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

Nitrous oxide (NO) gas transfer was studied in a full-scale process to correlate liquid phase NO concentrations with gas phase NO emissions and compare methods of determining the volumetric mass transfer coefficient, Ka. 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 NO and O from the same location. Ka was calculated with three methods: empirically, based on aeration superficial velocity, from experimentally determined O Ka, and using a static value of best fit. The findings of this study indicated trends in local emitted NO consistently matched trends in local dissolved NO, but the magnitude of NO emissions could not be accurately estimated without correction. After applying a static correction factor, the O method, using experimentally determined O Ka, provided the best NO emission estimation over the data collection period. NO emissions estimated using the O 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 Ka value, and therefore the method of Ka determination, had a significant impact on estimated emissions.