Nitrogen removal pathways during simultaneous nitrification, denitrification, and phosphorus removal under low temperature and dissolved oxygen conditions

Nitrogen removal pathways of simultaneous nitrification, denitrification, and phosphorus removal (SNDPR) at low dissolved oxygen (0.3 mg/L) and temperature (10 degrees C) were explored to understand nitrogen removal mechanisms. Biological nitrogen and phosphorus removal was sustained with total inorganic nitrogen removal, phosphorus removal, and simultaneous nitrification and denitrification (SND) efficiencies of 62.6%, 97.3%, and 31.2%, respectively. The SND was observed in the first 2 h of the aerobic phase and was attributed to denitrifying ordinary heterotrophic organisms using readily biodegradable chemical oxygen demand and denitrifying phosphorus accumulating organisms (DPAOs), which removed 15.1% and 12.2% of influent nitrogen, respectively. A phosphorus accumulating organism (PAO)-rich community was indicated by stoichiometric ratios and supported by 16S rRNA gene analysis, with Dechloromonas, Zoogloea, and Paracoccus as DPAOs, and Ca. Accumulibacter and Tetrasphaera as PAOs. Even though Ca. Competibacter (10.4%) was detected, limited denitrifying glycogen accumulating organism denitrification was observed.