The physical separation of pig and cattle slurry into its liquid and solid components will increase the rate of nitrogen losses to the atmosphere, according to research recently undertaken at Rothamsted Research in the UK.

This will serve to reduce the fertiliser value of the slurry while, at the same time, having a global warming impact on the environment.

The work, which was carried out in conjunction with scientists at the University of Milan’s, centred on containers of unseparated, solid, or liquid fractions of either pig or cattle digested slurries. These were where set up in a temperature controlled room for over a month.

Once a week, chemical analyses for determining nitrogen and organic matter content were carried out on each container. Twice weekly each container was stirred, in order to give the effect of being disturbed or agitated, after which a sample from each container was collected and tested for potential ammonia and greenhouse gases emissions.

It was found that mechanical separation increased nitrogen losses by 35% in separated fractions of pig slurry and 86% in separated fractions of cattle slurry.

The liquid fractions for both pig and cattle were found to emit the most amount of ammonia, accounting for 75% or more of total emissions. The disturbance to slurry by mixing also caused a considerable increase in ammonia emissions.

Dr Francesca Perazzolo, from the University of Milan, confirmed that the results of the research highlight the need to adopt mitigation techniques when slurry is mechanically separated into liquid and solid fractions of storage.

“This will help us to reduce the increased environmental impact of emissions. In this instance, we would high recommend that the liquid fractions of slurry be contained within covered storage tanks and/or that the pH value of the liquid fraction is lower by the application of an acid treatment, as a means of reducing or mitigating ammonia emissions.

“It would also be recommended that disturbance of slurry, by crust destruction or tank filling and emptying, is restricted until the period just before slurry spreading.”

Rothamsed’s Dr Tom Misselbrook said that anaerobic digestion of livestock manures in combination with crops, food waste of other by-products is increasing as a renewable energy source across Europe.

“The resulting digestate is a rich source of nutrients for recycling to agricultural land, but also a potential source of ammonia and greenhouse gas emissions to the atmosphere.

“Mechanical separation of the digestate into a liquid and solid fraction can help farmers to utilise the nutrient content more effectively, but this study has shown that the potential for emissions is greater from the storage of the separated fractions than for the whole digestate.

“It is important therefore that appropriate mitigations are included as part of the management process to minimise environmental effects and maximise the potential for nutrient utilisation.”