Insights into microbial removal of ammonia and nitrous oxide from biological air purification reactors used for livestock farming: a review

A large portion of ammonia (NH₃) originates from the agricultural sector, and intensive livestock farming is one of the fundamental causes of the airborne nitrogen (N) load. Treatment of NH₃ is therefore crucial for the better management of livestock farming. Biological air purification technology is one of the best options for mitigating NH₃ emissions from livestock farms. Inorganic nitrogen often accumulates in the filtering material and water inside the air purification reactor if they are not washed out because of requirements for wastewater reduction. They usually need to be removed and subsequently treated elsewhere to avoid inhibition of nitrification of free ammonia and free nitrous acid in drain water. Furthermore, nitrous oxide (N₂O) is emitted from the air purification reactor as a result of microbial N transformations. Because these concerns are all related to N cycles driven by microorganisms in the purification reactor, proper operations require informed understanding of microbiology related to NH₃ removal. The relationship between key operational parameters and the dynamics of microbial communities relative to nitrification, denitrification, and N₂O emissions from a biological air purification system have not been updated. Current knowledge of the mechanisms of NH₃ removal, the N cycle, and the complex network of microbes involved in N transformations that are affected by operational conditions in such a system is reviewed, with special emphasis on practices at livestock farms. Biodegradation might be a limiting factor, even if the load is lower than the critical inlet load at full-scale operation. Although biological air purification reactors are constantly aerated, a certain amount of N could be lost because of denitrification. After treatment of livestock air, up to 40% of the input NH₃-N can be transferred to N₂O. The operational parameters affecting N₂O emissions are packing material moisture content, pH, and nitrite concentration. The functional microbial community might change if it adapts to the surrounding environment in full-scale operation. Apart from water control, the effects of different properties of the packing material on the growth and activities of microbes have to be clearly elucidated to provide optimum conditions for microorganisms to both remove NH₃ and reduce N₂O emissions.