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

Microalgae can synthesise the ozone depleting pollutant and greenhouse gas nitrous oxide (NO). Consequently, significant NO 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 NO emissions during centralised aerobic wastewater treatment (0.016-4.5% of the influent nitrogen load). Critically, the ability of microalgae to synthesise NO challenges the IPCC's broad view that bacterial denitrification and nitrification are the only major cause of NO emissions from wastewater plants and aquatic environments receiving nitrogen from wastewater effluents. Significant NO 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 NO synthesis, further research must urgently seek to better quantify NO emissions from microalgae-based wastewater systems and eutrophic ecosystems receiving wastewater. This future research will ultimately improve the prediction of NO emissions from wastewater treatment in national inventories and may therefore affect the prioritisation of mitigation strategies.