Methods of measuring GHG fluxes at a full-scale Swedish WWTP: : A focus on nitrous oxide, methane and carbon dioxide in the SHARON treatment

Abstract: The Stable high rate ammonia removal over nitrite (SHARON) at Nykvarnsverket in Linköping is a relatively new kind of biological treatment. Fluxes of nitrous oxide (N2O) has not been fully mapped at Nykvarnsverket and additional efforts are needed for increased knowledge about possible emissions. The primary goals of the study were to measure and compare fluxes of N2O in the SHARON and to do a general greenhouse gas (GHG) flux comparison to those of the Biological treatment, the Chemical treatment and the Second denitrification at Nykvarnsverket. Secondary goals were to evaluate the use of two gas sensors, a SiC-FET sensor for N2O emissions, a CO2 Engine ELG sensor for carbon dioxide (CO2) emissions and their applicability in a WWTP environment. The measurements of GHG fluxes were performed by measuring the temporal change of GHG concentrations in the headspace of floating flux chambers placed in treatment tanks. The two gas sensors were tested either via tests in lab or via field measurements. The flux chamber method made it possible to estimate the fluxes at three out of four targeted tanks. The total daily GHG flux estimations (mmol m-2 d-1 ) in the SHARON were 6900 CO2, 320 methane (CH4) and 35 N2O. The estimations (mmol m-2 d-1) in the Biological treatment were 22 000 CO2, 120 CH4 and 23 N2O for 75% of the time. The estimations (mmol m-2 d-1) in the Chemical treatment were 110 CO2, 0.073 CH4 and 0.60 N2O. The largest N2O emissions were found to occur during nitrification processes in the SHARON. The fluxes in the SHARON were also the largest compared to those in the Biological treatment and the Chemical treatment, except for the CO2 flux that was larger in the Biological treatment. The CO2 sensor could be used during measurements over shorter time periods were CO2 levels did not exceed 10 000 ppm. Further tests on the SiC-FET sensor are needed to evaluate the sensor for measurements of N2O.