## 2 Biogas Production in AnMBRs  
### AnMBR Configurations and Operating Conditions  
Our systematic literature review indicated that investigations on biogas production in AnMBRs have been carried out at the pilot [70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82], semi-industrial/semi-pilot [83; 84], and laboratory (bench) scales [77; 85; 86; 87; 88; 89; 90; 91; 92; 93; 94; 95; 96; 97; 98; 99; 100; 101; 102; 103] (Table 1).  
Two AnMBRs configurations were used: external (side-stream) and submerged (Figure 4). In the case of the external system, the membrane filtration unit was separated from a biological reactor, while in the submerged installation, the membrane was submerged inside the reactor (or inside an external tank). Most of the above-mentioned works have been performed with the use of submerged AnMBRs. Undoubtedly, this is related to the fact that this configuration has a significantly lower energy requirement [104; 105; 106], which is crucial for industrial implementations of the technology.  
Moreover, an analysis of the collected data showed that polymeric membranes, such as polyvinylidene fluoride (PVDF), polyethersulfone (PES), polypropylene (PP), polyethylene (PE), and polyethylene terephthalate (PET), were the most commonly used. Only a few papers [87; 89; 92; 100; 103] have investigated the suitability of ceramic membranes. This can be attributed to the fact that the application of ceramic membranes leads to an increase in the cost of AnMBR technology. Indeed, although a decrease in the cost of ceramic membranes over the last 20 years has been noted [107], their fabrication is complex and they are more expensive than polymeric ones [108; 109; 110; 111]. However, ceramic membranes have several significant advantages, including high porosity and hydrophilicity, excellent separation properties, and high thermal, mechanical, and chemical stability [112; 113; 114; 115; 116; 117; 118]. Hence, additional experimental investigations are needed to determine the process performance in AnMBRs equipped with ceramic membranes.  
Generally, the digestion process can be performed at psychrophilic (<25 \({}^{\circ}\)C), mesophilic (30-40 \({}^{\circ}\)C), or thermophilic (50-60 \({}^{\circ}\)C) conditions. The studies performed in the literature have been carried out under temperatures in the range from 10 \({}^{\circ}\)C [76] to 37 \({}^{\circ}\)C [81; 84]; nevertheless, mesophilic temperatures are the most frequently applied.  
Worthy of note, in the analyzed studies, wide ranges of HRT and SRT were used. Indeed, the values of HRT, defined as a ratio of the reactor volume to the influent flow rate in time, ranged from 2.2 h [71] to 47 days [84]. In turn, SRT, which indicates the average time of the activated-sludge solids in the AnMBR, was between 20 days [72; 80] and 1000 days [99].  
The optimum pH for the methanogenesis process is between 6.8 and 7.4 [119; 120; 121]. In most of the investigations, the feed pH was kept in the above-mentioned range. For this purpose, feed neutralization with the use of chemicals such as sodium bicarbonate (NaHCO\({}_{3}\)) [77; 86; 102; 103] and sodium hydroxide (NaOH) [98] solutions was carried out.  
The process performance is described by the biogas (methane) yield, which is defined as the ratio of the total volume of biogas (methane) produced (Vgas) to the chemical oxygen demand (COD) removed (difference between the COD fed (COD\({}_{I}\)) from the reactor and the COD of the permeate (COD\({}_{\text{p}}\))) according to the following equation:  
\[\text{biogas (methane) yield }=\frac{\text{V}_{\text{gas}}}{\text{COD}_{I}- \text{COD}_{\text{p}}} \tag{11}\]  
The theoretical biogas and methane yields at standard temperature and pressure conditions are equal to 0.5 L/g COD and 0.35 L/g COD, respectively [123].