## 3 Fouling Mitigation Strategies
### Physio-Chemical and Chemical Cleaning  
Chemically enhanced backwashing is an essential process for the effective mitigation of the fouling phenomenon in AnMBRs. This strategy has been performed in several studies [74; 75; 76; 77; 78; 80; 82; 84; 87; 89; 95; 96] wherein both submerged and external AnMBRs have been applied. During backwashing, liquid flows from the permeate side to the feed side, leading to the removal of the matter loosely attached to the membrane surface and deposition inside its pores [140; 141]. In most of the analyzed studies, the applied backwashing was enhanced with the use of sodium hypochlorite (NaClO) and citric acid (C\({}_{6}\)H\({}_{8}\)O\({}_{7}\)). Worthy of note, in industrial installations, backwashing is performed fully automatically [137]. Chemical membrane cleaning, including soaking, has been successfully used as a fouling mitigation strategy in numerous studies [90; 91; 92; 92; 93; 94; 95; 96; 97; 98; 99; 100; 101; 102]. Our literature review indicated that among the most commonly used chemical agents are alkalis (sodium hydroxide, NaOH), oxidants (NaClO), chelates (ethylene diamine tetraacetic acid, EDTA), as well as acids (citric acid and nitric acid, HNO\({}_{3}\)) (Figure 6). As mentioned by [PERSON] et al. [142], the efficiency of the above-mentioned agents depends mainly on the following factors: agent chemical properties, membrane characteristics, and cleaning operating conditions. Furthermore, the choice of proper cleaning products requires knowledge of the feed nature [143]. Importantly, in membrane technology, a blend of various cleaning agents is often used. Figure 6: Chemical agents used for membrane cleaning in AnMBRs technology applied for the treatment of municipal and domestic wastewater. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{2}{c}{**AnMBR**} & \multicolumn{3}{c}{**Membrane Characteristics**} & \multicolumn{2}{c}{**Time**} & \multirow{2}{*}{**Ref.**} \\
**Scale** & **Configuration** & **Type** & **Material** & **Configuration** & **(s) or (h)** & \\ \hline pilot & submerged & UF & NI & hollow fiber & 5 s & [72] \\ pilot & submerged & UF & NI & hollow fiber & 10 s & [73] \\ pilot & submerged & UF & PES & flat sheet & 30 s & [74] \\ pilot & submerged & UF & PES & flat sheet & 30 s & [75] \\ pilot & submerged & UF & NI & hollow fiber & 50 s & [78] \\ pilot & submerged & MF & PVDF & hollow fiber & 60 s & [79] \\ pilot & external & UF & NI & hollow fiber & 50 s & [82] \\ bench & submerged & UF & ceramic & NI & 12 h & [87] \\ \hline \hline \end{tabular} Ni-no information, PES-polyetherswilofence, PVDF-polyvinylidene.  
\end{table} 
Table 5: Membrane relaxation time applied in AnMBRs technology as a fouling mitigation strategy. For instance, [PERSON] et al. [85] applied the soaking of PES microfiltration membrane in a NaClO solution overnight followed by thorough flushing with deionized water. NaClO is the most used oxidant employed for membrane cleaning. It allows for the removal of organic and biological foulants via oxidation and disinfection processes [68; 69]. It may degrade functional groups of natural organic matter (NOM) into ketonic, carbonyl, and aldehyde groups, leading to their hydrolysis at high pH levels [144]. Moreover, NaClO enhances the detachment of organic molecules from the membrane surface by increasing their hydrophilicity [138]. In another study [101], a PVDF ultrafiltration membrane was physically and chemically cleaned. The chemical cleaning included submerging the membrane into the NaClO solution for 2 h. Regarding ceramic membranes, [PERSON] et al. [102] demonstrated that the permeate flux can be successfully restored by the use of NaOH. Worthy of note, in the above-mentioned study, the chemical cleaning was conducted under a temperature of 70 \(\pm\) 1 \({}^{\circ}\)C. Clearly, NaOH is a well-known chemical cleaning agent which acts by both disintegrating large organic particles (e.g., colloids) into fine particles and hydrolyzing organic matters into small molecules [145].