## 4 Discussion
This application would seem to be a promising direction, given the push towards upgrading and enrichment of biogas for subsequent harvesting as biomethane and its use outside of the treatment plant [74]. However, this biogas use strategy requires alternative heat sources to provide optimum temperature parameters in digesters. Electromagnetic microwave radiation (EMR) may prove to be such a competitive heating method, given its well-documented features and benefits [75; 76]. Research to date indicates that MW can boost biogas production and quality. This was demonstrated, for example, by treating dairy effluent in a multisection horizontal flow reactor (HFAR) equipped with microwave and ultrasonic generators [77]. A conventionally heated reactor loaded with 2.0 kgCOD/dm\({}^{3}\cdot\)d produced 42.9 \(\pm\) 1.1 dm\({}^{3}\) biogas/day with 64.4 \(\pm\) 1.0% CH\({}_{4}\). Anaerobic digestion performance proved to be significantly better when the digester was heated using microwaves, with the MW variant yielding 46.1 \(\pm\) 1.1 dm\({}^{3}\) biogas/day with a CH\({}_{4}\) fraction of 66.4 \(\pm\) 1.0% [77]. Another study concluded that the heating method had no significant effect on the productivity of gas-producing anaerobic bacteria. The biogas output was approx. 450 cm\({}^{3}\)/gVS in both of the heating variants (conventional- and EMR-heated). However, the authors did note increased activity of methanogenic bacteria within the microwave-exposed zones, as evidenced by the CH\({}_{4}\) fraction in the biogas of 69% (the value was almost 4% lower for the control system) [78]. This finding is corroborated by the present study, which found that MW heating boosted the CH\({}_{4}\) fraction in the biogas under mesophilic conditions (35 \({}^{\circ}\)C) as long as the OLR was maintained within 1.0 kgCOD/dm\({}^{3}\cdot\)d to 4.0 kgCOD/dm\({}^{3}\cdot\)d. Conversely, at OLRs of 5.0 kgCOD/dm\({}^{3}\cdot\)d and 6.0 kgCOD/dm\({}^{3}\cdot\)d, the heating method had no significant effect on CH\({}_{4}\) content of the biogas. Similarly, [PERSON] et al. [79] demonstrated a positive effect of microwaves on the efficiency of organic matter biodegradation in a hybrid anaerobic reactor. The MW-heated reactor showed increased biogas output 0.360 \(\pm\) 0.006 m\({}^{3}\)/kgCOD\({}_{\rm rem. }\) and higher CH\({}_{4}\) content of the biogas at 67.7 \(\pm\) 1.9%. The reactors operated at OLR = 1.0 kgCOD/m \({}^{3}\cdot\)d and 35 \({}^{\circ}\)C [79]. The temperature was also found to shape the counts and taxonomic evolution of the methanogenic Archaea in the anaerobic biomass. In addition, there was no need to maintain thermophilic conditions during digestion, since the application of MW ensured high abundance and diversity of methanogenic Archaea in the biomass, which in turn resulted in increased production of methane-rich biogas and stable reactor performance [79]. Another study, by [PERSON] et al., aimed to determine how thermal stimulation via EMR impacts anaerobic digestion of five selected energy crop species. The highest biogas production was achieved for maize silage and Virginia mallow silage, i.e., at 680 \(\pm\) 28 dm\({}^{3}\)/kgVS and 506 \(\pm\) 16 dm\({}^{3}\)/kgVS, respectively. Exposure to MW boosted CH\({}_{4}\) production from maize silage by 18%--from 361 \(\pm\) 2 dm\({}^{3}\)/kgVS to 426 \(\pm\) 14 dm\({}^{3}\)/kgVS. The differences in CH\({}_{4}\) production were nonsignificant for the other energy crop variants [17]. The results of previous studies on the use of MW to heat bioreactors for the process of methane fermentation are presented in Table 6.  
Researchers have posited that the nonthermal properties of MW contribute to an increased rate and efficiency of pollutant absorption and biogas production via metabolic processes [82; 83]. This is supported by multiple molecular taxonomic assays of anaerobic microflora. Genetic studies have shown clear differences in the taxonomic structure of the anaerobic microbe community between conventionally heated and EMR-heated reactors [84; 85].