Seasonal and diurnal variations of GHG emissions measured continuously at the Viikinmäki underground WWTP

The gaseous emissions of an underground activated sludge treatment plant with total nitrogen removal have been measured on-line since 2012. The continuous Fourier transform infrared (FT-IR) measurement of CO2, NO, NO2, N2O, NH3 and CH4 is situated in the single exhaust air pipe, thus covering the whole wastewater treatment process. The measurement data from 2015 showed that the seasonal and diurnal variations of N2O and CH4 are considerable. When comparing N2O and CH4 emissions as CO2 equivalents to CO2 formed in the activated sludge process, the impact of N2O was dominant. The CH4 emission was considerably
smaller, and the emissions of NOx and NH3 from biological treatment were small. The long term variations of N2O or CH4 production were not linked directly to load variations or other “apparent” factors such as temperature, nitrogen load, nitrogen reduction or anoxic volume. There was neither a clear seasonal pattern. The short interval variations of N2O were similar to but not identical with CO2 variations.