Endogenous denitrification using SBOM as a carbon source: an in-depth investigation of the rate and driving forces

Post-denitrification offers an effective means to further reduce nitrogen discharge from wastewater and mitigate the environmental risks associated with excess nitrogen release. Endogenous denitrification, which operates without the need for external carbon sources, holds promise for low-cost advanced nitrogen removal. However, its application has been constrained by a relatively slow reaction rate. In this study, an anaerobic-anoxic sequencing batch reactor (SBR) was developed to enhance endogenous denitrification using slowly biodegradable organic matter (SBOM) as the sole carbon source. Through optimization of hydraulic retention time and microbial enrichment, the activated sludge exhibited improved SBOM utilization. As the COD/total nitrogen (TN) ratio decreased from 7.5 to 4.5, the endogenous denitrification rate increased significantly from 2.78 to 8.85 mg N/(g VSS·h), achieving a total nitrogen removal efficiency of 98.51 %. The activated sludge converted SBOM into intracellularly stored compounds-such as polyhydroxyalkanoates (PHAs), glycogen (Gly), and amino acids-which, together with EPS-associated carbon sources, jointly drove denitrification under anoxic conditions. Furthermore, the enrichment of Paludibacter (32.01 % relative abundance) facilitated efficient SBOM conversion and supported intracellular carbon accumulation. The narG gene, encoding nitrate reductase from Denitratisoma, exhibited the highest expression level (43.30 %) and was identified as the key contributor to rapid nitrate reduction. Overall, the endogenous denitrification rate was enhanced by 2.2-fold, providing a viable strategy and theoretical foundation for the practical application of endogenous denitrification processes.