Using slurry acidification to reduce emissions

How slurry acidification can reduce greenhouse gas emissions and ammonia.

Although slurry acidification is not common practice on UK farms, it is more widely used in other countries; Denmark, for example, acidifies 20% of all its farm slurry.

Slurry storage is associated with methane (CH₄) and ammonia (NH₃) emissions. Adding acids to slurry to lower its pH value is a recognised technique for reducing the amount of ammonia it can emit. The process is used primarily to reduce ammonia emissions, but there is evidence that it also reduces greenhouse gas (GHG) emissions.

Slurry typically has a pH of 7, the aim of acidification is to lower it to around 5.5 by mixing it with strong acids. Acid is usually added at storage, with some stored slurry circulating back into the house to start the process at source. It can also be injected into the slurry flow during the field application process, however this method does not reduce methane emissions.

Studies have shown that the process can reduce methane emissions and indirect nitrous oxide (N₂O) emissions.

The acid most commonly used is sulphuric, but other mineral acids can be effective. The effect on CH₄ emissions depends on the acid used. For example, studies have reported decreases of 67–97% with sulphuric acid, but a more variable 17–75% with nitric acid.

Because acidification reduces ammonia emissions, it has been recognised as a technique for improving air quality under the Environmental Permitting Regulations (EPR).

Controlling ammonia emissions by reducing the pH of slurry could be used as an alternative to fitting a cover to a slurry store. This has yet to be approved by Defra but the Slurry Infrastructure Grant (SIG) did not require reduced pH slurry to be covered and it is a recognised technique for EPR. Bear in mind that this approach does not prevent rainfall from entering the store.

Acidification also helps retain more of the nitrogen (N) in the form of ammonium, which is beneficial for plant growth when the slurry is applied to land. Using low-emission spreading techniques will reduce losses at application.

This can potentially reduce your need for manufactured fertiliser, which, as well as GHG mitigation benefits, can help you meet sustainability and environmental targets.

Slurry acidification is likely to be a cost to your business, with no gains to livestock productivity, although there may be savings on fertiliser inputs and better slurry N efficiency. However, these will not necessarily offset the expenditure – the initial outlay for the systems required and the cost of the acid itself.

If you apply acidified slurry to your land, you will need to lime fields more frequently as soil pH can decline over time.

Overall, acidifying slurry may be worthwhile to meet air quality and GHG emissions targets, despite the cost.

Acidification won’t be the right option for all livestock systems. Decide whether or not it is appropriate for your farm, taking into consideration the capital expenditure to implement acidification and the wider implications. Seek expert advice from the outset to help with your decision-making.

There are numerous considerations:

• Which system will work best for your farm
• Whether you want to acidify slurry during storage or when it is being spread.
• Infrastructure – to acidify slurry in underground stores in slatted sheds, the infrastructure is best incorporated when the housing is built, although retrofitting is possible
• Flooring and site drainage
• The upfront investment and operating costs
• Health and safety requirements
• The soil nutrient demand and pH of farmland – monitor the soil pH and expect a higher requirement to lime fields
• Slurry analysis to enable targeted application
• Timing and method of application