Quantification of methane emissions from outdoor manure storage tanks using the tracer dispersion method.
Authors: Thygesen Vechi Nathalia , Scheutz Charlotte 
Published: March 2024

Methane (CH4) emissions from outdoor manure storage tanks, from cattle and pig production, are complex and difficult to predict, therefore, the development of methods to monitor these emissions is needed. The mobile tracer gas dispersion method (TDM) has been used to quantify CH4 emissions from entire facilities and can be used to discriminate emissions from different operations within a farm, although challenged by road limitations. To increase the use and flexibility of the TDM in quantification of CH4 manure tank emissions, an alternative is to instead of measure concentrations using a mobile platform, the CH4 and tracer gas concentrations can be sampled by using stationary sampling points. In this method (stationary TDM), a few parameters need to be examined to decrease the error in the emission quantification, for example, by considering the position of the sampling points within in the measured concentration plume. In comparison, by following methods&#8217; best practices, the stationary TDM produced results like the mobile TDM, with relative errors of approximately 5 and 7%, respectively (Vechi &amp; Scheutz, 2023).&#160;The mobile and stationary TDM methods were further used to quantify CH4 emissions from outdoor manure tanks at pig and cattle farms and identify the factors affecting these emissions. Quantifications (6 to 14 measurements per tank) were done over several months, covering the entire year. In total, eight tanks were investigated, two of them stored cattle manure and six stored pig manure. The manure tanks measured emissions varied from 0.01 to 14.3 kg h&#8722;1, which when normalized by the amount of manure stored corresponded to a range of 0.01 to 11.0 g m&#8722;3 h&#8722;1. In a yearly average, cattle farm manure tanks emitted 0.63 &#177; 0.09 g m&#8722;3 h&#8722;1, while pig emissions were higher, averaging 1.56 &#177; 0.93 g m&#8722;3 h&#8722;1 (Vechi et al., 2023). Seasonal variation patterns were clear and similar among the different tanks, with emissions peaking between July to September and lower during winter and spring. The manure temperature was a significant factor correlated to the CH4 emission fluctuations, followed by type of manure stored (cattle or pig) and tank cover (covered and uncovered). When comparing the amount of CH4 emissions from the outdoor storage tanks to emissions from the entire farm, emissions from cattle manure tanks corresponded to 14 % of the total farm emissions, whilst, in pig farms, outdoor manure tanks covered from 21 to 64 % of the total emissions. There was a large variability in CH4 emissions among pig manure storage tanks, likely caused by different management practices. To support further investigation, other factors such as microbial and chemical composition, combined with emissions quantification by TDM, which showed to be a simple and reliable method for CH4 emissions measurements from manure storage tanks.&#160;References:&#160;Vechi, N. T., &amp; Scheutz, C. (2023). Measurements of methane emissions from manure tanks, using a stationary tracer gas dispersion method. Biosystems Engineering.Vechi, N. T., et al. (2023). Methane emission rates averaged over a year from ten farm-scale manure storage tanks.&#160;Science of the Total Environment.