## 3 Fouling Mitigation Strategies
### Physical Cleaning  
In AnMBRtechnology used for biogas production, the physical cleaning of membranes is carried out via biogas sparging, manual membrane cleaning (sponge sweeping), relaxation, and ultrasound. biogas sparging has been used to control the membrane fouling in submerged AnMBRs [21; 72; 73; 74; 78; 79; 80; 87; 88]. Gas bubbles may affect the properties of the cake layer formed on a membrane surface. Finally, this strategy may result in the detachment of the cake layer and reduce its thickness and compactness [29], leading to back transport of foulants to the feed. In the literature, a wide range of gas velocities have been applied (Table 4). For instance, [PERSON] et al. [73] applied continuous biogas sparging with a superficial velocity between 8 m/h and 16 m/h at the bottom of hollow-fiber UF membranes mounted in a pilot-scale AnMBR. In turn, [PERSON] et al. [74] employed gas sparging to minimize particle deposition on the surface of a polyethersulfone (PES) ultrafiltration membrane during the whole operation of a pilot-scale AnMBR. For this purpose, the gas velocity of 62 m/h was applied. In a subsequent paper, [PERSON] et al. [78] applied a constant biogas sparging intensity of 0.23 Nm\({}^{3}\)/m\({}^{2}\)h to ensure suitable shear conditions over the surface of an industrial hollow-fiber UF membrane. It should be pointed out that this strategy is energy-consuming [80]; however, it facilitates the mass transfer of CH\({}_{4}\) to a reactor headspace, which may ensure a reduction of its losses in the liquid phase [133]. Manual cleaning is a well-known method of recovering membrane permeability, usually performed ex situ [68]. Several studies [89; 90; 92; 96; 101] have employed this strategy for the fouling control in AnMBRs used for biogas production. An interesting protocol for MF membrane cleaning was proposed by [PERSON] et al. [96]. In the above-mentioned study, the membrane was gently scrubbed with a sponge before backwashing and soaking. Likewise, in another study [92], before soaking the ceramic MF membrane in chemical agents, it was manually cleaned by a sponge. In turn, in [101], before submerging the polyvinylidene difluoride (PVDF) ultrafiltration membrane in a solution of NaClO, it was manually flushed with distilled water. Relaxation, known as the intermittent cessation of filtration [134; 135; 136], is a method of membrane cleaning based on diffusive back foulant transport from a membrane surface caused by a concentration gradient [68; 137]. It has been used as an intermediate step among other membrane cleaning methods in several studies [73; 74; 75; 78; 79; 82; 87]. Our literature review indicated that in pilot-scale AnMBRs, the relaxation time is generally conducted for 5 s [72] to 60 s [79] (Table 5). For instance, in [74; 75], in the operation mode of the AnMBs, a 30 s pause was applied to relax the polyethersulfone (PES) ultrafiltration membranes. In turn, [PERSON] et al. [79] proposed 1 min for relaxing PVDF microfiltration membranes. On the other hand, in [87], wherein a bench-scale AnMBR was used, the membrane relaxation time was significantly longer (12 h). This can be explained by the fact that long-duration membrane relaxation leads to a break in the operation of AnMBRs, leading to a reduction in the economic viability of the technology. Finally, although the use of ultrasound to restore membrane performance is a relatively new strategy [138], it has already been applied in AnMBRs used for biogas production. This method aims to dissolve and displace soluble and insoluble particles [139]. Hence, it is an