## 1 Introduction  
Mechanical Heat Treatment (MHT) is a relatively new and poorly investigated technology for processing municipal solid waste. MHT uses a combination of mechanical and thermal-based technologies to separate a waste stream into several component parts and enable further options for recycling and biological treatment. MHT sterilizes pathogens, deodorizes the waste stream, reduces waste mass (mainly by dehydration), compacts plastics and disintegrates labels on glass bottles, food packaging and cans. The separated utility fractions produced by MHT, including pre-residual derived fuel (pre-RDF), account for up to 80% of the initial waste mass [1]. The most common method of thermally treating municipal waste is autoclaving. This technology is used by the majority of existing European MHT plants operating on a technical scale [2]. Technologically, the MHT process consists of a grinding stage (I), autoclaving (II) and mechanical separation of material fractions (III). The largest fraction obtained during MHT is the organic remaining fraction (ORF), mechanically separated after full-scale autoclaving of unsorted municipal solid waste (Figure 1). Depending on the technological process and the composition of the waste, this fraction constitutes up to 61.5% of the initial weight of waste [3]. This fraction consists mainly of thermally processed organic waste, such as paper, kitchen and garden waste and other organics, which are turned into a very homogenous fibrous material under the influence of temperature [4]. According to [PERSON] [1], a properly conducted MHT process enables approximately 80% of the initial waste stream to be recycled/recovered. However, this level of effectiveness can only be obtained if the entire organic fraction remaining after the process of autoclaving is managed. Although autoclaving dissolves paints on cans and other packages and thus could contribute to heavy metal pollution of organic fiber [4; 5], contamination resulting from autoclaving is similar to that from the most effective mechanical-biological treatment systems [4]. The process of autoclaving disrupts the lignocellulosic structure in organic waste [6], which theoretically facilitates the decomposition of organic matter, both in aerobic and anaerobic conditions. A test performed by [PERSON] et al. [7] showed that the effect of waste sterilization during autoclaving is not permanent, and the organic remaining fraction converts to biologically unstable material that could not be landfilled without posttreatment (ca. AT4 = 25 g O\({}_{2}\) kg\({}^{-1}\) TS). The separated ORF can be also contaminated with glass and other foreign matter, which limits the production of high-quality compost [8]. For this reason, research on the aerobic treatment of the autoclaved organic fraction is being conducted with the goal of achieving aerobic stabilization, i.e., in passive aeration conditions [7]. Another test, performed by [PERSON] et al. [9], showed that stabilizer from autoclaved municipal solid waste might be a source of valuable humic substances (HS). The maximum HS production of 82-120 mg/g OM was similar to that in composts from different kinds of organic waste, which partly justifies the practical use of the autoclave process in a waste circular economy. Due to the above-mentioned problems with compost management after autoclaving, there is interest in anaerobic digestion of autoclaved organic wastes. However, research done so far has usually been carried out in batch conditions, with the use of agricultural waste or synthetically prepared organic waste. [PERSON] et al. [10] investigated the impact of the autoclaving process on anaerobic digestion of swine slurry and obtained 115% higher production of methane when the slurry was autoclaved at 120 \({}^{\circ}\)C. Similarly, when waste activated sludge was digested in a CSTR reactor, methane production was 12% and 25% higher after autoclaving at 135 \({}^{\circ}\)C and 190 \({}^{\circ}\)C, respectively, than without autoclaving [11].