Comparison of methods for nitrous oxide emission estimation in full-scale activated sludge

Nitrous oxide (N2O) gas transfer was studied in a full-scale process to correlate liquid phase N2O concentrations with gas phase N2O emissions and compare methods of determining the volumetric mass transfer coefficient, K(L)a. Off-gas and liquid phase monitoring were conducted at the Viikinmaki wastewater treatment plant (WWTP) over a two-week period using a novel method for simultaneous measurement of dissolved and off-gas N2O and O-2 from the same location. K(L)a was calculated with three methods: empirically, based on aeration superficial velocity, from experimentally determined O-2 K(L)a, and using a static value of best fit. The findings of this study indicated trends in local emitted N2O consistently matched trends in local dissolved N2O, but the magnitude of N2O emissions could not be accurately estimated without correction. After applying a static correction factor, the O-2 method, using experimentally determined O-2 K(L)a, provided the best N2O emission estimation over the data collection period. N2O emissions estimated using the O-2 method had a root mean square error (RMSE) of 70.5 compared against measured concentrations ranging from 3 to 1,913 ppm and a maximum 28% error. The K(L)a value, and therefore the method of K(L)a determination, had a significant impact on estimated emissions.