Enhancing Methane Yield in Anaerobic Co-Digestion of Sewage Sludge and Other Organic Wastes: Linking Feedstock Synergy, Engineering Design, and Carbon Performance

Anaerobic co-digestion (AcoD) is increasingly applied in sewage-sludge management and organic-waste treatment because it can improve methane recovery, stabilize mixed substrates, and reduce life-cycle greenhouse-gas emissions under appropriate feedstock and operating conditions. However, existing reviews still focus mainly on feedstock types or isolated enhancement measures and less often connect synergistic mechanisms with engineering implementation and carbon outcomes. The specific novelty of this review is to connect functional feedstock classification, mechanism boundaries, engineering controls, and carbon-performance evaluation within one sludge-centered AcoD framework. This review synthesizes recent progress in AcoD of sewage sludge, food waste, livestock manure, crop residues, and industrial organic streams through a chain from feedstock traits to substrate interactions, microbial responses, engineering performance, and carbon benefits. Feedstocks are reorganized by function rather than by waste name, highlighting how carbon-to-nitrogen contrast, buffering capacity, hydrolysis recalcitrance, and inhibitor profiles jointly define synergy potential. Key mechanisms, including C/N balancing, hydrolysis complementarity, inhibitor mitigation, and direct interspecies electron transfer (DIET), are discussed together with their applicability limits. Representative evidence shows methane-yield or methane-production increases of about 41–55% for selected food-waste–manure blends, approximately 45% for rice–straw–pig manure systems after cellulolytic pretreatment, and approximately 16–55% for selected additive strategies; these values are illustrative rather than directly comparable because the underlying studies differ in substrates, baselines, reactor configurations, pretreatment conditions, and operating parameters. The review then translates mechanism into practice through pretreatment, reactor-selection templates, operating windows, additive reinforcement, and artificial-intelligence-assisted monitoring. Representative cases and life-cycle evidence indicate that AcoD can improve methane productivity while lowering greenhouse-gas emissions relative to landfill or mono-digestion pathways when energy substitution and nutrient recycling are credibly counted. Remaining bottlenecks include incomplete kinetic integration, limited DIET quantification, insufficient reporting of quantitative operating ranges and additive dosages, and weak coupling of carbon, economics, and regional feedstock dynamics. The revised review therefore treats AcoD as a sludge-centered mechanism-to-engineering framework and highlights two transferability gaps that require stronger standardization: biodegradation/toxicity testing and local co-substrate logistics.