Overview of Enhancing Sludge Anaerobic Digestion Through Exogenous Materials: From Non-Recyclable to Recyclable Materials

The introduction of exogenous materials offers a highly promising technical approach for enhancing the performance of sludge anaerobic digestion (AD). This review systematically presents the mechanisms and application effects of carbon-based materials, metal-based materials, inorganic mineral materials, and metal–carbon composites in enhancing sludge AD. In-depth mechanism analysis revealed that these materials primarily enhance both methane yields and system stability through multiple synergistic mechanisms, such as mediating interspecies electron and proton transfer, promoting biofilm formation as microbial carriers, and adsorbing inhibitors while enhancing the buffering capacity of the system. However, a critical analysis indicated that the inherent limitations of various materials in engineering applications, particularly in terms of stability and recyclability, are the fundamental obstacles hindering the engineering implementation of this technology. Based on the above analysis, this review further proposes that the field is shifting from a sole pursuit of efficiency enhancement to a paradigm that balances efficiency and sustainability. Developing new recyclable and circular material systems with both high efficiency and excellent recycling potential is crucial for promoting the development of sludge AD technology towards a green, low-carbon, and resource-cycling direction, and also provides key scientific and technological support for the circular economy transformation of the sludge treatment system.