necessary to point out that there are two types of micro-organisms: aerobic bacteria and anaerobic bacteria. For the aerobic bacteria (pseudomonas faecalis) the operating conditions such as the high temperatures applied in MD are unfavorable to its growth. However, in the case of anaerobic bacteria (_Streptococcus faecalis_) and fungi (_Aspergillus fungi_) the high temperatures applied in  
Figure 2: Cross sectional SEM images after the 4 th week of seawater exposure for (**a,b**) PTFE membrane (with 0.2 \(\upmu\)m pore size and PP nonwoven support, purchased from Thermoscientific) and (**c,d**) PVDF membrane (with 0.45 \(\upmu\)m pore size and no support, purchased from Thermoscientific). Reproduced with permission from [41], Copyright Elsevier, 2013.  
MD are favorable to their reproduction and growth. In addition, _Streptococcus faecalis_ can penetrate through the membrane pores [64; 76]. Therefore, pretreatment by NF and addition of hypochloric acid (pH = 5) completely prevented this type fouling, even after long-term operation (i.e., after 1400 h of operation [76]).  
It is worth noting that EPS (i.e., extracellular polymeric substances) excreted by the bacteria are the major structural components of biofilms and are very difficult to remove [14; 77; 78]. The biofilm structure containing EPS with amphiphilic properties induces the membrane pore wetting leakage of feed solutes to the permeate side [15].  
MD bioreactor (MDBR) consists of a MD module submerged in an aerobic-thermophilic bioreactor where the bacteria break down and consume the inorganic and organic solutes, thereby maintaining the functionality of the MD membrane module. However, due to the biomass, biofouling is a problem in MDBR as it is in conventional membrane bioreactors (MBRs). [PERSON], et al. [79] who used an MDBR with submerged flat sheet PVDF membranes for the treatment of synthetic wastewater, observed a permeate flux decline of 5.9% over 3 days of operation and 51% over 23 days. Confocal microscopy (CM) indicated a biofouling layer thickness of 2-8 mm after 7 days of operation and 20 mm after 22 days [79]. It was concluded that the thin biofouling layer did not significantly affect the resistance to heat transfer but it is resistant to mass transfer [3]. In another study, [PERSON], et al. [80] analyzed biofouling in cross-flow MD considering two sludges having different hydrophilicities. Compared to distilled water used as feed, it was observed a permeate flux decline for both sludges of 60% over 180 h. CM indicated a thickness of the biofouling layer after 180 h of 7.4-15.1 mm for the more hydrophilic sludge and 8.1-14.4 mm for the less hydrophilic one.