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 (N O). Consequently, significant N O 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 N O emissions during centralised aerobic wastewater treatment (0.016-4.5% of the influent nitrogen load). Critically, the ability of microalgae to synthesise N O challenges the IPCC's broad view that bacterial denitrification and nitrification are the only major cause of N O emissions from wastewater plants and aquatic environments receiving nitrogen from wastewater effluents. Significant N O 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 N O synthesis, further research must urgently seek to better quantify N O emissions from microalgae-based wastewater systems and eutrophic ecosystems receiving wastewater. This future research will ultimately improve the prediction of N O emissions from wastewater treatment in national inventories and may therefore affect the prioritisation of mitigation strategies.