When animal slurry is stored in anaerobic (oxygen-free) conditions, bacteria begin to degrade its organic content, producing and releasing methane into the atmosphere.

This process can take place in slurry tanks and lagoons and is a significant contributor to agricultural greenhouse (GHG) emissions. However, there are a number of methods which can help to reduce these emissions.

Although bound by the regulations on slurry spreading and storage requirements, the quick removal of slurry from animal housing, where temperatures are typically higher, to outdoor storage is one method.

Outdoor temperatures are generally lower, which in turn reduces methane production.

Lagoon covers

Where slurry is stored in open lagoons, a substantial quantity of methane can still be released over time. Some suppliers are offering lagoon covers which can capture methane emissions for use in small-scale combined heat and power plants. Sealing the tank can be problematic, however.

Anaerobic digestion

Anaerobic digestion (AD) plants can provide a near perfect environment for bacterial fermentation of slurry – warm temperatures, a high level of nutrients, a neutral pH and an oxygen-free environment. The resulting biogas, typically made up of 65% methane and 35% carbon dioxide, has a range of uses. Depending on the AD plant’s retention time, the resulting methane content in the digestate is reduced.

Slurry treatment

Another emission reduction strategy includes the acidification of slurry to inhibit the growth and activity of bacteria. Studies have shown the addition of sulphuric acid resulted in a 69–84% reduction of cumulative methane emissions in slurry.