In the spring of 2024, Denmark housed almost 11.5 million pigs. Although many are exported, some are also consumed in Denmark. Pork is such a popular part of our daily diet that one in three participants in a 2021 Epinion survey with 6,228 respondents stated that they had eaten pork the night before.
But our meatballs or roast porks leave a climate footprint because of the pig manure, which during storage in slurry tanks produces methane – an extremely potent greenhouse gas. This has created an opportunity to test biofilters in pig production, as the technology has previously shown promising results in reducing methane emissions from landfills.
In 2020, a group of researchers from DTU Sustain therefore started designing and constructing a biofilter which they in 2022 installed at a pig Danish producer in Gilleleje, north of Copenhagen.
Bacteria break down methane
A biofilter is established by excavating an area at a depth of about half a metre, which is filled with stones that act as gas distribution layers. An 80-cm thick layer of compost is placed on top of the stones. The compost holds a naturally occurring group of bacteria that feed on methane, which means that the bacteria break down the unwanted greenhouse gas in their encounter with the methane. The filter works completely without the addition of chemicals, and the solution can therefore be applied in both organic and conventional production.
At the Gilleleje test farm, a 400 m2 filter was created and placed close to the farm's 4,400 m3 covered slurry tank. The entire biofilter plant was also fitted with measuring equipment so the researchers could monitor and document the fate of the methane and plant efficiency.
The biofilter system works by pumping slurry gas from the top of the slurry tank, after which it is led through pipes under the compost layer outside. From here, the methane-filled gas leaks out and up through the compost. Along the way, the methane is broken down by the methane-eating bacteria.
It was the first full-scale test of a biofilter's ability to reduce methane emissions from a pig herd, and the results were excellent, says Charlotte Scheutz, Professor at DTU, who headed the project.
"The filter was able to break down 92 per cent of the methane that was passed through it in the first year. Our hopes were that it would break down 80 per cent, so the biofilter's efficiency far exceeded our expectations," says Charlotte Scheutz.
