## 1 Introduction  
The negative impact of human activity on the environment is associated not only with the increasing consumption of natural resources but also, to a greater extent, with the for mation of huge amounts of organic waste from agricultural and processing industries [1; 2; 3]. The problem of disposal of sewage sludge (SS) generated in municipal wastewater treatment plants (WWTP) is becoming increasingly important and amounts to up to 50% of the current operating costs of WWTP [4]. SS is formed as a byproduct of the physical, chemical and biological processes used in wastewater treatment. The currently accepted daily rate of SS formation per population equivalent (p.e.) varies from 60 to 90 g of total solids (TS), which for the EU is about 10 million tons of TS per year [4]. In the USA, Canada and Japan, about 8, 0.4 and 2.2 million tons of TS are generated annually, respectively [5]. In Russia, according to various estimates, from 2 to 4.5 million tons of TS are formed annually in municipal WWTPs [6; 7; 8; 9; 10]. To reduce the volume, improve the characteristics and reduce the potential hazard to public health, primary and secondary SS should be treated to ensure the possibility of their disposal or storage. Annually, 60 million tons of municipal solid waste (MSW) are produced in Russia as well. Organic components constitute up to 50%-60% of MSW, about half of the organic fraction is biodegradable and consists mainly of paper and food waste [11]. About 97% of the MSW produced in Russia is disposed of at landfills [11]. Among all the possible methods for the treatment and disposal of SS, such as aerobic stabilization, composting, incineration, gasification, pyrolysis, etc., the method of anaerobic digestion (AD) is one of the most preferred [12]. AD of organic fraction of MSW (OF-MSW) is a promising option to reduce the anthropogenic impact on the environment. AD process is associated with the production of biogas of higher quality than landfill gas [4], as well as biofertilizers. Anaerobic codigestion of SS and OFMSW has several benefits, including increased economic scale through increased waste, as well as increased microbial community diversity through feeding on more diverse organic waste and diluting inhibitory compounds, which helps to stabilize the digester ecosystem [13; 14; 15]. AD is a microorganism-driven process that occurs in the absence of oxygen. The AD process includes four main stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis; products formed at one stage are used as substrates at the next stage [16]. The AD process is usually carried out in one reactor, where all metabolic reactions take place [17]. According to [PERSON] [18], typical pressures in digesters range from 4 to 16 inches of water column (101.6-406.4 mm of water column; 0.001-0.004 MPa). Most microorganisms in the digester are insensitive or tolerant to pressure, which means that their growth rate is practically not inhibited at pressures up to 200 kPa [19]. The pressure sensitivity of bacteria participating in the process of anaerobic fermentation and the effect of increased pressure on methanogenic activity have not been practically studied. Nevertheless, methanogens are known from deep-water basins, where the pressure can reach 100,000 kPa [20]. After the technical feasibility of anaerobic bioconversion of organic waste in digesters operating at elevated pressure was proved, further research was aimed at increasing the efficiency of the new AD systems, namely, through increasing the organic load under elevated pressure [19]. It should be noted that when compressed gas is supplied to the digester, the concentration of dissolved carbon dioxide increases. Dissolved carbon dioxide is an electron acceptor from hydrogen in the process of hydrogenotrophic methanogenesis (the main route of methanogenesis in thermophilic fermentation). Thus, dissolved carbon dioxide reduces the partial pressure of hydrogen and thus improves the interaction between syntrophic bacteria and methanogenic archaea.