Manure from cows and pigs is a surprisingly large source of greenhouse gases (GHGs) in the United States — over 1% of the country’s total emissions. The vast majority (about 80%) of these emissions are methane, with the rest coming from nitrous oxide.
Most dairy and swine farms in the U.S. currently manage manure using “wet systems.” These use water jets to flush manure into storage structures such as lagoons or tanks, creating a liquid or slurry. Wet storage systems create ideal conditions for methane-producing microorganisms to thrive, allowing them to emit large quantities of the gas into the atmosphere.
Methane is over 80 times more potent than carbon dioxide over a 20-year period, and reducing it is one of the fastest ways to slow global warming. Yet current efforts to tackle manure methane are lagging in the U.S., both in terms of testing and adoption. It is time to widen the lens and invest in a broader set of solutions for manure methane — solutions that can offer tangible benefits for farmers as well as the climate. Our new analysis provides a close look at both the problem and potentially promising solutions.
US Manure Emissions Are Underestimated — and Rising
The U.S. Environmental Protection Agency (EPA) estimates that manure management contributes around 1.3% of the country’s total GHG emissions, with most of this coming from dairy and swine operations. But recent satellite data shows that methane emissions are being undercounted in the U.S. livestock sector.
Our own analysis suggests that methane emissions from dairy and swine manure may be at least 40% higher than EPA estimates. This reflects more recent data from the 2022 U.S. Census of Agriculture, which shows that herd sizes are growing, and how that impacts manure storage systems. As livestock operations continue to consolidate and increase in herd size, they become more reliant on storing manure in liquid form, generating more methane. This means manure emissions will likely keep rising unless targeted action is taken.
Biogas Digesters Are Not a Universal Solution
Biogas digesters are currently a leading solution to reduce methane emissions from livestock manure. In the dairy sector, more than 11% of manure is now treated in digesters. These systems capture biogas (a mixture of methane and carbon dioxide) during manure storage and convert it into either electricity or vehicle fuel, popularly called “renewable natural gas.” But while they receive the most policy and financial support, digesters offer only modest methane reductions from manure itself. Most of their climate benefits come from offsetting fossil fuel emissions by generating energy, rather than directly reducing emissions from manure storage.
The main concern is what happens after digestion. Most digesters in the U.S. remove less than a third of the methane-generating material from manure. The leftover material, known as “digestate,” is typically stored in uncovered lagoons or pits that continue to emit methane into the atmosphere. Without gas-tight storage or additional treatment, much of the benefit of methane capture is lost. This means that digesters reduce methane from manure storage by only about 25%. The benefits may rise to 35% if the digestate is stored in covered, gas-tight systems that prevent further methane release.
Methane can also leak from other parts of the digester system, including the cover, biogas flaring units, feedstock tanks and gas upgrading equipment. Most digesters in the U.S. are not designed to fully break down manure or to manage emissions from digestate. They operate with relatively short treatment times and typically lack sealed storage. This results in lower methane reductions and additional environmental trade-offs, such as increased ammonia and nitrous oxide emissions when digestate is stored or applied as fertilizer in crop fields.
Cost is another challenge. Digesters are expensive to purchase and operate, often costing over $500 per cow, per year. This may be financially feasible for farms that have thousands of animals. But without policy support and subsidies, digesters are prohibitively expensive for small and medium farms and substantially increase the costs of milk or pork production.
These limitations underscore the need to strengthen measurement, monitoring, reporting and validation for existing digesters. Better data can help identify underperformance, detect methane leaks and ensure systems operate as intended.
However, given the high cost and relatively low overall mitigation potential of biogas digesters, manure management interventions should also begin shifting toward more efficient and cost-effective alternatives.
Alternative Technologies Show Promise, but Need More Support
Compared to anaerobic digesters, other technologies — including solid-liquid separation, aeration and acidification — offer more cost-effective options for manure methane mitigation. These approaches often cost less than $80 per cow, per year, and can be applied across various farm sizes, making them more versatile than large-scale digesters. Yet despite their practical advantages, they have received limited policy and funding support.
Solid-liquid Separation
Most methane from manure storage is emitted by the breakdown of methane-producing solids under wet, oxygen-free (“anaerobic”) conditions. Solid-liquid separation reduces emissions by removing a large share of these solids before the manure is stored. The solids, once separated, are drier and more exposed to air, inactivating methane-producing microorganisms. The remaining liquid has far fewer methane-producing solids and, therefore, releases much less methane during storage in lagoons or tanks.
Mechanical equipment such as screw presses, screens or centrifuges can be used to perform this separation on the farm. Depending on the separation system used, methane emissions from manure storage can be reduced by up to 65%. Solid-liquid separation also helps reduce odor, improves the fertilizer value of manure, and prevents solids from accumulating in storage lagoons (which reduces the maintenance cost of removing sludge).
Yet adoption remains limited. Cost is partly to blame: While solid separators are significantly cheaper than anaerobic digesters, they still require upfront investment in equipment, installation and maintenance, and few programs offer financial or technical support to help farmers adopt them. California’s Alternative Manure Management Program has supported the installation of several solid separators in the state. However, while USDA offers cost-share programs for general manure management, states rarely provide targeted support for non-digester technologies.
In addition, farmers don’t have a clear picture of the return on investment. More detailed quantification of solid separators’ economic value — including cost savings from bedding, improved lagoon function and nutrient recovery — could help make a stronger case for adoption.
Manure Acidification
Another promising technology is acidification, or mixing manure with an acid. Acidic manure creates unfavorable conditions for methane-forming microorganisms, thereby reducing methane emissions. This approach is most commonly used in Denmark as part of the country’s ammonia control regulation.
Literature strongly supports the use of acidification to reduce methane, nitrous oxide and ammonia emissions. However, the extent of these benefits depends on the frequency, type and dosage of the acid used. Researchers report up to an 89% reduction in methane using higher acid doses and a 46% reduction with lower doses of acid. While other methane-reduction methods (including anaerobic digestion and solid-liquid separation) can risk increasing nitrous oxide emissions, acidification stands out as the only approach consistently shown to reduce both nitrous oxide and ammonia emissions alongside methane.
Our analysis shows that acidification can be implemented at a much lower cost than most other manure mitigation technologies. We also found that it is easier to integrate into small and medium-sized farm operations with minimal changes to existing practices. However, there is virtually no farm-level data on its effectiveness in the U.S., and only a few trials are underway.
A technology this promising warrants more pilot projects and on-farm trials. These trials should assess effectiveness as well as practical barriers — such as odor, infrastructure corrosion risk and safety concerns — while quantifying co-benefits, like ammonia reduction. Engaging farmers early in the process will be key to building collective trust and informing adoption.
Manure Aeration
Manure aeration involves bubbling air through manure storage using a circulator or pump. The presence of oxygen creates “aerobic” conditions that suppress methane-producing microbes. Complete aeration, where manure is continuously mixed with air over extended periods, can virtually eliminate methane emissions. However, this approach is energy-intensive and costly, requiring continuous operation of pumps or blowers. As a more affordable alternative, partial aeration involves introducing air intermittently or targeting only a portion of the storage volume. While less intensive, partial aeration can still reduce methane emissions by 40% to 57% and may be more feasible for many farms.
Beyond methane mitigation, aeration offers several practical benefits for farmers. It helps reduce odor by minimizing the smelly byproducts of anaerobic decomposition. It also improves manure handling by breaking down solids into a more uniform slurry that requires less agitation before land application.
Aeration has been widely used for decades in wastewater treatment to manage human waste, but it’s rarely used on livestock farms, likely due to high operating costs. Still, it may be a good fit for swine farms, where manure is often stored in deep pits that are compatible with aeration systems. Aeration is less practical for very large farms that use lagoons, as it becomes more expensive and less efficient at that scale.
While manure aeration has not been widely adopted, advancements in system design and components are starting to make it a more attractive option by lowering costs, improving energy efficiency, and making systems easier to operate and maintain. Some farmers are recognizing the value proposition of aeration, particularly in terms of odor control and ease of manure pumping.
Comparing Manure Methane Mitigation Options
| Manure management option | Methane mitigation potential | Cost per dairy cow, per year |
|---|---|---|
| Anaerobic digestion |
25% to 35% (manure storage emissions only)
|
$317 to $643 |
| Solid-liquid separation | 12% to 65% | $3 to $24 |
| Acidification | 46% to 89% | $6 to $20 |
| Aeration | 40% to 57% | $24 to $79 |
Note: Cost estimates are preliminary.
Adoption Is Limited by Funding and Field Data
Despite promising results in early trials, alternatives to biogas digesters remain underused on U.S. farms. A key barrier is the lack of farm-scale data to build confidence in how these technologies perform in real-world conditions. Most evidence comes from international case studies or controlled trials, leaving major information gaps regarding costs, emissions outcomes and day-to-day management needs in the U.S.
At the same time, limited funding and narrow eligibility requirements prevent many farmers from accessing the support they need to adopt these technologies. Most manure-related funding is concentrated on large-scale biogas systems. Alternatives that may work better for small- and mid-sized farms receive little attention, few incentives, and limited technical assistance and training. As a result, the most accessible and potentially cost-effective solutions remain out of reach for most producers.
Bringing Solutions to Farmers
Cost and technical barriers aside, on-farm solutions will not work without farmer buy-in. Methane reductions will contribute to a more stable climate and improved agricultural outcomes in the long term. However, farmers often do not see these technologies as offering immediate financial gains or increased productivity. This can make widespread adoption challenging, especially among those managing tight budgets and unpredictable incomes.
The good news is that these technologies can offer tangible benefits for farmers. For instance, solid-liquid separation can provide practical and financial advantages: Reducing lagoon volume lowers cleanout frequency and cost; enhancing fertilizer value allows more targeted nutrient use and reduces the need for synthetic fertilizers; and separated solids can be reused as animal bedding, avoiding additional purchases. Acidification can reduce ammonia levels in the air, which can lower respiratory stress for animals and promote better health for workers and nearby communities.
Promoting these co-benefits and boosting adoption will require involvement from a wide range of stakeholders, including farmers, extension specialists, manure services contractors, companies with food and agriculture supply chains, and policymakers.
- Farmer associations can help raise awareness and connect members to available incentive programs or pilot opportunities.
- Companies with food and agriculture supply chains aiming to reduce scope 3 emissions can support adoption by engaging their suppliers, incorporating these practices into their procurement standards, funding trials through supplier programs, and participating in broader partnerships.
- Policymakers and funders should prioritize grant programs and public-private partnerships that support on-farm trials, technical training and open sharing of results.
- Manure services contractors will play a central role in implementing technologies like acidification. They should be engaged early through training, equipment support, and targeted incentives to help deliver solutions that work in practice.
- Environmental organizations and governments will need to work together to coordinate these efforts, helping to simultaneously reduce GHG emissions from manure management and improve economic benefits for farmers. They can also help ensure that tools and standards are in place to measure and track emissions reductions across various systems in a consistent way.
Manure management is a growing climate challenge that demands more scalable and cost-effective solutions. While digesters have dominated investment in the U.S., their high costs and limited effectiveness underscore the need for alternatives. Promising options like solid-liquid separation, acidification and aeration offer potential for meaningful methane reductions and some on-farm benefits. Yet these technologies remain underutilized and still involve net costs. Expanded piloting is needed to evaluate real-world performance and economic viability. Broader adoption will require targeted financial incentives, and private sector investment will be essential to accelerate progress.
To learn more, read our new working paper.