## 2 Biogas Production in AnMBRs
### Process Yield  
#### 2.2.1 Impact of HRT  
The key factor affecting the process of biogas production is HRT. It is a well-accepted fact that microorganisms occurring in the feed that convert organic substrates into biogas require sufficient retention time. In addition, performing wastewater treatment in AnMBRs under a short HRT may significantly affect the microbial community, which is important for the performance of the anaerobic process [97; 124]. Importantly, HRT should also be selected according to the expected quality of the effluent and the costs of reactor maintenance [77]. Roughly speaking, a small-volume bioreactor is related to a short HRT and, consequently, to lower capital costs [125]. The literature reports related to the impact of the HRT on the biogas yield in the AnMBRs used for the treatment of municipal and domestic wastewater are ambiguous in  
Figure 4: Schematic diagrams of the AnMBR configurations used for biogas production: (**a**) external; and (**b**) submerged. terms of results. In studies [70; 79; 85; 91; 96; 99] it has been demonstrated that biogas production increased with the decrease in HRT. More specifically, [PERSON] et al. [91] investigated biogas production in a laboratory-scale submerged AnMBR inoculated with anaerobically digested sludge from the WWTP. The above-mentioned authors noted the average values of the biogas production rate as equal to 0.14 L/L/day; 0.28 L/L/day; 0.42 L/L/day; 0.56 L/L/day; and 0.89 L/L/day at HRT equal to 48 h; 24 h; 16 h; 12 h; and 8 h, respectively. Moreover, it has been demonstrated that the CH\({}_{4}\) content in biogas increased from 70% to about 80% with HRT decreasing from 48 h to 12-8 h. [PERSON] et al. [85] studied the effect of HRT and SRT on the treatment performance and membrane fouling in a submerged AnMBR used for the treatment of low-strength wastewater. In the above-mentioned study, the authors applied the HRT in the range from 8 h to 12 h. The highest value of the CH\({}_{4}\) yield equal to 0.25 \(\pm\) 0.041 L/g COD was obtained for an HRT of 8 h. On the other hand, the lowest value, equal to 0.138 \(\pm\) 0.031 L/g COD, was noted for an HRT of 12 h. The authors pointed out that this finding can be attributed to the fact that at shorter HRT, the biomass concentration in AnMBR is higher, which resulted in a higher methane production. These results are in good agreement with the results of [79], wherein a large pilot-scale submerged AnMBR was used. The system was firstly operated under an HRT equal to 48 h and, subsequently, HRT was gradually decreased to 24 h; 12 h; 8 h; and 6 h. For an HRT of 24 h, the biogas yield was equal to 0.16 L/g COD, while for HRT between 12 h and 6 h, it was equal to 0.25 L/g COD. In turn, investigations performed with the use of the external AnMBR were presented by [PERSON] et al. [99]. Wastewater treatment was studied for the HRT of 6 h and 12 h. For an HRT of 6 h, the CH\({}_{4}\) yield was in the range from 0.175 \(\pm\) 0.006 L/g COD to 0.386 \(\pm\) 0.035 L/g COD; meanwhile, for the HRT of 12 h, values between 0.129 \(\pm\) 0.004 L/g COD to 0.359 \(\pm\) 0.010 L/g COD were noted. Overall, the above-presented results indicate that in the initial period of the wastewater treatment process, the bioconversion rate and CH\({}_{4}\) yield are higher. Subsequently, due to substrate loss and increases in metabolite concentrations, the process efficiency decreases and more by-products are formed. As a result, the average value of gas production expressed as [L/g COD] decreases with an increase in the retention time. Interestingly, the study presented by [PERSON] et al. [70] does not support the above-discussed findings. The above-mentioned authors investigated the treatment of municipal wastewater in pilot-scale submerged AnMBRs at a low temperature (15 \({}^{\circ}\)C). It was shown that extending the HRT had a positive effect on the biogas yield.