Nitrous oxide production under toxic conditions in a denitrifying anoxic filter

Production of nitrous oxide in a multifed anoxic filter was found during denitrification of wastewaters containing nitrite and nitrate. In the same reactor three different actions take place: Denitrification of nitrite and nitrate, degradation of formaldehyde (which is used as carbon source for denitrification) and urea hydrolysis. Nitrous oxide was detected during overloading periods being its concentration increasing in the gas phase when formaldehyde accumulates. 1.0% of nitrous oxide in the gas phase was measured when formaldehyde concentration was increased up to 1500 mg l−1 in the feeding and the loading rate from 1.5 to 6 g l−1 d−1. Formaldehyde was found to be toxic or inhibitory for denitrification, also observing a 90% urea hydrolysis inhibition at formaldehyde concentration over 300 mg l−1. The presence of nitrous oxide in the gas phase might be used for detecting overloading or anomalous working of denitrifying reactors.