Nitrous oxide emissions from high rate algal ponds treating domestic wastewater

• Under normal operation, HRAP treating wastewater released <2 nmol N2O/g TSS h.
• These N2O emissions (⩽0.0047% N input) generated a low carbon footprint.
• External supply of nitrite significantly boosted N2O production in darkness.
• Ammonium overloading was associated with N2O emissions up to 11 nmol/g TSS h.
• C. vulgaris was the most likely significant N2O producer in the HRAP microcosms.

This study investigated the generation of N2O by microcosms withdrawn from 7-L high rate algal ponds (HRAPs) inoculated with Chlorella vulgaris and treating synthetic wastewater. Although HRAPs microcosms demonstrated the ability to generate algal-mediated N2O when nitrite was externally supplied under darkness in batch assays, negligible N2O emissions rates were consistently recorded in the absence of nitrite during 3.5-month monitoring under ‘normal’ operation. Thereafter, HRAP A and HRAP B were overloaded with nitrate and ammonium, respectively, in an attempt to stimulate N2O emissions via nitrite in situ accumulation. Significant N2O production (up to 5685 ± 363 nmol N2O/g TSS h) was only recorded from HRAP B microcosms externally supplied with nitrite in darkness. Although confirmation under full-scale outdoors conditions is needed, this study provides the first evidence that the ability of microalgae to synthesize N2O does not affect the environmental performance of wastewater treatment in HRAPs.

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