Release of nitrous oxide (N2O) from denitrifying activated sludge: Verification and application of a mathematical model

The simple mathematical model described in v. Schulthess et al., 1994 [Wat. Sci. Technol., 30(6), 123–132] could only be used for the identification of the most important mechanism leading to nitrous oxide (N2O) accumulation in denitrifying activated sludge systems. In this report the mentioned model is extended with the consumption of organic substrate and with nitrification to predict the nitrous oxide accumulation in continuously fed full scale waste water treatment plants. In full scale waste water treatment plants the nitrous oxide emission cannot be measured directly. Therefore, a gas stripper for measuring the dissolved nitrous oxide in the anoxic and aerobic reactor was built. Four experiments with different operating conditions were performed to verify the model. In these experiments oxygen concentration as well as the nitrogen and organic substrate load were varied. As it was possible to properly describe the experiments with the extended mathematical model it could be used to calculate the N2O emission of a nitrifying/ denitrifying activated sludge system. Even under undesirable operating conditions (low oxygen concentration in the first reactor) the nitrous oxide emission is very low as compared to the total nitrogen in the inlet of the plant (<0.1%).