Effect of Nitrite and Temperature on Autotrophic Denitrification in Anammox Granular Biomass from a Partial Nitritation–Anammox Reactor

Anaerobic ammonium oxidation (anammox) is a key process in the removal of nitrogen from wastewater, in which episodes of substrate inhibition may occur. In this study, the effect of nitrite on anammox in the short and long term was investigated using granules from a full-scale SBR reactor in operation. In the short term, maximum activity was achieved at 100 mg N-NO2−/L, with higher concentrations being inhibitory. It was determined that the biomass behavior is well interpreted (R2 = 0.955) by a non-competitive substrate inhibition model (Andrews model), with a KS of 55.6 mg N-NO2−/L and a KI of 116.7 mg N-NO2−/L, and also well interpreted by the Edwards model (R2 = 0.957), with a KS of 36 mg N-NO2−/L and a KI of 287 mg N-NO2−/L. In the long term, the biomass retained its anammox activity at 15 mg N-NO2−/L over a three TRH horizon; however, at 30 mg N-NO2−/L, anammox activity decreased by 50% at the end of the experiment. Finally, the effect of temperature on the activity of the anammox granules from a different source was studied, revealing that the activity increases with temperature within the range of 25–35 °C, which can be useful if a rapid increase in activity is desired. Operationally, maintaining nitrite below 30 mg N-NO2−/L ensures stability, while exceeding 100 mg N-NO2−/L causes immediate SAA inhibition and slower recovery.