## 1 Introduction  
Nowadays, special research focus is being placed on the concept of the environmental protection and sustainability development. In this context, the increasing interest in biogas is related to the fact that it is an alternative to fossil fuels. In particular, biogas technology may contribute to reducing both the waste generation and emission of greenhouse gases. Therefore, in recent years, a continuous increase in biogas production has been noted all over the world. Moreover, since in the energy sector of many countries biogas production is a significant option, it is envisioned that its worldwide market size will increase from USD 55.84 billion in 2022 to USD 78.8 billion by 2030 [1] (Figure 1). Biogas primarily consists of methane (CH\({}_{4}\), 50-75% by volume), carbon dioxide (CO\({}_{2}\), 25-50% by volume), and minor amounts of other gases, such as nitrogen (N\({}_{2}\)), oxygen (O\({}_{2}\)), hydrogen (H\({}_{2}\)), hydrogen sulfide (H\({}_{2}\)S) as well as water vapor (H\({}_{2}\)O)(g) [2; 3; 4]. It is a green energy carrier characterized by a calorific value between 20 and 32 MJ/m\({}^{3}\)[5] depending on the CH\({}_{4}\) content. However, it is important to note that to find various applications, the methane concentration in biogas should exceed 90% [4]. After upgrading and cleaning with the use of commercially available technologies [6; 7; 8; 9], biogas may be used for power and heat generation, transportation, as well as thermal energy generation for industrial applications. Moreover, methane-rich biogas can be used as a feedstock in material and chemical production [10]. The final biogas applications depend not only on its composition but also on the factors such as the upgrading process and national frameworks [11]. Worthy of note, the largest share of biogas is produced in Europe [12] (Figure 2), where it is mainly used for the generation of heat and electricity [6; 13]. The biogas perspectives in Europe were thoroughly discussed in the review paper [14]. Biogas is produced via anaerobic digestion (AD) process including instantaneous and continuous complex phases: hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Table 1) as a result of organic matter decomposition by various groups of synergistically acting facultative or obligatory anaerobic microbial species [15; 16; 17; 18; 19; 20; 21; 22]. The detailed characteristics of the subsequent steps in AD have been presented in many previously published studies [3; 10; 15; 20; 23; 24; 25; 26; 27; 28]. Roughly speaking, in the first phase, organic matters, mainly proteins, polysaccharides, and lipids, are hydrolyzed into simple components, such as glucose (C\({}_{6}\)H\({}_{12}\)O\({}_{6}\)), amino, and fatty acids. This phase is relatively slow and is considered a rate-limiting step in the AD process due to the complex floc structure of the sewage sludge. Key bacteria involved in the hydrolytic phase include, for instance, _Bacteriocides_, _Clostridia_, _Bifidobacteria_, _Streptococci_, and _Enterobacteria_. In the following stage, known as the fermentation phase, amino acids, lipids, and glucose are degraded into methanogenic compounds, such as hydrogen (H\({}_{2}\)), alcohols, carbon dioxide (CO\({}_{2}\)), carbon acids, and ammonia. It should be pointed out that when pH is higher than 5, the production of volatile fatty acid is favored. On the other hand, for pH lower than 5, ethanol production (CH\({}_{3}\)CH\({}_{2}\)OH) is enhanced. During this phase, bacteria such as _Streptococcus_, _Staphylococcus_, _Escherichia_, _Bacillus_, _Sarcina_, and _Desulfovibrio_ are active. During the acetogenesis, volatile fatty acids are converted into CO\({}_{2}\), H\({}_{2}\), and acetate. Importantly, it determines the biogas production efficiency since the acetate ion reduction forms about 70% of CH\({}_{4}\). Among the involved bacteria are mainly _Desulfovibrio_, _Syntrophobacter solini_, and _Syntrophomonas_. Finally, during methanogenesis, acetic acid is converted into methane and CO\({}_{2}\).