Nitrous oxide (N₂O) emissions from biotrickling filters used for ammonia removal at livestock facilities

Recently several manufacturers of nitrifying biotrickling filters for ammonia (NH₃) removal at animal houses have started to add a denitrification step to the installation, aiming to reduce the amount of discharge water by conversion of NH₃ to nitrogen gas (N₂). The aim of this research was to quantify the possible formation of nitrous oxide (N₂O), which is a potent greenhouse gas, in three of these farm-scale installations. Furthermore, the removal efficiency of NH₃ and odor was determined. All installations were successful in reducing the amount of discharge water. The average NH₃ removal efficiency for the three locations was 85, 71 and 86%, respectively. However, a significant part of the NH₃ removed from the inlet air was not converted to N₂ but to N₂O, which is a potent greenhouse gas. The part of the inlet NH₃-N that was converted to N₂O-N amounted to 17, 66 and 24%, respectively. The high N₂O production might have been caused by a too low scarcity of biodegradable carbon/N ratio for complete denitrification. The average odor removal efficiency was 21, 32 and 48%, respectively. Further research is necessary to explore how process conditions can be adjusted and controlled in order to reduce the production and emission of N₂O from these types of systems.