Nitrous oxide emissions from denitrifying activated sludge of urban wastewater treatment plants, under anoxia and low oxygenation

Abstract

Batch experiments were made to better understand the mechanisms of N 2O emissions from activated sludge in denitrifying conditions found in urban WWTPs, i.e. under anoxic and low oxygenation conditions. The results showed that in completely anoxic conditions, denitrification, related to a periplasmic nitrate reductase activity, is the major producer of N 2O (100% of the N 2O production), whereas the nitrate ammonifying activity is not significant. In a gradient of low oxygenation, the highest N 2O emissions (49.7 ± 3.8 μg N 2O–N/g SS/h on average) occurred at a dissolved-oxygen concentration of around 0.3 mg O 2/L. Below 0.3 mg O 2/L, heterotrophic denitrification appeared to be the major process responsible for the N 2O emission (100% at zero oxygenation). From 0.4 to 1.1 mg O 2/L, N 2O emissions were due to two processes: (i) heterotrophic denitrification that represented about 40% of the N 2O production, and (ii) autotrophic nitrifier denitrification that accounted for about 60%. The N 2O emissions from activated sludge represented on average 0.4% of reduced NO 3 - in anoxic conditions. The N 2O emissions associated with denitrification of entire nitrogen load would amount to 155 T N 2O–N/year, if all the Paris wastewater was treated by a process using activated sludge.