Abstract
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 (N(2)O) at wastewater treatment plants (WWTPs). However, the detailed understanding of N(2)O 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 (NO(3) (-))+ nitrite (NO(2) (-))+N(2)O condition, few electrons (i.e., 12.6 %) distributed to N(2)O reductase (Nos), significantly decreasing the N(2)O 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 NO(3) (-)+NO(2) (-)+N(2)O 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 N(2)O emissions. The higher abundance of Bacteroidota and Comamonadaceae ensured the stable carbon source release and nitrogen conversion states.