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Biogas Collection: Anaerobic Digester with Co-Digestion

alternative practice names:

Digester; Biodigester; Co-Digestion

An anaerobic digester is a facility which utilizes a biological process. Many dairy producers install anaerobic digesters to provide added revenue from the biogas produced during this process. For general information on anaerobic digestion, see Biogas Collection: Anaerobic Digester


Some dairy operations desire to add commercial or post-consumer food waste to an anaerobic digester system. This digestion of multiple organic wastes is often called co-digestion. Since dairy manure has been partially digested, as it has passed through the stomachs of the cow, it has a relatively low gas production potential. When food wastes are combined with dairy manure, biogas production is increased significantly. The dairy manure also provides a buffering effect when co-digested material is added to the system, reducing large spikes and providing for more consistent steady-state gas production from the high concentrations of volatile organic compounds found in food wastes. With the increased gas production when co-digesting food waste, smaller dairies might be economically viable when compared to no food waste. 


There are various types of feedstocks that can be added to dairy manure, including materials from restaurants and grocery stores; fats, oils, and grease (FOG) from food processing facilities; energy crops; and crop residues. Many of these food waste providers pay “tipping” fees to the dairy operation to take their material, which provides an added revenue source. Some states have greatly limited or completely eliminated adding food waste to landfills. This provides added incentives for food waste sources to find other alternatives, such as anaerobic digesters, to take their excess material. 


When food waste is co-digested with dairy manure, the nutrients from the food waste must also be taken into account. As mentioned in the Biogas Collection: Anaerobic Digester document, the amount of nutrients entering the digester equals those exiting the digester. The increased nutrient load from the food waste must be addressed in the nutrient management plan.

When used, in what regions in the U.S. is the practice found: 

West, Upper Midwest, Northeast, Northwest

FARM SIZE 

When used, typically found on farms of the following sizes:

Over 1000 cows

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Practice Benefits 

Operational expansion: With co-digestion of food wastes, an evaluation will need to be made as to whether the increased nutrient content from the food waste would allow the dairy to expand and still meet the nutrient management plan requirements. If the food wastes are properly addressed (from an odor standpoint) before co-digesting with dairy manure, the dairy could expand and not impact neighbor relations. 


Diversified income: Many food waste providers pay "tipping" fees to the dairy operation to take their material. The biogas can generate electricity for on-farm use, be sold to the energy grid, or be converted to renewable natural gas.

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Implementation Insights

Site-specific or Farm-specific requirements 

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  • Material limits: In many locations around the country, limits are established for the amount of co-digested materials that can enter an anaerobic digester. Exceeding these limits may cause the system to fall under other regulatory requirements. Dairies interested in co-digestion should contact local jurisdictions to determine co-digestion limits and permit requirements.

Required Capital Expenditures (CapEx)

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See Biogas Collection: Anaerobic Digester for details on capital expenditures. Additional considerations when incorporating food waste are as follows:

  • Feasibility and engineering: The feasibility study should also look at the impact of adding food waste to the system on biogas production and determine how the system may need to be modified to account for the food waste. 

  • Infrastructure: Besides the components associated with a typical anaerobic digester, food waste collection bays, and a mixing tank or basin are needed to properly mix the food waste and dairy manure to the right consistency and proportion. The flare capacity should be evaluated to ensure it can handle the potential increased biogas generated from the food waste. The system must be designed to handle the increased biogas production from the addition of food waste.

Required Operational Expenditures (OpEx)

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See Biogas Collection: Anaerobic Digester for details on operational expenditures. Additional considerations when incorporating food waste are as follows:

  • Monitoring: Food waste will increase biogas production rates. Biogas production rates should be monitored to watch for “spikes” or if the system becomes “upset.” Higher biogas production with the addition of food waste could increase the system's explosion potential.

  • Maintenance: Spikes or lower biogas production rates from food waste can impact biogas clean-up technologies and may require additional system maintenance.

Implementation Considerations

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  • Co-digestion products: Different food waste products produce different methane production potentials. The amount and type of co-digested material added to a digester over time should not drastically change. Changing co-digestion materials can “upset” the digester, requiring it to be emptied and restarted.

  • Location: The food waste storage and mixing areas should also be close to the anaerobic digester. This will make it easier to inspect and maintain the digester system. 

  • Nutrient management plan: The amount of nutrients leaving a digester equals those entering. Food wastes provide additional nutrients to the waste stream. The nutrient management plan must address these added nutrients to ensure that excess nutrients are not applied, leading to leaching and runoff of these nutrients in cropped fields. Also, a portion of the nutrients are converted from organic to inorganic, so the nutrient management plan should be adjusted further to account for the increased nutrient availability of the effluent following anaerobic digestion.

Financial Considerations and Revenue Streams

PROFIT POTENTIAL

Tipping fees associated with co-digestion are charges levied on external waste producers for disposing of their organic waste at an anaerobic digester facility. These fees provide an additional revenue stream for the farm, helping to offset the costs of operating the digester.


FEDERAL COST-SHARE PROGRAM

Funding is available for this practice through USDA's Natural Resources Conservation Service (NRCS) Environmental Quality Incentives Program (EQIP).

Notes:

  • Check with the local NRCS office on payment rates and practice requirements relevant to your location.


FEDERAL CONSERVATION FUNDING

USDA's Rural Development Offices provide grants and loans for renewable energy systems, includingdigesters through the Rural Energy for America Program (REAP). Grants are awarded throughout the year and and a formal application must be submitted for review. Contact the state Rural Development Office for more information.


CARBON MARKETS

Practicing co-digestion may not prequalify for carbon credit programs, but if the specific food waste is considered an eligible feedstock per state clean fuel standards, projects may apply to establish a pathway to quantify carbon intensity from co-digested food waste and dairy manure.


For additional information, see Biogas Collection: Anaerobic Digester.

Additional Resources

► See the Newtrient Solutions Catalog to learn more about Anaerobic Digesters and related solution providers.

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Environmental Impacts

REDUCES FARM GREENHOUSE GAS FOOTPRINT

Anaerobic digesters capture methane-rich biogas produced during the decomposition of manure in an oxygen-free environment. When the captured methane is combusted to create energy, it is converted into carbon dioxide and water. While carbon dioxide is still a greenhouse gas (GHG), it is significantly less impactful than methane in terms of global warming potential.¹ Also, anaerobic digesters mitigate nitrous oxide emissions by stabilizing manure in an oxygen-limited environment. This reduces the conditions that promote nitrification and denitrification, which are responsible for nitrous oxide production. Thus, by capturing and combusting the methane and minimizing the formation of nitrous oxide, anaerobic digesters can significantly reduce GHG emissions associated with manure storage.²


See the research highlights below:

  • Greene et al. (2024) quantified the GHG emission reduction potential of the adoption of AD technology on applicable large-scale dairy farms in the contiguous United States. They estimated that, at the farm level, AD technology may reduce GHG emissions from manure management systems by 58.1−79.8%, depending on the region.


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Note: CH₄ emissions from digestate

After digestion, manure is sometimes transferred to holding lagoons for secondary storage, where it can continue to decompose anaerobically and release CH₄. However, modern systems often include technologies like lagoon covers or biogas capture systems that collect this CH₄, allowing it to be used as a renewable energy source.


¹ CH₄ has a global warming potential approximately 27 times higher than CO₂ over a 100-year period based on IPCC Global Warming GWP100 AR6 Potential Values.

² After digestion, manure is sometimes transferred to holding lagoons for secondary storage, where it can continue to decompose anaerobically and release considerable quantities of CH₄ (Baldé et al., 2016). However, modern systems often include technologies like lagoon covers or biogas capture systems that collect this CH₄, allowing it to be used as a renewable energy source. 

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Alignment with FARM Program

FARM Environmental Stewardship (ES) V2-V3 Alignment

FARM ES Version 2 and Version 3 include anaerobic digestion as a manure management system.

Contents

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We're always eager to update the website with the latest research, implementation insights, financial case studies, and emerging practices. Use the link above to share your insights. 

We're always eager to update the website with the latest research, implementation insights, financial case studies, and emerging practices. Use the link above to share your insights. 

Contents

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Practice Overview

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Practical Insights.png
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Research Results.png
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An anaerobic digester is a facility which utilizes a biological process. Many dairy producers install anaerobic digesters to provide added revenue from the biogas produced during this process. For general information on anaerobic digestion, see Biogas Collection: Anaerobic Digester


Some dairy operations desire to add commercial or post-consumer food waste to an anaerobic digester system. This digestion of multiple organic wastes is often called co-digestion. Since dairy manure has been partially digested, as it has passed through the stomachs of the cow, it has a relatively low gas production potential. When food wastes are combined with dairy manure, biogas production is increased significantly. The dairy manure also provides a buffering effect when co-digested material is added to the system, reducing large spikes and providing for more consistent steady-state gas production from the high concentrations of volatile organic compounds found in food wastes. With the increased gas production when co-digesting food waste, smaller dairies might be economically viable when compared to no food waste. 


There are various types of feedstocks that can be added to dairy manure, including materials from restaurants and grocery stores; fats, oils, and grease (FOG) from food processing facilities; energy crops; and crop residues. Many of these food waste providers pay “tipping” fees to the dairy operation to take their material, which provides an added revenue source. Some states have greatly limited or completely eliminated adding food waste to landfills. This provides added incentives for food waste sources to find other alternatives, such as anaerobic digesters, to take their excess material. 


When food waste is co-digested with dairy manure, the nutrients from the food waste must also be taken into account. As mentioned in the Biogas Collection: Anaerobic Digester document, the amount of nutrients entering the digester equals those exiting the digester. The increased nutrient load from the food waste must be addressed in the nutrient management plan.

Practices and technologies

Biogas Collection: Anaerobic Digester with Co-Digestion

alternative practice name:

Digester; Biodigester; Co-Digestion