Dynamics of N2O production from activated sludge

N2O emission from activated sludge basins is the net result of microbial production and consumption of N2O. In this study semicontinuous and batch laboratory scale experiments were carried out with activated sludge from a Danish waste water treatment plant in order to elucidate important parameters in N2O production under oxic and anoxic conditions. To distinguish between N2O production from NH4+ and NO2− oxidation under oxic conditions, C2H2 and ClO3− were used as selective inhibitors. Our study showed that most of the N2O produced was reduced to N2 under anoxic conditions. Shorter N2O residence times in the sludge resulted in more N2O produced. Most of the N2O emission occurred under oxic conditions, probably because the reduction to N2 was inhibited by O2. We suggest a coupling between NH4+ oxidation and NO2− reduction (nitrifier–denitrification) as responsible for the majority of the N2O produced under oxic conditions, since NH4+ oxidation and high NO2− concentrations stimulated N2O production. Increasing temperature, NH4+ and dissolved O2 concentrations enhanced this effect.