Ground-based remote sensing of CH and NO fluxes from a wastewater treatment plant and nearby biogas production with discoveries of unexpected sources.

This study is an attempt to assess CH and NO emissions from all the treatment steps of a wastewater treatment plant (WWTP) in Sweden, serving 145 000 persons, and an adjacent biogas production facility. We have used novel mid-IR ground-based remote sensing with a hyperspectral camera to visualize and quantify the emissions on 21 days during a year, with resulting yearly fluxes of 90.4 ± 4.3 tonne CH/yr and 10.9 ± 1.3 tonne NO/yr for the entire plant. The most highly emitting CH source was found to be sludge storage, which is seldom included in literature as in-situ methods are not suitable for measuring emissions extended over large surfaces, still contributing 90 % to the total CH emission in our case. The dominating NO source was found to be a Stable High rate Ammonia Removal Over Nitrite reactor, contributing 89 % to the total NO emissions. We also discovered several unexpected CH sources. Incomplete flaring of CH gave fluxes of at least 30 kg CH/min, corresponding to plume concentrations of 2.5 %. Such highly episodic fluxes could double the plant-wide yearly emissions if they occur 2 days per year. From a distance of 250 m we found a leak in the biogas production facility, corresponding to 1.1 % of the CH produced, and that loading of organic material onto trucks from a biofertilizer storage tank contributed with high emissions during loading events. These results indicate that WWTP emissions globally may have been grossly underestimated and that it is essential to have effective methods that can measure all types of fluxes, and discover new potential sources, in order to make adequate priorities and to take effective actions to mitigate greenhouse gas emissions from WWTPs.