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_La. Off-gas and liquid phase monitoring were conducted at the Viikinmäki wastewater treatment plant (WWTP) over a two-week period using a novel method for simultaneous measurement of dissolved and off-gas N₂O and O₂ from the same location. K_La was calculated with three methods: empirically, based on aeration superficial velocity, from experimentally determined O₂ K_La, 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 O₂ method, using experimentally determined O₂ K_La, provided the best N₂O emission estimation over the data collection period. N₂O emissions estimated using the O₂ 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_La value, and therefore the method of K_La determination, had a significant impact on estimated emissions.