Development Directions for High-Efficiency Anaerobic Digestion Technology with High Cold Tolerance, High Solid Content, and High Stability for Sewage Sludge Methane Production

Low-carbon treatment and resource utilization of urban sludge are important pathways for sewage treatment to achieve "zero-carbon" or even "negative-carbon" goals. With increasing global attention to climate change and the announcement of China's dual-carbon goals, the exploration and application of new technologies for urban sludge treatment and resource utilization have become a critical challenge in the current sewage treatment industry. In this study, we systematically review the current research and development directions of technologies for enhancing the anaerobic digestion of sewage sludge, focusing on three aspects: high solid content, treatment in cold regions, and operational stability. Due to the low efficiency of anaerobic digestion of sewage sludge in cold regions, mainly caused by geographic and climatic factors, we discuss the effects of various pretreatment methods. These include microwave and ultrasonic treatment, acid-alkali and oxidative pretreatment, the addition of conductive materials such as biochar, and heat preservation methods, particularly the use of ground-source heat pump systems. We emphasize the impacts of calcium peroxide-coupled freeze-thaw pretreatment and the addition of biochar derived from magnetic oil sludge on the anaerobic digestion of sewage sludge in cold regions. We summarize the optimization of operating conditions, such as stirring intensity, organic loading, temperature, and C/N ratio, as well as the design of a high-efficiency, heat and mass transfer-enhanced anaerobic reactor using computational fluid dynamics (CFD) simulation to enhance the anaerobic digestion of sewage sludge with high solid content. Furthermore, we conduct an in-depth analysis of the mechanisms of anaerobic co-digestion of sewage sludge and other substrates, including food waste and agricultural residues (such as pig manure, wheat straw, and cow dung), as well as molecular weight substrate compounding technology to alleviate system inhibition caused by intermediate products (volatile fatty acids and free ammonia), thereby maintaining high stability in anaerobic digestion systems. We outline applicable technologies and index systems for different directions and clarify the mechanisms of heat and mass transfer involved in methane production within anaerobic digestion systems from multiple perspectives. Additionally, we propose future optimization directions for anaerobic digestion technology from the perspectives of microbial ecology, innovative reactor design, intelligent control systems, and integrated multi-energy systems. Finally, we discuss the prospects for the development and application of anaerobic digestion technology for sewage sludge in China, based on its specific characteristics. This study aims to serve as a reference for the future development of high-efficiency anaerobic digestion in cold regions, with a focus on high solid content and operational stability.