Release of nitric and nitrous oxides from denitrifying activated sludge

Activated sludge was examined in a continuously stripped batch reactor and in closed serum bottles for the production of the volatile denitrification intermediates nitric oxide (NO) and nitrous oxide (N2O). During the experiments in the reactor all compounds of the denitrification pathway were monitored and a mass balance was made. From the time profiles of the concentrations the production and consumption rates were calculated. During denitrification with nitrate (NO−3) as the first electron acceptor neither NO nor N2O is accumulated in significant amounts. An increased gas stripping shows an increased production of the volatile intermediates. Pulsing nitrate- and acetate saturated denitrifying organisms with nitrite (NO−2) causes an accumulation of NO and N2O and a severe inhibition of all enzymes of the denitrification respiratory chain. Further pulsing nitrite to microorganisms with reduced denitrification metabolism does not result in NO and N2O accumulation and no severe inhibition of denitrification is observed. Experiments in serum bottles show that the bacteriostatic denitrification intermediate NO causes the inhibition. Experiments with NO addition confirm the proposed inhibition mechanism. However, slow transport mechanisms between the bulk media and the cell surface influence the concentration of NO in the immediate environment of the denitrifying organisms. It makes a difference whether NO is brought from the outside to the cells or whether it is produced by the organism itself.