Nitrous oxide emissions during microalgae-based wastewater treatment: current state of the art and implication for...

Microalgae can synthesise the ozone depleting pollutant and greenhouse gas nitrous oxide (N₂O). Consequently, significant N₂O emissions have been recorded during real wastewater treatment in high rate algal ponds (HRAPs). While data scarcity and variability prevent meaningful assessment, the magnitude reported (0.13-0.57% of the influent nitrogen load) is within the range reported by the Intergovernmental Panel on Climate Change (IPCC) for direct N₂O emissions during centralised aerobic wastewater treatment (0.016-4.5% of the influent nitrogen load). Critically, the ability of microalgae to synthesise N₂O challenges the IPCC's broad view that bacterial denitrification and nitrification are the only major cause of N₂O emissions from wastewater plants and aquatic environments receiving nitrogen from wastewater effluents. Significant N₂O emissions have indeed been repeatedly detected from eutrophic water bodies and wastewater discharge contributes to eutrophication via the release of nitrogen and phosphorus. Considering the complex interplays between nitrogen and phosphorus supply, microalgal growth, and microalgal N₂O synthesis, further research must urgently seek to better quantify N₂O emissions from microalgae-based wastewater systems and eutrophic ecosystems receiving wastewater. This future research will ultimately improve the prediction of N₂O emissions from wastewater treatment in national inventories and may therefore affect the prioritisation of mitigation strategies.