Assessing nitrous oxide emissions from algal-bacterial photobioreactors devoted to biogas upgrading and digestate...

Nitrous oxide (N₂O) emissions in High Rate Algal Ponds (HRAP) can negatively affect the sustainability of algal-bacterial processes. N₂O emissions from a pilot HRAP devoted to biogas upgrading and digestate treatment were herein monitored for 73 days. The influence of the pH (7.5, 8.5, and 9.5), nitrogen sources (100 mg L^-1 of N-NO₂^-, N-NO₃^-, and N-NH₄⁺) and illumination on N₂O emissions from the algal-bacterial biomass of the HRAP was also assessed in batch tests. Significantly higher N₂O gas concentrations of 311.8 ± 101.1 ppm_v were recorded in the dark compared to the illuminated period (236.9 ± 82.6 ppm_v) in the HRAP. The batch tests revealed that the highest N₂O emission rates (49.4 mmol g^-1 TSS·h^-1) occurred at pH 8.5 in the presence of 100 mg N-NO₂^-/L under dark conditions. This study revealed significant N₂O emissions in HRAPs during darkness.