From Single-Objective Optimization to Dynamic Decision-Making: Pathway Trade-Offs and Synergies in Anaerobic Digestion for Bioenergy and Bioproduct Production

Anaerobic digestion (AD) is evolving from a conventional waste-to-energy technology toward a multiproduct biorefinery platform capable of producing bioenergy and value-added bioproducts. However, production pathways compete for organic carbon and energy resources, and their technical, economic, and environmental performance depends on feedstock characteristics, operating conditions, downstream recovery requirements, and market demand. This narrative and critical review synthesizes methane, biohydrogen, volatile fatty acid (VFA), polyhydroxyalkanoate, single-cell protein, and digestate-derived product pathways, together with emerging options such as microbial chain elongation, biological methanation, and biogas CO2 utilization. It critically compares pathway-specific performance evidence and examines two-stage conversion, metabolic regulation, downstream integration, techno-economic analysis, life cycle assessment, multi-objective optimization, multi-criteria decision analysis, and data-driven prediction. Based on this literature synthesis, a conceptual closed-loop decision-support framework is proposed that connects monitoring, state prediction, feasible pathway generation, trade-off evaluation, preference-based selection, switching-feasibility assessment, operational adjustment, and feedback. A hypothetical numerical scenario illustrates how changes in VFA market conditions can alter pathway rankings, while technical constraints, transition costs, and biological recovery times can prevent implementation of a higher-scoring alternative. The framework is not an experimentally validated autonomous control system; its application requires plant-specific data, uncertainty analysis, and validation of transition feasibility and long-term process stability.