Are Manure Digesters the Ultimate Solution for Reducing Dairy Farm Emissions?

During World War II, farmers in Germany and France found innovative ways to harness energy. They covered fertilizer reservoirs to capture methane and secure their own fuel supplies. Today, anaerobic digesters, the advanced version of that technology, are promoted by governments as a means to mitigate greenhouse gas emissions from dairy farms. However, researchers warn that investments in these digesters may lead to unintended consequences for both climate and public health.

Rebecca Larson from the University of Wisconsin-Madison asks, “Is this funding more effective for climate change mitigation than other strategies, like solar panel installations?” She recognizes that although digesters are effective for livestock emissions, it’s crucial to explore all options.

Agriculture is responsible for approximately one-third of global human emissions, with cow burps contributing one-third of that in the U.S. alone. Industrial dairy farms manage large quantities of cow manure and often flush it into lagoon systems.

The commercial-scale use of digesters began in the 1970s. Now, there are over 17,000 digesters primarily on farms in the European Union, while the U.S. and England each have around 400. In China, millions of small farms use brick digesters to optimize waste management.

When organic waste decomposes anaerobically, it not only releases methane but also carbon dioxide. This is typical in sewage treatment plants and waste lagoons. However, utilizing sealed digesters allows for the recovery of biogas, which can be generated more efficiently and utilized for heat, electricity, or even as natural gas for vehicles. This process mitigates methane emissions, a greenhouse gas significantly more potent than CO2. The digested waste is then repurposed as fertilizer and bedding.

Following digestion, methane emissions from stored fertilizers can be reduced by up to 91%. Yet, a recent analysis of methane emissions from 98 California dairy farms indicates a more complex scenario. Approximately 1.7 million dairy cows are housed in factory farms across the state, which has invested $389 million in digester construction—more than anywhere else in the U.S.

Although digesters reduced point-source methane emissions from 91 kg to 68 kg per hour across two-thirds of the participating farms, emissions spiked temporarily during construction. This anomaly remains unexplained, potentially tied to fertilizer slurry disturbances during installation.

Due to their heated environments, digesters may produce methane at a faster rate than traditional lagoons and can occasionally leak. The study found that in some cases, leakage rates were over 1000 kg per hour, highlighting a potential risk in efficiency.

Alyssa Valdez from the University of California, Riverside, emphasizes that high leak rates serve as a warning. Despite this, California’s leak notification program remedied 20% of identified issues. Studies suggest digesters can still reduce fertilizer emissions by about half.

“When gas leaks occur, operators incur financial losses, creating an incentive to minimize emissions,” states Angela Bywater from the University of Surrey, UK. However, digesters can also lead to increased ammonia production from fertilizers, raising environmental contamination concerns.

The prevailing debate focuses on how aggressively governments should support digesters. California’s favorable policies appear to encourage the growth of factory farms, as incentives are linked to biogas production under the Low Carbon Fuel Standard. According to a preprint study, receiving such incentives can increase dairy herd size by an average of 860 cows.

Brent Kim of Johns Hopkins University warns, “Taxpayer funding that inflates fertilizer value may distort market dynamics, making fertilizers more valuable than milk. We should consider viable climate change solutions that don’t inadvertently sustain harmful industry practices.”