Reduce pH of slurry via acidification
System: Dairy Cattle
Applicability
Mainly applicable for: Liquid manure, manure separation (acidifying liquid fraction), very large farms
Not or less applicable for: Solid manure, small and medium farms, organic farms, concrete tanks (risk of corrosion)
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. For cattle slurry approximately 6 kg sulfuric acid (H2SO4) per ton of slurry is used to achieve the target pH of 5.5. 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.
Effects on GHG emissions
Reference situation: No acidification of slurry
Effect on total greenhouse gas (GHG) emissions
| Mean effect and range in kg CO2-equivalents | per kg product | per farm | |||
| Mean | Min-Max | Mean | Min-Max | Level of evidence | |
| Acidification of stored slurry | ●● | ● – ●● | ●● | ●–●● | High |
Legend
| ● – Small effect (<5%) | o – No effect | ? – Effect unknown |
| ●● – Medium effect (5-20%) | ● – Unfavourable effect | |
| ●●● – Large effect (>20%) | ● – ● – Variable effect (depending on farm characteristics or way/level of implementation) | |
Effect per emission source
| Mean effect on absolute emission from | Animal | Manure storage | Feed and forage production | Barn | |||
| CH4 | CH4 | N2O | 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’)
Legend
| ● – Small effect (<5%) | o – No effect | ? – Effect unknown |
| ●● – Medium effect (5-20%) | ● – Unfavourable effect | |
| ●●● – Large effect (>20%) | ● – ● – Variable effect (depending on farm characteristics or way/level of implementation) | |
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.
Other Effects
Effects on yield and cost-effectiveness
| Yield | Labor | Costs and revenues | ||||
|---|---|---|---|---|---|---|
| Animals | Crops | Time | Capital investment | Operational Costs | Revenues | |
| Acidification of stored slurry | o | o | ●● | ●● | ●● | o |
Legend (thresholds differ per indictor and can be found in the tooltip)
| ● – Small favorable effect | o – No effect | ? – Effect unknown |
| ●● – Medium favorable effect | ● – Unfavourable effect | |
| ●●● – Large favorable effect | ● – ● -Variable effect (depending on farm characteristics or way/level of implementation) | |
Effects on other sustainability aspects
| Risks of trade-offs | Potential synergies | |
|---|---|---|
| Acidification of stored slurry | Water quality, Soil health, Biodiversity, Farm labour safety | Ammonia emission |
| Literature references | Acidification of stored slurry |
|---|---|
| Ambrose et al., 2023 | Additives and methods for the mitigation of methane emission from stored liquid manure |
| Hou et al., 2014 | Mitigation of ammonia, nitrous oxide and methane emissions from manure management chains: a meta-analysis and integrated assessment |
| 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 | Acidification of slurry to reduce ammonia and methane emissions: Deployment of a retrofittable system in fattening pig barns |
| Kupper et al., 2020 | Ammonia and greenhouse gas emissions from slurry storage – A review |
| Vechi et al. 2022 | Methane emissions from five Danish pig farms: Mitigation strategies and inventory estimated emissions |
| SEGES, 2024 | Klimavirkemidler Til Dansk Landbrug (Climate Agents for Danish Agriculture) |