Reducing Greenhouse Gas Emissions through Improved Manure Management (FS-2023-0689)

Livestock generate large amounts of manure that is normally stored on-farm and then spread on fields as fertilizer at certain times of the year to match crop nutritional needs. Livestock manure from dairy and swine is a liquid slurry that can be collected directly via a scrap system or by adding water in a flush-type system within the barn to push liquid manure to a lagoon or pit. The storage of liquid slurry creates anaerobic (i.e., without oxygen) conditions.

Decomposition of manure or other organic waste source, such as food waste in a landfill, in anaerobic conditions produces a naturally occurring biogas containing about 60% methane. If the lagoon is not covered with a biogas recovery system, this biogas is released directly into the atmosphere and increases the global warming potential.

When the solids in dairy and swine manure are separated from the liquid manure slurry, the solids tend to decompose aerobically (i.e., with oxygen) and produce much less methane than the separated liquids stored in anaerobic lagoons. Poultry manure mixed with wood or straw bedding shavings or other moisture absorbing materials is known as poultry litter, with the amount of methane produced lower due to the lack of anaerobic conditions forming. The quantity of methane released from liquid or solids streams depends on climate (temperature and rainfall) and the conditions under which manure is managed, including the oxygen level, water content, pH (acidic or basic qualities), and nutrient availability for the microbes responsible for methane production when decomposing waste under anaerobic conditions.

Researchers at the University of Maryland performed an analysis of greenhouse emissions from manure production in the state (using 2022 data). The data showed that the poultry industry account for 83% of animals in Maryland (percentage based on animal units (AU) where 1 AU is equivalent to one 1,000 lb. beef cattle), yet dairy manure accounts for 78% of the total manure produced in the state. Dairy manure is stored as a liquid slurry in open manure lagoons, resulting in higher GHG emissions from dairy manure than any other manure source in the state (see Figure 1 and Table 1). In Maryland, 51% of the large dairy farms are in Washington or Frederick counties, resulting in more global warming potential (measured as million tons of carbon dioxide equivalents (MtCO₂ₑ) annually) in these two counties than all other counties. The full data analysis is available in the “Maryland Animal Waste Technology Assessment and Strategy Planning” report available at https://go.umd.edu/AWTF.

Anaerobic digestion transforms biomass into renewable energy and beneficial fertilizer while reducing odors and greenhouse gas emissions. During anaerobic digestion, biogas is produced from a naturally occurring microbes that degrade organic materials inside a sealed, oxygen-free reactor, called an anaerobic digester. The sealed reactor allows for the capture of biogas that can be used as a renewable energy source for heating, electricity, or upgraded to renewable natural gas. Find more information on anaerobic digestion at https://go.umd.edu/AWTF.

Anaerobic digestion can:

• Capture and mitigate GHG emissions, especially methane, from open manure lagoon.
• Process a variety of wastes, including manure or co-digestion of manure with food waste, leading to substantial GHG emission reductions associated with both open lagoon storage and landfill processing.
• Produce non-intermittent renewable energy from organic waste that offsets fossil fuel usage.
• Allow nutrients from off-farm wastes, such as food processing residuals and food waste, to be digested with manure, with the digester outflow used as a beneficial fertilizer to off-set the use of commercial fertilizer and GHG emissions associated with commercial fertilizer production.

Composting is a natural, biological process that transforms organic materials into a nutrient-rich soil conditioner called “compost.” Composting involves the controlled aerobic (i.e., with oxygen) decomposition of organic waste. The composting process produces heat, which decreases pathogens, viable seeds, and the volume of the organic materials. Composting can reduce GHG emissions and allows nutrients from waste to be returned to the agricultural production systems. Find more information on composting at https://go.umd.edu/AWTF.

Composting can:

• Utilize the solid fraction of wet manure from dairy and swine or drier sources like poultry litter and food waste
• Achieve GHG emission reductions, although the exact reduction value will depend on the waste type and ensuring that operational conditions maintain aerobic conditions throughout the composting period.
• Allow nutrients from off-farm wastes, such as low-moisture food processing residuals and food waste, to be composted with solid manure fractions, with the compost used as a beneficial fertilizer to off-set the use of commercial fertilizer and GHG emissions associated with commercial fertilizer production.

Thermochemical processing is a waste to energy alternative to field application of drier manure sources, such as poultry litter. Thermochemical processing transforms solid, low-moisture organic materials into value-added products, including renewable energy. Thermochemical processing includes gasification, pyrolysis, combustion, and incineration. Each process not only reduces the total volume of biomass but also can reduce GHG emissions associated with anaerobic decomposition. Depending on the specific thermochemical processing utilized, the following byproducts could be formed: 1) biochar, which is a stable carbon product enhancing soil health, water retention, and acts as a carbon sink, 2) bio-oil, which is a viscous liquid with potential applications in biofuel production, chemical processes, and heating, and 3) syngas, which is a gas mixture mainly of hydrogen, carbon monoxide, and methane that can be used directly for renewable electricity generation. Learn more about thermochemical processing at https://go.umd.edu/AWTF.

Thermochemical processing can:

• Be used on dry manure like poultry litter.
• Achieve near-total GHG emission reduction.
• Produce syngas that can be used as a renewable heating or electricity production sources.
• Produce biochar, which can enhance soil properties including carbon sequestration and binding of nutrients within the soil matrix.

Researchers at the University of Maryland compared various manure management techniques, including composting, thermal conversion, and anaerobic digestion based on their annual GHG emissions per ton of manure as they related to different livestock manure feedstocks and poultry litter (Lansing et al., 2023). Figure 2 shows that reduction of greenhouse gas emissions (measured as million tons of carbon dioxide equivalents (MtCO₂ₑ) from employing these technologies to reduce the baseline emissions from existing uncovered lagoon and poultry litter storage in each Maryland county (2022 data). The data is explained below based on the analyses of all manure storage in Maryland (Figure 1 and Table 1) and processing all daily manure produced through one of the three animal waste technologies currently employed in Maryland (Figure 2 and Table 2).

Dairy Manure:

Researchers at the University of Maryland determined that dairy manure lagoon storage with no cover in the state results in annual GHG emissions of 70.4 MtCO₂ₑ annually per daily ton of manure produced. Most dairy manure in Maryland is stored using open lagoon storage. The calculated reductions in GHG emissions from employing manure technologies to dairy manure in the state are shown below (Lansing et al., 2023).

• Composting dairy manure solids can lead to 23.5 MtCO₂ₑ annually emissions per daily ton of manure produced, which is a 66.7% reduction from the baseline (open lagoon storage).
• Anaerobic digestion of dairy manure slurry (liquids and solids) would have negative value of -3.9 MtCO₂ₑ annual emissions per ton of manure produced daily, indicating a 106% reduction from the baseline (open lagoon storage) due to the production of renewable energy offsetting fossil fuel emissions from energy use.

Poultry Litter (Broiler and Layer):

Broiler and layer litter open storage has annual baseline emissions of 13.8 MtCO₂ₑ GHG and 12.6 MtCO₂ₑ GHG, respectively, per daily ton of litter produced in Maryland. The baseline emissions are lower due to the lower moisture content of poultry litter compared to dairy and swine manure slurries. The calculated reductions in GHG emissions from employing manure technologies to poultry litter in the state are shown below (Lansing et al., 2023).

• Composting reduces broiler litter storage annual emissions to 1.7 MtCO₂ₑ and layer litter storage annual emissions to 1.6 MtCO₂ₑ, an 84.1% reduction from the baseline per daily ton of litter produced.
• Thermochemical conversion nearly eliminates emissions, with annual emission values close to 0.1 MtCO₂ₑ per daily ton of litter produced.
• Anaerobic digestion of poultry and broiler litter results in an annual reduction of over 206% from the baseline due to the production of renewable energy offsetting fossil fuel emissions from energy use.

Swine Manure:

Swine manure lagoon storage with no cover has annual GHG of 154 MtCO₂ₑ per daily ton of manure produced in the state. Most swine manure in Maryland is stored using open lagoon storage. The calculated reductions in GHG emissions from employing manure technologies to swine manure in the state are shown below (Lansing et al., 2023).

• Composting swine manure solids results in 23.5 MtCO₂ₑ GHG annually emissions per daily ton of manure produced, which is an 84.7% reduction from the baseline (open lagoon storage).
• Anaerobic digestion of swine manure slurry (liquids and solids) would have a negative value of -8.58 MtCO₂ₑ annual GHG emissions per ton of manure produced daily, indicating a 106% reduction from the baseline (open lagoon storage) due to the production of renewable energy offsetting fossil fuel emissions from energy use.

Anaerobic digestion had the largest greenhouse reductions due to the ability to process both the solid and liquid portions of the manure slurries and the production of renewable energy.

• Anaerobic digestion of poultry litter resulted in greater GHG emissions reductions (206% reduction) than composting (102%), yet the baseline value for GHG emissions for layer litter (12.6 MtCO₂ₑ) was much lower than dairy manure (70.4 MtCO₂ₑ) due to the dry nature of the litter. This reduces the anaerobic conditions in which methane is produced from waste.
• Uncovered manure lagoons and stored litter emit GHGs in each Maryland county. Manure management technologies could be employed to mitigate these emissions and help Maryland reach the climate change reduction goals.

• Climate Nexus, 2019. Animal agriculture’s impact on climate change. Available at: https://climatenexus.org/climate-issues/food/animal-agricultures-impact-on-climatechange/.
• Lansing, S., Dill, S., Everts, K. Hassanein, A., Hendricks, M., MacDonald, J., Moyle, J., Nunn, N., Potts, S., Rhodes, J., Ruppert, D., Semler, J., Thilmany, E., Alves, P.B.R., Bolster, B., Brito, H., Burnstein, E., Delp, D., Franceschi, R., Sanford, M., Wietelman, D., 2023. Maryland Animal Waste Technology Assessment and Strategy Planning. Final Report to the Maryland Department of Agriculture. Available at: https://go.umd.edu/AWTF.
• US EPA, 2023. Global greenhouse gas overview. Available at: https://www.epa.gov/ghgemissions/global-greenhouse-gas-overview#Reference%201.

• Dr. Amro Hassanein: ahassane@umd.edu
• Dr. Stephanie Lansing: slansing@umd.edu
• Dr. Danielle Delp: ddelp@umd.edu

University of Maryland, Department of Environmental Science & Technology, College Park MD, 20742

For more information on the Animal Waste Technology factsheet series and the Maryland Animal Waste Technology assessment submitted to the Maryland Department of Agriculture go to https://go.umd.edu/AWTF.

This material is based upon work supported by the Maryland Department of Agriculture under Grant # MDA-2072- FY22