Nitros Oxide (N2O) Emissions During Wastewater Treatment in High-Rates Algae Ponds

Algae-based wastewater treatment processes are often heralded for their high sustainability. However, this view is challenged by recent demonstrations of the ability of axenic microalgae to synthesize nitrous oxide (N2O), a powerful greenhouse gas and ozone-depleting atmospheric pollutant. To determine if N2O can also be released during algae-based wastewater treatment, N2O emissions from high rate algae ponds (HRAPs) treating synthetic wastewater were quantified under various operating conditions. Low emissions rates (0-7 nmole/g TSS-h) with minor impact of the process carbon footprint were recorded under ‘normal’ operation. In contrast, considerable emissions (up to 500 nmole/g TSS-h) occurred in batch assays supplied with exogenous nitrite (12 mM). Despite evidence of denitrification and nitrification occurring when the HRAPs were operated under high loadings of nitrate and ammonium, respectively, the accumulation of nitrite in the reactors was not followed by an increase in N2O emissions. Instead, the changes in operational regimes were followed by significant changes in the rates of N2O emissions in the presence of a high concentration of exogenous nitrite. Given the high sensitivity of N2O emissions to process operation, further research is needed to improve the understanding of the mechanisms involved and monitor HRAP treating real wastewater outdoor. Refinement of impact assessment is also needed, especially to quantify the impact of N2O release of ozone depletion potential.