Evaluation of nitrous oxide reduction in solid carbon source-driven counter-diffusional biofilm denitrification system

Solid carbon-driven biofilm system can provide sufficient carbon source for denitrification, while its counter-diffusional structure could inevitably induce the delayed carbon-nitrogen contact and electron transport, further affecting carbon footprints mainly contributed by nitrous oxide (N₂O) at wastewater treatment plants (WWTPs). However, the detailed understanding of N₂O dynamics during solid-phase denitrification (SPD) has not been disclosed. In this work, a fixed bed bioreactor driven by polycaprolactone (PCL) was constructed and operated over 180 days, achieving 97 %-99 % of total nitrogen (TN) removal efficiency. Biochemical results indicated that under the condition that each nitrogen oxide (NO_x) concentration was maintained at 30 mg-N/L, the electron competition between upstream and downstream electron pools was still observed during PCL-driven denitrification even providing sufficient carbon source. For example, under the coexistent nitrate (NO₃^-)+ nitrite (NO₂^-)+N₂O condition, few electrons (i.e., 12.6 %) distributed to N₂O reductase (Nos), significantly decreasing the N₂O reduction rate (i.e., 1.42 mg/g VSS/h). Under the condition that TN concentration was maintained at 30 mg-N/L, the TN removal rate in the scheme containing NO₃^-+NO₂^-+N₂O was observed to be 1.75–2.3 times higher than that of the scheme with sole NO_x of 30 mg-N/L. This suggested that when treating wastewater containing multiple NO_x, the PCL-driven biofilm denitrification system can not only relatively improve the total nitrogen removal efficiency, but also relatively alleviate N₂O emissions. The higher abundance of Bacteroidota and Comamonadaceae ensured the stable carbon source release and nitrogen conversion states.