## 1 Introduction
As a result, the yield of methane increases [20; 21]. Therefore, increasing CO\({}_{2}\) concentration in bulk sludge may enhance AD productivity. An increase in CO\({}_{2}\) concentration can be achieved by introducing fine gases, as well as increasing the pressure in the digester [20; 21]. Organic loading rate (OLR) is an important operating parameter that represents the bioconversion capacity of an AD system [22]. Typically, AD of waste is carried out at an OLR of 3 to 5 kg of volatile solids (VS) per 1 m\({}^{3}\) of the reactor per day (kg VS/(m\({}^{3}\)-day)) [23; 24; 25; 26; 27]. At low OLR values, the digester works ineffectively, and at high values, there is a risk of system failure due to overload [23; 24; 25; 26; 27]. It should be noted that the optimal OLR value mostly depends on the type of substrate and the TS concentration [24; 26; 28; 29; 30]. Even in modern anaerobic processes, the OLR significantly affects the performance of the digester [31; 32; 33; 34]. In [35], it was shown that the maintenance of the minimum limiting ratio of the total amount of dissolved carbon dioxide and OLR (1:40) is necessary to ensure a stable and efficient AD process. To secure this, it is necessary to reduce the outflow of gaseous carbon dioxide and increase the amount of dissolved carbon dioxide in one of the above ways. By raising the pressure in the digester to 0.15 MPa, it was possible to ensure good thermophilic AD performance at 2-3 times higher OLRs [35]. However, there are few studies on the effect of increased pressure on the performance of digesters [36]. Modern research is mainly focused on the use of high pressure in biogas technology to increase the calorific value of biogas and its further use in gas distribution networks [19; 36; 37; 38]. In [19; 33], it was compared the efficiency of a digester (anaerobic filter) operating at two different pressures of 9 bar (900 kPa) and 1.5 bar (150 kPa) with a stepwise increase in the OLR from 5 to 17.5 kg COD/(m\({}^{3}\) -day). It was found that pressure does not directly affect the stability of the AD process. However, the low pH value in bulk liquid, caused by the high partial pressure of CO\({}_{2}\), limited the performance of the digester at high OLRs. In terms of gas quality, biogas collected in the digester at 9 bar contained a higher percentage of methane. The authors noted the need to improve the stability of the digester operating under high pressure and consider it promising to use the technology of anaerobic digestion under pressure in a large-scale plant [19; 33]. However, maintaining such high overpressure values undoubtedly increases the requirements for the explosion safety of such digesters, and there are also limitations in the available technologies and facilities suitable for full-scale operation. In this work, we aimed to evaluate the possibility of using overpressure in the digester to improve the efficiency of codigestion of SS and OF-MSW using block-modular anaerobic bioreactors based on railway tanks (60-100 m\({}^{3}\)), which are widely used as digesters in the Russian Federation (Figure 1) [39]. The investigated range of overpressure in the digester was 100-200 kPa due to the maximum operating pressure in the railway tank of 250 kPa and the nominal pressure of valves and pipeline elements of 600 kPa. Figure 1: Typical for the Russian Federation block-modular anaerobic bioreactors based on railway tanks (with the permission of SelkhozBioGaz LLC) [39].