Insights into the addition of exogenous materials to enhance anaerobic digestion

Anaerobic digestion (AD) is a globally recognized technology for treating organic waste, providing key environmental benefits, including renewable energy generation. Over the years, significant research efforts have focused on optimizing AD under diverse conditions to enhance its feasibility and broaden its application to various substrates. Among the promising approaches for improving AD performance, using exogenous materials (EMs) has garnered considerable attention. This review presents a comprehensive overview of the recent advances in using EMs as promoters to enhance the efficiency of the AD process. The EMs were classified based on their chemical composition into seven categories, namely, carbon-based, metal-based, zeolites, natural, biological, hybrid, and miscellaneous materials. This review aims to deepen the understanding of how various promoters influence AD, focusing on their specific functions, dosage effects, underlying mechanisms, and their role in mitigating both conventional inhibitors and emerging contaminants (e.g., antibiotic resistant genes), thereby enhancing the AD process. Notably, this review explores natural and waste-derived materials as exogenous promoters in AD, an overlooked topic, highlighting their role in improving AD while promoting circular economy principles and sustainable development goals. This review highlights the economic and recovery potential of EMs in AD, offering novel insights not covered in existing literature. Finally, the review provides insights into the possibility of using exogenous materials in AD systems, highlighting the challenges and opportunities for future research.