Acidification of liquid manure (i.e. slurry) can significantly reduce ammonia volatilisation. Acids can be added to slurry during storage but also during application of slurry to land (see (figure 1). Acids commonly used include sulphuric acid, but also phosphoric acid and nitric acid. Acidification of manures is one of the most effective measures to reduce ammonia emissions, and comparable in terms of ammonia reductions to those achieved by liquid manure injection (UBA 2019). Acidification can be carried out in the barn, during slurry storage or during spreading.
The pH value of slurry at the time it is applied to land is typically higher than 7.0 and can be higher than 8.0. Reducing slurry pH value from 7.5 to 6.5 has been shown to reduce NH3 emissions by over 60%. Although field measurements do not always show such a clear relationship between slurry pH and total NH3 emission this measure is a reliable mean to reduce ammonia emissions (European Commission, 2017).
Most field trials have used sulphuric acid to reduce the pH value of slurry. The amount of sulphuric acid needed for a tonne of slurry is approximately 2.5 to 3 litres. This corresponds to about 4.6–5.5 kg of acid for each tonne of raw slurry, to reduce the pH to between 5.5 and 6. However, adding too much sulphuric acid result in the production of hydrogen sulphide, which can cause odour problems and health and safety issues. Acidification with sulphuric acid (or phosphoric acid) can add nutrient concentrations (i.e. sulphur or phosphorus) in the slurry and should be monitored to ensure excess nutrient are not applied to soils (UBA, 2019).