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

[Display omitted] • Advanced nitrogen removal was achieved despite the gradual decrease in C/N. • NRE increased from 43.8% to 91.5% while NAR decreased from 94.1% to 60.7% • Nitrogen removal aerobically by the PNA pathway was enhanced as C/N decreased. • Nitrite supplied by EPD (by GAOs) for anammox compensated the nitrite deficiency. • Enhanced aerobic and anoxic nitrogen removal by enriched AnAOB ensured superior NRE. 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 organisms (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. [ABSTRACT FROM AUTHOR]