Ecological Selection of Anammox Bacteria Driven by Endogenous Carbon in a Low-Oxygen SBR Biofilm System Without External Carbon Addition

This study investigated the ecological selection and enrichment of anaerobic ammonium-oxidizing bacteria (AnAOB) driven by endogenous carbon cycling in a low-oxygen SBR biofilm system without external carbon addition. The system was operated using dried biofilm inoculation, continuous low oxygen (DO < 0.1 mg/L), and complete drainage. After 117 days, AnAOB were enriched to 8.14% relative abundance and became the dominant functional group. At an influent total nitrogen (TN) of 25 mg/L, the average effluent TN and NH4+-N were 6.37 and 3.75 mg/L, respectively, corresponding to a TN removal efficiency of 75% and meeting the Class A discharge standard. Metagenomic and metatranscriptomic analyses revealed that anammox was the primary nitrogen removal pathway, with nitrite supplied through partial nitrification and endogenous partial denitrification. Higher expression of nitrate reductase genes than of nitrite reductase genes favored nitrite accumulation through endogenous partial denitrification, thereby creating a self-sustaining internal cycle between nitrate reduction and anammox. Extracellular polymeric substances (EPS) served as the key internal carbon source driving this process. This ecological regulation strategy provides an energy-efficient and stable strategy for mainstream low C/N municipal wastewater treatment without external carbon addition.