Reduce pH of slurry via acidification
System: Pigs
Mainly applicable for: Liquid manure, manure separation (acidifying liquid fraction), very large farms or where ammonia emissions are regulated.
Not or less applicable for: Solid manure, small and medium farms, organic farms
Description
Reducing the pH of stored slurry by adding either inorganic acids (e.g., sulphuric acid) and organic acids (e.g., acetic acid). Acidification was developed to reduce ammonia emissions but is also very effective at reducing methane emissions when applied to stored slurry. To avoid recovery of pH, the acid treatment should be repeated at regular time intervals. Acidification is expensive, and using sulphuric (strong) acid on farms requires strict health and safety measures. For weak organic acids large quantities are often needed. There is a risk of foaming with the addition of strong acids, which should be avoided. Using sulphuric acid and phosphoric acid adds nutrients to the slurry that may cause over-fertilization with sulphur (S) and phosphorus (P) (Bittman et al, 2014).
Mechanism of effect
A pH below 6 inhibits methanogenenis as the optimum pH range of methanogic microbes is 6.5-7.6, thereby reducing methane (CH4) emissions. Lowering the pH of slurry also delays nitrification, which reduces nitrous oxide (N2O) emissions. The use of nitric acid (HNO3), however, is not recommended because the pH value should be kept very low (~4) to avoid a potential increase in N2O production due to nitrification and denitrification processes (TFRN, in prep.). Nitric acid treatment is therefore considered unsuitable for reducing CO2-eq (Dalby et al., 2022). Manufacturing acids requires some energy, hence CO2.
Reference situation
No acidification of slurry.
Legend
| ● – Small effect (<5%) | o – No effect | o – no effect |
| ●● – Medium effect (5-20%) | ● – Unfavourable effect | N/A – unknown effect |
| ●●● – Large effect (>20%) | ● – ● – Variable effect (depending on farm characteristics or way/level of implementation) |
Effect on total greenhouse gas (GHG) emissions (LCA)
| Mean effect and range in kg CO2-equivalents | per kg product | per farm (absolute) | Level of evidence | ||
| Mean | (min-max) | Mean | (min-max) | ||
| Acidification of stored slurry | ●● | ●●–●●● | ●● | ●●–●●● | High |
Effect per emission source
| Mean effect on emission from | Manure | Animal | Feed and forage production | Barn & farm inputs | |||
| CH4 | N2O | CH4 | CO2 | N2O | LUC | CO2 | |
| Acidification of stored slurry | ●●● | o* | o | o | o* | o | ● |
*risk of an adverse effect (see ’cause of variable or unfavourable effect’)
Cause of variable or unfavourable effect
Acidification of stored slurry
The size of the effect depends on the type and dose of acidifier that is applied (e.g. H2SO4 shows a large effect) and the pH of acidified slurry. Nitric acid reduces ammonia emissions, but increases emissions of N2O (TFRN, in prep.). The effect also depends on the composition and characteristics of the slurry (e.g. level of degradable organic compounds, buffer capacity), and the manure temperature and storage duration. If the level of degradable organic compounds is low, (weak) organic acids (acetic acids) can be used as acidifiers, or a combination of acetic acid and sulfuric acid. In case of high buffer capacity higher doses of acid or more frequent acidifications are required.
| Literature references | Acidification of stored slurry |
|---|---|
| Ambrose et al., 2023 | Greenhouse Gas and Ammonia Emissions from Different Stages of Liquid Manure Management Chains: Abatement Options and Emission Interactions |
| Hou et al., 2014 | Total methane emission rates and losses from 23 biogas plants |
| Emmerling et al., 2020 | Meta-Analysis of Strategies to Reduce NH3 Emissions from Slurries in European Agriculture and Consequences for Greenhouse Gas Emissions |
| Overmeyer et al., 2023 | Klimavirkemidler Til Dansk Landbrug (Climate Agents for Danish Agriculture) |
| Kupper et al., 2020 | Ammonia and greenhouse gas emissions from slurry storage – A review |
| Vechi et al. 2022 | Danish pork production – An environmental assessment |
| SEGES, 2024 | LCA of comprehensive pig manure management incorporating integrated technology systems |