###### Abstract  
Studies on harnessing solidified carbon dioxide (SCO\({}_{2}\)) for municipal sewage sludge (MSS) pre-treatment have been conducted exclusively in batch reactors. This makes it difficult to accurately assess how long-term SCO\({}_{2}\) treatment affects anaerobic digestion (AD) conditions and performance. The aim of our study was to evaluate the effect of long-term MSS pre-treatment with SCO\({}_{2}\) on AD conditions, anaerobic bacterial community, and biogas composition and yields. The presented experiments are the first studies on the effect of pre-treatment with SCO\({}_{2}\) on the efficiency of AD of MSS in continuous reactors. So far, the impact of the organic load rate (OLR) on the efficiency of MSS methane fermentation has not been assessed, which is also a novelty of the conducted research. The AD process was conducted in continuous-stirred, continuous-flow anaerobic with an active volume of 20 dm\({}^{3}\). The digestion process was run at 38 \(\pm\) 1 \({}^{\circ}\)C. The experiment was divided into two stages. Raw (non-pretreated) MSS was used in stage 1, whereas the MSS used in stage 2 was pre-treated with SCO\({}_{2}\). The SCO\({}_{2}\)/MSS ratio was 1:3. Each stage was sub-divided into four variants, with different levels of the OLR ranging from 2.0 to 5.0 \(\mathrm{\SIUnitSymbolCelsius  
AD is a well-established technology, highly optimized on the basis of extensive data from experimental research and commercial deployment [5; 6; 7]. According to many researchers and operators, the prerequisite to improving the AD process and its performance lies in implementing effective and commercially/environmentally informed methods of MSS pre-treatment [8; 9; 10]. Many such methods have been identified and tested, including hydrothermal depolymerization, ultrasonic disintegration, hydrodynamic cavitation, as well as treatments using acids, bases, strong oxidizing agents, enzymes, and other biologically active substances [11; 12; 13]. Though pre-treatments have seen substantial improvements over time, and many have been deployed in large scale, new, competitive solutions are still being sought that would combine commercial viability with environmental benefits. Studies, reviews, and experimental work conducted so far point to solid carbon dioxide (SCO\({}_{2}\)) as a promising option for MSS pre-treatment [14; 15]. The ability to harvest SCO\({}_{2}\) via the biogas conditioning/upgrading process greatly bolsters the attractiveness and potential of the technology, bringing it fully in line with material recycling, greenhouse gas reduction, climate change mitigation, and the principles of a circular economy [16; 17; 18]. SCO\({}_{2}\) can damage cell walls by way of ice crystal formation in the microbe-growing medium and in the cytoplasm [19]. This leads to the destruction of the sludge biomass and dissolution of organic matter, increasing its availability to anaerobic bacteria and improving AD performance [20]. Using SCO\({}_{2}\) for sludge disintegration has been shown to markedly increase soluble nutrient levels (orthophosphates (P-PO\({}_{4}\)), ammonia nitrogen (N-NH\({}_{4}\))) and organic content (total organic carbon (TOC), chemical oxygen demand (COD), volatile fatty acid (VFA)) [21], improve biogas output by up to 40% and more [14], increase methane production [14; 15], and provide commercial benefits if certain conditions are met [14; 20]. Research so far on harnessing SCO\({}_{2}\) for MSS pre-treatment has focused exclusively on fine-tuning and optimizing SCO\({}_{2}\) doses for the best biogas/methane yields [14; 21; 22]. All such experiments have been performed exclusively in batch reactors. The data and results from respirometric studies are limited and mainly focused on the biogas output of the tested process variants. This alone cannot give a full and accurate picture of how long-term SCO\({}_{2}\) treatment affects anaerobic digestion conditions, trends in reactor physicochemical parameters, the evolution of anaerobic bacteria composition, and performance over time [20].