Effective biological nitrogen process and nitrous oxide emission characteristics for the treatment of landfill leachate with low carbon-to-nitrogen ratio

Biological nitrogen removal via nitrite has been applied extensively to treat low chemical oxygen demand to nitrogen (COD/N) ratio wastewater, e.g. landfill leachate. It is known that landfill leachate has a very complex organic and nutrients profile. During the treatment, the transformation behavior of organic pollutants and nitrogen, as well as the side-product of nitrous oxide (N₂O) emission in this process has not been fully explored. In this study, a typical integrated anoxic-aerobic (low dissolved oxygen (0.1–1.0 mg O₂ L⁻¹)) process with sludge recirculation was employed for landfill leachate treatment. Results show that most of low molecular weight acids, building blocks and low molecular weight neutrals can be removed in the anoxic unit due to denitrification. A total nitrogen removal efficiency up to 84% was achieved with influent COD/N ratio of about 4.5, while the overall N₂O emission was 2.8 ± 0.5% of the incoming nitrogen load. It was found that N₂O emission was negligible from anoxic unit under COD/(NO₂ ⁻-N + NO₃ ⁻-N) ratio of 5.51 ± 0.83. However, reduced COD/(NO₂ ⁻-N + NO₃ ⁻-N) ratio in the anoxic unit caused enhanced N₂O emission where nitrite accumulation was also observed. When COD/(NO₂ ⁻-N + NO₃ ⁻-N) ratio in the anoxic unit was decreased to 2.66 ± 0.76 and 1.39 ± 0.19, N₂O emission in the process increased to 5.6 ± 1.8% and 9.5 ± 1.6% of incoming nitrogen load, respectively. Quantification of functional genes suggests that N₂O emission was negatively correlated to the abundance of nirS, nosZ genes and nosZ/narG ratio, highlighting the role of effective denitrification potential and more N₂O-consuming bacteria in lowering N₂O emissions. This study shed light on conversion of different fractions of organic matter in the landfill leachate treatment, and provided strategies to minimize N₂O emission.