An introduction to large-scale manure management carbon projects

Tuesday, 14 May 2024


Livestock play an important role in global food production, as well as agricultural and rural economies worldwide. In addition to being one of the fastest-growing subsectors of agriculture, the livestock sector is also a notable source of anthropogenic greenhouse gas (GHG) emissions. Specifically, manure and its management contribute to approximately 10% of total emissions from livestock[1].

What is manure management?

Integrated manure management involves the comprehensive management of both dung and urine. It encompasses activities such as excretion, collection, housing, storage, anaerobic digestion, treatment, transport, and application of manure and addressing any potential losses or discharge throughout the entire manure management process.

What is a manure management carbon project?

Manure management carbon projects occur on large-scale livestock farms (cattle, swine, buffalo, sheep, goats, and/or poultry). They reduce emissions by capturing GHGs (such as methane and nitrous oxide) released from animal manure, preventing them from entering the atmosphere. This is done by flaring the gas without utilization or, more often, converting it to heat/electricity. These projects collect and pump livestock manure into a closed system, the biodigester, that captures the biogas produced when microorganisms break down organic material. When the captured gas is converted into energy it could lead to to additional reductions in emissions by decreasing the reliance on fossil fuels for producing electricity/heat. Additionally, these projects often contribute to sustainable development goals, for example, by producing affordable clean energy and creating job opportunities, stimulating the local economy. 

Manure management projects and the carbon market

According to the Berkeley Carbon Trading Project Voluntary Registry Offsets Database[2], there are currently over 400 such projects in the carbon market and about 70% of projects are located in either the United States or China. As of December 2023, over 23 million credits have been issued for manure management projects across the four registries: ACR, Climate Action Reserve, Gold Standard and Verified Carbon Standard.

This photo depicts cows inside a large-scale dairy cow farm.

What is a commercial-scale biodigester?

To process large volumes of manure, commercial-scale biodigesters are used; they function similarly to household biodigesters, which are detailed in our introduction to household biodigesters blog post. In this case, the input into the biodigester is always animal manure. Once in the digester, bacteria begin to break down the waste in a process called anaerobic (“without oxygen”) digestion. This process produces two main bioproducts: biogas, which is high in methane, and effluent (i.e., liquid waste). If the volume and quality of biogas (methane) generated is sufficient and suitable, it can be used to produce electricity or heat in a generator engine. The effluent, meanwhile, can be disposed of or dispersed onto a nearby field, as fertilizer.

There are various types of digesters used in waste management, and while all reactors serve the same fundamental purpose, each operates most effectively with a specific manure consistency.

This diagram shows the generic function of large-scale biodigesters.

This photo shows a large biodigester on a farm in Switzerland.

How manure management projects claim emission reductions

Even in the absence of carbon projects, most farms in developed countries are required to have basic manure management practices. In most cases, manure in the baseline scenario (also known as business as usual) is stored in an uncovered anaerobic lagoon. Here, the manure breaks down and the gases produced (methane and nitrous oxide) are released directly into the atmosphere.

This image depicts the greenhouse gases - methane (CH4) and nitrous oxide (N2O) being emitted from an uncovered lagoon at Oregon Dairy Farm in Pennsylvania, U.S.

In the project scenario, the manure undergoes treatment stages, which may include aerobic ponds, anaerobic digestion, sludge removal and land application. The uncovered anaerobic lagoon is replaced by an anaerobic biodigester, which captures the gases generated. These gases are then destroyed or used to generate heat and/or electricity. 

This photo depicts large-scale biodigesters on a farm.

Since methane and nitrous oxide are captured and utilized (or destroyed) instead of emitted into the atmosphere, the amount of emissions that would normally occur from the lagoon can be claimed as avoided emissions by the project.

The methodologies applied by these projects (e.g., ACM0010) introduce a secondary pathway (energy production) from avoided fossil fuel use to be claimed; however, these emission reductions are typically not claimed as they are very minor in comparison to avoided methane/nitrous oxide emissions.

How manure management carbon projects have the potential to benefit the planet
1. Avoided emissions

These projects aim to reduce the emissions from uncovered lagoons by capturing and utilizing the gases emitted by the decomposition of manure waste.

2. Energy production

In the project scenario, the captured methane (biogas) is transformed into electricity or heat using a biogas generator. Thus, there is an opportunity to claim emission reductions for energy produced, as it is potentially replacing fossil fuels. However, a large number of projects choose to use the energy produced for internal animal waste management systems and farm operations rather than supplying it to the electrical grid, and the emissions avoided from replacing fossil fuels are not very significant. Therefore, projects do not typically claim emission reductions from the energy produced.

3. Naturally produced fertilizer

After the manure undergoes treatment in the biodigester, the remaining solids must be extracted and disposed of. This effluent contains a high level of nitrogen, perfect for fertilizer. Many farms will disperse the effluent to fertilize their own field, or provide neighboring farms and fields with this organic fertilizer. Any excess is typically sold to the market and can be the main source of revenue for these projects. By using the effluent from the biodigester, projects are maximizing their potential to provide benefits to local communities.

Common environmental and social risks in manure management carbon projects

With all its benefits, manure management also has its challenges. Improper management practices and the absence of biodiversity, environmental and social strategies in project planning and implementation can lead to environmental and air pollution, habitat destruction, and biodiversity loss, notably linked to intensive livestock farming and its large-scale manure production. Moreover, changes in agricultural practices from these projects may disrupt community livelihoods and traditional knowledge systems, exacerbating inequalities in benefit distribution. 

Without mitigation measures, manure management projects can have adverse human health impacts from exposure to pathogens associated with manure handling, as well as odor problems, affecting local communities' well-being. Finally, manure management projects can create ethical conflicts for vegans and animal rights activists, who may support sustainability efforts but oppose projects indirectly supporting animal exploitation or normalizing animal agriculture. Outside of potential ethical dilemmas, with thoughtful management, most of these risks can be mitigated. When considering manure management carbon credits, environmental and social risks should be a part of quality evaluation.

Calyx Global ratings of manure management projects
This week, Calyx Global rated our first large-scale manure projects and will continue expanding our assessments of this project type. If you’re interested in viewing our ratings of large-scale manure management projects, contact us about a subscription to the Calyx Global platform.


[1] Global Assessment of Manure Management Policies and Practices, Livestock Research Report 844, Wageningen UR Livestock Research Wageningen, December 2014

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Calyx Global

This article includes insights and input from multiple experts in Calyx Global.