Pathways to mitigating nitrous oxide emissions throughout the nutrient wastewater circular economy

Nitrous oxide (N 2 O) is one of the world’s most potent greenhouse gases (GHG) originating from microbial metabolic interactions during nitrification and denitrification processes. Despite the significance of this gas from wastewater treatment plants (WWTPs), the efforts of GHG reduction are lacking in this domain. Combined with the need to recover critical nutrients for fertilisers, combat climate change, stabilise food prices from fertiliser price shocks, and improve financial channels for WWTP processes - the nutrient circular economy (CE) is proposed as a method for mitigating and sequestering GHG emissions. This paper proposes a variety of pathways to combat N 2 O emissions. To effectively mitigate N 2 O emissions, proper monitoring methods such as the use of machine learning (ML), geospatial systems to map emissions, sharing of data for robust modelling, and sensor accuracy improvements are needed. Mitigation strategies include embedding respiring microorganisms, biochar inductions, use of recovered struvite as fertiliser, enriching denitrifying microorganisms, introducing beneficial microbes during wastewater recovery, and promoting the use of slow-release mechanisms in CE fertilisers. Controls should be taken to mitigate downstream emissions occurring at the soil-application stages via appropriate fertiliser compositions and modified farming practices, which remains the single largest source of emission. Estimations for N 2 O emissions will continue to be challenging due to the heterogenous nature of wastewater, different plant processes, and varying presence and concentrations of diverse microorganisms. • Benefits of several N 2 O reduction pathways are discussed. • Major emission sources come from storage and soil nitrification-denitrification. • Opportunities for machine learning and soil analysis to drive down N 2 O emissions. • Fertilisers and soil amenders play an important role in N 2 O mitigation. • Adopting struvite and slow-release fertilisers are promising steps forward.