## 3 Results and Discussion
### Microbial Compositions in Anaerobic Digester
The class Bacilli was mainly represented by Lactobacillus, the abundance of which in all samples was about 3%. Lactobacillus utilize sugars as a carbon source and produce either mainly lactate (homofermentation) or lactate, acetate, and ethanol (heterofermentation). Lactobacillus sp. is often used in biological pre-treatment or as an additive in the anaerobic digestion of complex substrates [86; 87]; therefore, its natural presence in the anaerobic digestion community is favorable for stable methane production. The abundance of the phylum Chloroflexi was the highest in V1 (10.3%); then, increased OLR markedly reduced its abundance to 2.4% in V3 and 1.7% in V5 (Figure 6). Chloroflexi grow during stable anaerobic digestion performed at high HRT [88] and are responsible for the fermentation of hydrocarbons and amino acids. In the present study, Chloroflexi was also observed in the digester with the high HRT. The phylum Chloroflexi was dominated by the class Anaerobic. [PERSON] and co-workers [89] conducted anaerobic digestion of oil-related substrates and observed that Anaerobic was the most dominant group of bacteria during all experiments. Among Proteobacteria, the most abundant were Deltaproteinobacteria (7.3% in V1, 3% in V3, and 9.1% in V5), Gammaproteobacteria (1.6% in V1, 1.3% in V3, and 0.7% in V5) and Betaproteobacteria (1.1% in V1, 0.4% in V3, and 1.4% in V5) (Figure 7). The phylum Proteobacteria was commonly found in digesters treating dairy wastewater [77; 90; 91]. The abundance of 3.7% of Actinobacteria was the highest in V3, followed by 3% in V1, and the lowest, 1.7%, in V5 (Figure 6). This phylum is involved in the degradation of complex carbohydrates and proteins [92]. [PERSON] and co-workers [93] pointed out that Actinobacteria and Bacteroidetes were growth promoters of syntrophic bacteria and acetoclastic archaea. The abundance of Synergistates represents the same tendency as Actinobacteria (3% in V1, 3.9% in V3, and 1.3% in V5) (Figure 6). This phylum was dominated by Synergistia, to which belong syntrophic acido- and acetogens involved in amino acid fermentation [94]. Probably due to interspecies hydrogen transfer between Synergistia and methanogens, [PERSON] and co-workers [95] observed that the abundance of Synergistales highly correlated with the rate of methane production from oil during the anaerobic digestion for treatment of synthetic lipid-rich wastewater. As mentioned at the beginning of this section, Thermotogae was observed mainly in V5 (constituted almost 7%). Members of this phylum were observed in mesophilic anaerobic reactors [96] where they were responsible for complex polysaccharide fermentation and hydrogen production [97], which might promote interactions with hydrogenotrophic methanogens. [PERSON] and co-workers [98] observed a high abundance of Thermotogae in a reactor at pH 5.7, which is in consensus with the present study, in which they were observed at pH 6.49. The community of anaerobic granular sludge in all variants of experiments was represented by various hydrolytic and fermentative bacteria. Based on the microbial composition, it might be assumed that the methane was mainly produced in acetoclastic methanogenesis; however, the increased presence of Methanomicrobia and Thermotogae at the highest OLR suggests that this condition enhanced hydrogenotrophic methanogenesis. The study revealed classes that were sensitive to increasing OLR, which were Flavibacteriia, Anaerobic, Nitrospira, Methanobacteria, and Gammaproteobacteria, and classes that preferred increasing OLR, which were Clostrridia, Methanomicrobia, and Thermotogae.