Inorganic carbon in anaerobic digestion: Carbonate speciation, stability interpretation, and monitoring.

Anaerobic digestion (AD) stability is commonly assessed using empirical indicators such as pH, total alkalinity, intermediate to partial alkalinity and volatile fatty acids (VFA) to total alkalinity ratios. Although operationally useful, these indicators provide limited mechanistic resolution because similar bulk values may arise from different combinations of carbonate buffering, dissolved CO2 dynamics, ammonia speciation, VFA accumulation, gas-liquid transfer, and microbial adaptation. This review examines inorganic carbon (IC)-related physicochemical states as interpretive links between carbonate speciation, gas-liquid CO2 partitioning, process stability, and monitoring. Evidence from modelling, thermodynamic, microbiological, hydrogen-assisted CO2 conversion, high-pressure, ammonia-rich, and full-scale monitoring studies shows that the IC pool, including total IC, dissolved IC, dissolved CO2, and bicarbonate, together with associated CO2 partial pressure dynamics, should not be interpreted as isolated variables. Instead, their effects depend on pH, hydrogen partial pressure, organic loading, ammonia concentration, reactor hydrodynamics, and microbial community history. IC-related chemistry can influence VFA dissociation, free ammonia formation, syntrophic oxidation, hydrogenotrophic methanogenesis, homoacetogenesis, and syntrophic acetate oxidation, but pathway shifts and recovery dynamics cannot be attributed to IC alone. Full-scale evidence further indicates that conventional indicators may detect symptoms of imbalance without resolving the underlying physicochemical and biological state. Integrating carbonate speciation with gas-phase measurements, temperature, VFA profiles, ammonia speciation, and model-assisted estimation can therefore strengthen process interpretation and support condition-specific operational decisions during gas management, bicarbonate addition, H2 dosing, and pressure-regulated operation. Future progress requires standardized IC measurements, validated soft sensors, and full-scale assessment of IC-informed monitoring under realistic operating conditions.