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Evaluation of nitrous oxide reduction in solid carbon source-driven counter-diffusional biofilm denitrification system
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Researchers evaluated nitrous oxide reduction in a solid carbon source-driven counter-diffusional biofilm denitrification system used in wastewater treatment, finding that the structural separation of carbon and nitrogen in these biofilms increases nitrous oxide emissions — a potent greenhouse gas — and identifying operational strategies to mitigate this carbon footprint.
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.
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