Install anaerobic digester
System: Dairy Cattle
Mainly applicable for: Large farms, slurry systems (with frequent removal), and farms with a demand for electricity or heat on site
Not or less applicable for: Farms with (not-pumpable) solid manure and full-grazing systems
Description
Installing an anaerobic digester (AD) in which animal manure is decomposed by microorganisms in the absence of oxygen (‘anaerobic’). In this factsheet only mono-digestion (the use of 100% manure or slurry as a feedstock) is included, as there is a risk of higher emissions (fugitive methane, nitrous oxide, ammonia) when other feedstocks are digested with animal manures. The AD process produces biogas and digestate. The biogas, consisting of biomethane and carbon dioxide, can be used in various ways: combusted to produce heat; converted to green electricity using a CHP (Combined heat and Power generator); or separated and the biomethane upgraded to replace natural gas. Digestate can be used as a fertilizer or further processed (e.g. for animal bedding).
Mechanism of effect
Anaerobic digestion (AD) captures methane from manure that would otherwise be released to the atmosphere. It also produces biogas containing biomethane which can be used to generate heat or electricity, or can be purified and used as a replacement for natural gas. In life cycle assessments the benefit of replacing fossil fuel use is only attributed to the farm if the renewable energy is used on the same farm and not sold. In this factsheet we assume that the biogas is combusted in a combined heat and power unit (CHP) to generate electricity and heat. For emission reduction the main benefit of AD is the capture of methane; the benefit of replacing grid electricity will decline as grid supplies are decarbonised.
Nitrous oxide emissions from stored digestate are higher than from undigested slurry in open stores in summer, but this can be avoided by a gas-tight cover. When applied to soils, nitrous oxide emissions from digestate can be higher or lower than from undigested slurry, depending on specific soil conditions. Some CO2 emission is associated with the manufacturing and transport of the AD installation, but this is small compared to the reduction in emissions by AD.
Climate Impacts
| ● – Small effect (<5%) | o – No effect |
| ●● – Medium effect (5-20%) | ● – Unfavourable effect |
| ●●● – Large effect (>20%) | ● – ● – Variable effect (depending on farm characteristics or way/level of implemention) |
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 | |
| Mono-digestion | ●● | ● – ●●● | ●● | ●–●●● | 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 | |
| Mono-digestion | ●●● | ●● | o | o | N/A | o | ●● |
*risk of an adverse effect (see ’cause of variable or unfavourable effect’)
Legend
| ● – Small effect (<5 %) | o – No effect |
| ●● – Medium effect (5-20%) | ● – Unfavourable effect |
| ●●● – Large effect (>20%) | ● – ● -Variable effect (depending on farm characteristics or way/level of implemention) |
Effect on soil organic carbon (SOC) stocks
| Relative change (%) in SOC | mean (min – max) | Level of evidence | ||
| Mono-digestion | o ( ) | Low | ||
Legend
| ● Small increase (<10 %) | ● Small decrease (<5%) | o – No effect |
| ●● Medium increase (10-25%) | ●● Large decrease (>= 5%) | N/A – effect unknown |
| ●●● Large increase (>25%) | ● – ● -Variable effect (depending on farm characteristics or way/level of implemention) |
Cause of variable or unfavourable effect
Mono-digestion
Frequent removal of manure to the digester is important for a high reduction of methane emissions. Anaerobic digestion is less effective when slurry is held in pits for a long time. The reduction in greenhouse gas emissions also depends on the composition and characteristics of slurry, and storage temperature. Improper management and functioning of the system can result in leakage of methane from the system, which can offset the GHG mitigation benefits of anaerobic digestion on farms. Incomplete digestion can result in methane emissions from digestate during storage or spreading. There is also a risk of higher nitrous oxide emissions due to increased pH, higher concentrations of available nitrogen, crust formation, and higher temperatures during digestion. Digestate stores should be covered and any further methane emissions captured. If the biogas is used to replace on-farm electricity, heat or natural gas from fossil energy sources, this results in lower carbon dioxide emission, but the size of this effect depends on the quantity and carbon intensity of the energy replaced.
Other Effects
Effects on yield and cost-effectiveness
| Yield animals | crops | Labor time | Costs and revenues Investment | Costs | Revenues | |
| Mono-digestion | o | o | ● – ●● | ●● – ●●● | ● – ●●● | ●-●●● |
Legend
| Literature references | Mono-digestion |
|---|---|
| Sajeev et al., 2018 | Greenhouse Gas and Ammonia Emissions from Different Stages of Liquid Manure Management Chains: Abatement Options and Emission Interactions |
| Zhang et al., 2021 | The potential of dairy manure and sewage management pathways towards a circular economy: A meta-analysis from the life cycle perspective |
| Moller et al., 2022 | Agricultural Biogas Production—Climate and Environmental Impacts |
| Miranda et al., 2015 | Meta-Analysis of Greenhouse Gas Emissions from Anaerobic Digestion Processes in Dairy Farms |
| Scheutz and Fredenslund, 2019 | 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 |
| Kupper et al., 2020 | Ammonia and greenhouse gas emissions from slurry storage – A review |
| Aguirre-Villegas et al., 2017 | Grazing intensity affects the environmental impact of dairy systems |
| Rivas-García et al., 2015 | Environmental implications of anaerobic digestion for manure management in dairy farms in Mexico: a life cycle perspective |
| Battini et al., 2014 | Mitigating the environmental impacts of milk production via anaerobic digestion of manure: Case study of a dairy farm in the Po Valley |
| Vida et al., 2017 | The carbon footprint of integrated milk production and renewable energy systems – A case study |
| Setoguchi et al., 2022 | Carbon footprint assessment of a whole dairy farming system with a biogas plant and the use of solid fraction of digestate as a recycled bedding material |
| Thomas et al., 2017 | Nitrous Oxide Emitted from Soil Receiving Anaerobically Digested Solid Cattle Manure |
| Shen et al., 2018 | Modeling nitrous oxide emissions from digestate and slurry applied to three agricultural soils in the United Kingdom: Fluxes and emission factors |
| Eickenscheidt et al., 2014 | Short-term effects of biogas digestate and cattle slurry application on greenhouse gas emissions affected by N availability from grasslands on drained fen peatlands and associated organic soils |
| Maciel et al., 2022 | Life cycle assessment of milk production system in Brazil: Environmental impact reduction linked with anaerobic treatment of dairy manure |
| Šimon et al., 2015 | The effect of digestate, cattle slurry and mineral fertilization on the winter wheat yield and soil quality parameters |
| Barlog et al., 2020 | Effect of Digestate on Soil Organic Carbon and Plant-Available Nutrient Content Compared to Cattle Slurry and Mineral Fertilization |