Invited review: Combined mitigation of methane and ammonia emissions from dairy barns through barn design, ventilation and air treatment systems.

Removal of contaminants and gases from air in cattle housing systems makes a positive contribution to in-house air quality, thereby benefiting both animal and human welfare as well as reducing the environmental pressure from cattle. In this review, we aimed to combine knowledge of the capture of ammonia and methane from dairy cattle facilities by removing and oxidizing these gases from the ventilation air in one process. For cattle housing, several techniques are currently available and in various stages of development and application to capture ammonia from the air. A central component of these approaches is an air scrubber with acid to remove the ammonia from the ventilation air flow. In this review, we focused particularly on enteric methane from ruminants, because that represents the largest methane output on the dairy farm (∼80%) and remains the biggest challenge. We sought suitable physicochemical and microbiological methods and applications that absorb or oxidize methane from the air. Our literature review showed that the concentration of methane in modern, naturally ventilated, open cattle barns is relatively low (averages of 5 to 100 ppm), and at present, no cost- and climate-effective technology is available that can capture and oxidize methane at this low concentration. It was apparent that most techniques may only be able to capture or convert methane at concentrations above 500 ppm (and preferably >2,000 ppm). The limiting factor is the diffusion speed of methane and the competition with other gases in case filtering or adsorption techniques are used. Therefore, we formulated the ventilation challenge, which refers to smart ventilation techniques that would allow capturing methane from the barn air at higher concentrations. In addition, technologies that have the capacity to capture ammonia and methane from the air are described separately and in combination. Then, inventive designs are presented to picture capturing of ammonia and methane in a one-process approach. Applying selected innovations, promising results are expected in lowering methane emissions from barns and storage areas (∼one-fourth reduction at the farm level). The development of more efficient adsorbents and bio and soil filters to optimize the process of adsorption and oxidation at low concentrate levels would enhance this reduction.