Effect of the carbon source on N2O emissions during biological denitrification

During biological denitrification in Waste Water Treatment Plants (WWTPs), many parameters (chemical, physical and biological) are responsible for greenhouse gas emissions such as nitrous oxide (N2O) and nitric oxide (NO). The present study intends to investigate the impact of the carbon source more specifically on N2O emissions, but also on NO emissions. The experiments were done in a bioreactor performing batch denitrification at a laboratory scale. Three sources of carbon were tested: ethanol and acetate as short carbon chain compounds and a mixture composed of ethanol and acetate and two long carbon chain compounds: casein extract and meat extract. The nitrogen source was always nitrates (NO3−) and the ratio COD/N was set to three. Current nitrite and nitrate ions, nitric and nitrous oxide levels were monitored during experimentation. The results principally show that the acetate carbon source generates the highest N2O and NO emissions (74% and 19% of denitrified N-NO3−, respectively). The results of this work suggest that the type and length of the carbon source used are responsible for nitrogen emissions but not in the expected way. While the literature always focuses on the inhibitory effect of nitrites on N2O emissions, this work has singled out that NO may also exert inhibitory effects on the N2O reductase enzyme. These results may be explained by the diversity of denitrifying bacteria and their distinct metabolic pathways towards the added carbon substrates (influents).