The effect of free ammonia on ammonium removal and N₂O production in a consortium of microalgae and partial nitritation cultures

The coupling of microalgae and partial nitritation (PN) cultures was a promising technology to treat ammonium-rich sidestream wastewater. However, limited information is available on nitrogen transformation pathways, N₂O production mechanisms and their association with free ammonia (FA) in this symbiotic system. Enhanced ammonium removal (up to 100%) by the consortium was achieved mainly through algal assimilation (3.2–21.9%) and ammonia oxidation (33.2–72.7%) under autotrophic conditions, while bacterial assimilation (11.6–45.4%) had an important contribution under mixotrophic conditions as it was the least inhibited by FA. Higher FA concentration at the range of 6.53–45.70 mg/L would lead to an increase of N₂O accumulation from 0.56 to 1.09 mg N/L, but a decrease of N₂O emission factor from 0.6 to 0.3% in the mixotrophic consortium. At each FA level (6.53–58.76 mg/L), mixotrophic algal-bacterial consortium contributed to significant reductions of N₂O accumulation by 60.5–83.8% and of N₂O emission factors by 60.5–92.7% compared to the single PN cultures, with optimal mitigation scenario achieved at FA level of 45.70 mg/L. Herein, heterotrophic denitrification was deemed as the major sink for N₂O production, while microalgal assimilation played an important role in limiting N₂O emission factors as it restrained the nitrogen distribution to nitritation and denitritation. This might imply the potential of the consortium of microalgae and PN cultures in achieving higher ammonium removal while reducing greenhouse gas emission during sidestream wastewater treatment.