Achieving endogenous partial denitrification by cultivating denitrifying glycogen-accumulating organisms

Although anaerobic ammonia oxidation (anammox) is considered a promising process due to its high efficiency and low energy in nitrogen removal, nitrite inadequacy was one of the bottlenecks for the application of anammox. However, endogenous partial denitrification (EPD) has been emerging as a stable pathway to provide nitrite for anammox. Furthermore, denitrifying glycogen-accumulating organisms (DGAOs) are believed to be associated with EPD. In this study, firstly, GAOs were gradually enriched in a sequencing batch reactor (SBR) with the dual strategy of influent phosphorus limitation and withdrawal after the anaerobic stage. DGAOs were successfully induced by adding sodium nitrate solution at the end of the anaerobic stage, resulting in NO₃^--N concentration increasing from 15 to 30 mg/L. During a typical SBR cycle, DGAOs contributed up to 96% of the conversion of intracellular carbon sources and up to around 95% of nitrate reduction during the anoxic stage. The maximum nitrate-to-nitrite transformation ratio (NTR) of the system reached 80%. Microbial community analysis demonstrated that the <i>Ca.</i> Compatibactors were the dominant functional bacteria for EPD, with a relative abundance of 31.12%. However, the relative abundance of phosphorous-accumulating organisms (PAOs) was only 1.02%. This study reveals the important role of DGAOs in the EPD process, which can provide a stable nitrite for anammox.