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How Should the Valuable Resource that is Livestock Slurry be Stored?

25 September 2015

New research from the UK's Rothamsted Research and the University of Milan, Italy, considers the impact of mechanically separating livestock slurry into a liquid and solid fraction during storage on ammonia and greenhouse gas emissions.

Livestock slurry is valuable source of free, organic fertiliser, which farmers can spread on farmland. However, gases which can be lost from slurry, during collection, storage and spreading, are of environmental concern.

Scientists at Rothamsted Research and the University of Milan, Italy, have examined the effect that mechanically separating anaerobically digested cattle and pig slurries into their liquid and solid fractions during storage has on ammonia and greenhouse gases emissions.

A main finding of the research, which is published in the Agriculture, Ecosystems and Environment journal, was that storage of the separated fractions of pig and cattle slurries increases nitrogen and ammonia emissions, compared with unseparated pig and cattle slurries.

During storage ammonia and greenhouse gases are emitted from slurry. Emissions of ammonia can cause excessive richness of nutrients in our lakes and rivers, resulting in dense growth of plant life. Emissions of greenhouse gases trap heat in the atmosphere, which makes the Earth warmer.

However, slurry can also be an asset to the wider population, at a time when we need renewable energy resources to combat climate change.

Anaerobic digestion, whereby microorganisms break down slurry in the absence of oxygen, results in the production of biogas - a renewable energy with a small carbon footprint. A byproduct of producing this energy is the digested, leftover slurry. But what is the best way of storing this digested slurry before spreading on land?

For this experimental research, containers of unseparated, solid, or liquid fractions of either pig or cattle digested slurries 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 Department of Agricultural and Environmental Science, University of Milan, said: “The results of our experimental 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.”

Dr Tom Misselbrook, a Principal Research Scientist at Rothamsted Research, who is strategically funded by the BBSRC, concluded: “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.”

September 2015

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