Abstract
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.