Anaerobic Digestion as an Electron Economy: An Objective-Oriented Electron Allocation Framework

Abstract Anaerobic systems are increasingly operated for objectives beyond methane production, such as volatile fatty acid production, chain elongation, biogas upgrading, and reductive treatment. Mechanistically, these systems largely share the same reaction network; operationally, the same pathway can be the target in one system and a competing pathway in another. Electron balances quantify allocation at the mechanistic level but leave its operational meaning implicit, making low performance difficult to diagnose. This study proposes an objective-oriented electron allocation framework (E-framework) that adds an operational level to electron balances by assigning roles to electron-consuming pathways, here termed electron sinks, according to the objective. Low target recovery has three possible causes, each requiring a different intervention point and approach: electrons not reaching the terminal sinks, electrons allocated to competing sinks, and an allocation opportunity limited by influent conditions. The first two are separated by decomposing target recovery into sink expression and target alignment. Across 104 literature cases from seven system types, the two indicators varied within system families, and identical target recovery arose from opposite combinations. The third is identified against a nominal benchmark defined by thermodynamic priority and electron-accepting capacity. Applied to sulfate-reducing methanogenic systems (52 cases), the benchmark showed that sulfate reduction typically received fewer electrons than allowed, and separated interventions that helped systems reach the benchmark from those that changed it. Tracking across successive analysis windows revealed whether low target recovery was transient, persistent, or reversible, indicating whether to accelerate realization, act on the single-window diagnosis, or restore operating conditions. The E-framework turns electron balances into a diagnostic basis for intervention.