## 1 Introduction  
Anaerobic digestion (AD) of organic waste is an attractive biotechnology, in which the microorganisms degrade the complex organic matter to simpler components under anaerobic conditions to produce biogas and fertilizer [1, 2]. After AD processes, the biogas can be upgraded to biomethane, which is considered an attractive renewable energy alternative to natural gas. Furthermore, the use of the resulting digestate for enriching agricultural soils also contributes to creating carbon sinks. AD has many environmental benefits, such as: green energy production; treatment of heavily polluted with organics wastewater, industrial and municipal waste; environmental protection; and greenhouse gas (GHG) emissions reduction. To become climate neutral by 2050, the transport sector must decrease current GHG emissions by 90%. In this context, the use of biofuels, such as bioethanol, biodiesel, and biogas, may contribute to overcoming this challenge. In the European Union, the long-term decarbonization strategy expects an increase in biomass and waste-derived energy from 140 million tons of oil equivalent (Mtoe) in 2015 to over 250 Mtoe in 2050. From them, biogas will represent about 54-71 Mtoe. Biomethane produced from waste streams via AD has currently the lowest cost (40-50 EUR/MWh) among advanced biofuels. The full use of the potential production of biogas and biomethane could cover up to 20% of the worldwide gas demand [3]. It is known that more than 95% of industrial biogas plants operate with so-called bioreactors with continuous stirring (CSTR) [4]. The biodegradation of the complex organic matter undergoes four main steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis [1,5]. Biogas consists mainly of methane (CH4) and carbon dioxide (CO2); however, hydrogen (H2) and hydrogen sulfide (H2S) also exist in low quantities. In traditional AD, H2 is not high in general (around 1% or less) and it is consumed by hydrogenotrophic bacteria immediately in the production of CH4 and CO2 [1]. During the fermentation process, energy-rich reducing compounds (i.e., NAD(P)H and FADH) are generated during metabolic reactions, followed by reoxidation through respiratory chains with a terminal electron acceptor. Thus energy-rich molecules (ATP) are formed. In aerobic respiration, an oxygen molecule is a final electron acceptor and ATP is generated with concurrent regeneration of the reducing powers. Anaerobic respiration uses a variety of organic and inorganic compounds (e.g., NO3\({}^{-}\) and SO4\({}^{2-}\), among others) as terminal electron acceptors with their concurrent reduction and regeneration of reducing powers. Glycolysis is the key metabolic pathway in which a substrate can be transformed into pyruvate, a central metabolic intermediate. Under anaerobic conditions, pyruvate enters into the acidogenic pathway coupled with H2 production, which results in the formation of volatile fatty acids (VFAs) [6]. The oxidation of organic acids such as propionic, butyric, valeric acids, and alcohols leads to acetate production. It is driven by heterotrophic microorganisms. Acetate formation by autotrophic acetogenic microorganisms (homoacetylogens) through CO2 reduction with H2 is named homoacetylogenesis. Briefly, acetogenesis produces H2 and CO2 while homoacetylogenesis consumes H2 and CO2, although both processes lead to acetate production [7]. At the same time, it has long been known that acidogenic and methanogenic microorganisms in mixed anaerobic culture in AD differ in many respects and the optimal conditions for their growth and development are different [5]. For example, optimal pH and organic load for acidogens (optimal pH 5.0-5.5) can lead to inhibition of methanogens (optimal pH 6.5-8.5). Therefore, in AD in a single BR (single-phase process), they are selected taking into account the slow-growing methanogens at the expense of fast-growing acidogens, which affect the efficiency of the whole process.