###### Abstract  
The most common technology for the recovery of energy and valuable materials from sewage sludge is anaerobic digestion (AD). Ensuring thermophilic conditions during AD has been proven to cause process intensification and an improvement in its final outcomes. Nonetheless, the search is underway for other methods to bolster the effectiveness of the AD of aerobic granular sludge (AGS), which is characterized by a compact and complex structure. A prospective AGS pre-treatment technology entails the use of solidified carbon dioxide (SCO\({}_{2}\)). The present study focused on an evaluation of the AGS pre-treatment with SCO\({}_{2}\) on the thermophilic AD technological effects. It evaluated the effect of the SCO\({}_{2}\) pre-treatment method on changes in the concentrations of organic and biogenic compounds in the dissolved phase and the yield and kinetics of biogas and methane production in periodical reactors, as well as enabled the development of an empirical organizational model of biogas production. SCO\({}_{2}\) introduced to AGS caused an increase in the content of COD, N-NH\({}_{4}\)+, and P-PO\({}_{4}\)3- in the AGS dissolved phase at SCO\({}_{2}\)/AGS volumetric ratios ranging from 0 to 0.3. A further increase in the SCO\({}_{2}\) dose did not cause any statistically significant differences in this respect. The highest biogas and methane yields were obtained at SCO\({}_{2}\)/AGS of 0.3 and reached 482 \(\pm\) 21 cm\({}^{3}\)/gVS and 337 \(\pm\) 14 cm\({}^{3}\)/gVS, respectively. The higher SCO\({}_{2}\) doses used led to a significant decrease in the pH value of the AGS, which, in turn, contributed to a decreasing CH\({}_{4}\) concentration in the biogas. Footnote 3: Accepted: 3 July 2023  
The most common technology for the recovery of energy and valuable materials from sewage sludge is anaerobic digestion (AD) [7]. It is a well-known and optimized method, as indicated by a vast number of closed digesters exploited in the technical scale. It is noteworthy, however, that despite high technological advancement and the immense diversity of the available solutions for sewage sludge AD, the search is still underway for solutions ensuring AD intensification [8]. The main focus of interest among scientists, technologists, and operators of such bio-energetic systems is enabling the undisturbed exploitation of digesters and ensuring high technological effectiveness of AD at a high organic load rate (OLR) and short hydraulic retention time (HRT) [9]. Optimization works conducted in this respect would enable bio-methane production and the stabilization of significantly greater volumes of sewage sludge in small-cubature digesters, which, in turn, would directly contribute to an increased energy gain [10; 11]. Increasing the temperature of methane fermentation and conducting it under thermophilic conditions are well-known technological means that ensure expected outcomes [12; 13], like, e.g., an increased rate of biochemical reactions, improved effectiveness of the biodegradation of organic substances and the mineralization of sewage sludge, increased effectiveness of biogas and methane production, significantly shortened HRT and increased OLR, and also partial hygienization of fermented substrates [14]. The process weaknesses relate to the sensitivity of anaerobic microflora to slight fluctuations in technological parameters and changes in environmental conditions, which necessitates the use of substrates with a stable quantitative and qualitative composition [15]. The implementation of thermophilic fermentation is facilitated by the possibility of using the usually sparingly manageable and often lost waste heat generated in commonly applied co-generation systems [16]. The continuous search for, and the observed evolution of, AD technology is associated primarily with the increasingly common co-digestion of sewage sludge with other organic waste in biorefinery systems, as well as new wastewater treatment technologies, resulting in modified characteristics of the sewage sludge produced [17; 18].