Enhanced nitrogen removal in partial nitrification-anammox (PNA) suspended sludge system for real municipal wastewater treatment at extremely low carbon to nitrogen ratio

The chemical oxygen demand (COD) to total inorganic nitrogen ratios (C/N) fluctuations under mainstream conditions has a crucial influence on the performance of partial nitrification-anammox (PNA) systems. In this study, a PNA sequencing batch reactor was applied for real municipal wastewater treatment with a continuous decrease in C/N (4.3 to 1.4). The results showed that with the decrease of C/N, the nitrite accumulation ratio (NAR) decreased from 94.1% to 60.7%, and nitrogen removal by endogenous denitrification was attenuated. However, the nitrogen removal efficiency (NRE) significantly increased from 43.8% to 91.5%, which was attributed to the enhanced performance of PNA under aerobic conditions and additional nitrite provided by endogenous partial denitrification (EPD) for anammox under anoxic conditions. Glycogen accumulating or-ganisms (GAOs) that dominated the EPD process increased remarkably from 0.42% to 2.78% exhibiting a competitive advantage over phosphorus accumulating organisms (PAOs) at low organic loads. Co-existence of highly enriched anammox bacteria (0.91%) and GAOs (2.78%) ensured the enhanced nitrogen removal during the aerobic and anoxic phases. This study provides in-depth perspectives on the PNA combing with EPD under varied C/N and broadening the flexibility of PNA applications.