Optimization Strategies, Synergistic Effects, and Emerging Technologies for Enhanced Biogas Production Through Anaerobic co-digestion: A Review

Abstract

Anaerobic co-digestion (AcoD) has been found to be one of the most promising and technically feasible approach for maximizing the production of biogas from various organic feedstocks. AcoD allows simultaneous digestion of two or more feedstocks, thus improving the complementary nutrient profile, balancing the carbon to nitrogen (C/N) ratio, and enhancing the microbial syntropy, all of which results in biogas outputs consistently greater than mono-digestion. This review aims to analyse the state-of-the-art in anaerobic co-digestion technologies with a focus on optimization strategies, synergistic effects and new technologies. In particular, the review covers: (i) the selection criteria and characterization parameters for feedstock that are important to ensure the compatibility of the substrate; (ii) the biochemical and microbiological mechanisms that underlie synergistic interactions in co-digestion; (iii) the mixing ratio of co-substrates and its effect on methane production; (iv) the optimization parameters of the co-digestion process such as temperature, pH, hydraulic retention time (HRT), and the organic loading rate (OLR); (v) advanced pre-treatment technologies before the co-digestion process; and (vi) new digital and biotechnological innovations, such as machine learning for process control, bio electrochemical systems, and multiple-stage reactor configurations. This review systematically synthesizes peer-review findings from 51 selected articles, key findings mostly published between 2018 and 2025, including pilot-scale, bench-scale, and full-scale operations. Key findings indicate that optimum methane production can be achieved with a C/N ratio of 20:1 to 30:1, mesophilic temperature (35-38°C), and strategic pairing of co-substrates such as food waste and sewage sludge, animal manure and agricultural residues that can raise methane output by 30-120% above the mono-digestion baselines. Innovative technologies such as biochar addition, ultrasound pretreatment, and catalysts with nanoparticles have opened a new window of hope for even further enhancing the process efficiency. The review highlights the main research gaps and outlines prospective pathways to make AcoD processes sustainable and economically viable in a circular bioeconomy.