Internally stored carbon-driven full and partial denitrification: mechanisms and implications for wastewater treatment applications

Intracellularly stored carbon (ISC) plays a central role in enabling carbon-efficient nitrogen removal, yet its contributions to full denitrification (FdN) and partial denitrification (PdN) remain incompletely understood. This review synthesizes current knowledge of ISC anabolism and catabolism across phosphate-accumulating organisms (PAOs), glycogen-accumulating organisms (GAOs), PHA-storing organism (PSOs), glycogen-storing organism (GSOs), and oleaginous microorganisms under feast-famine, nutrient limitation, and variable redox regimes. Major ISC storage and utilization pathways are discussed alongside two conceptual models of ISC metabolism (sequential storage-growth versus parallel storage-growth) in the context of mainstream biological nutrient removal. Subsequently, the influence of wastewater characteristics, reactor configuration, feast-famine structure, dissolved oxygen control, sludge age, biomass concentration, and ISC composition on ISC-driven denitrification is analyzed. For ISC-FdN, evidence from bench- to full-scale shows that post-anoxic ISC utilization can achieve low effluent nitrogen while greatly reducing external carbon dosing and mixed-liquor recirculation. For ISC-PdN, slow ISC depolymerization, nitrate residuals, and nitrite sinks via anammox are highlighted as key kinetic drivers of denitritation suppression and/or nitrite detouring to shortcut N removal pathway, linked to the activity of taxa including Candidatus Competibacter, Candidatus Accumulibacter, and Thauera. Finally, the integration potential of ISC-driven PdN with mainstream deammonification and sludge densification is evaluated, and priority research needs are identified.