Are manure digesters the real solution to dairy farm emissions?

When fuel ran out during World War II, some farmers in Germany and France made own fuel by covering manure tanks and capturing the methane that is produced.

Now governments are pushing an improved version of this technology, called anaerobic digesters, as a way to reduce greenhouse gas emissions on dairy farms. But some scientists say spending on hoods could have unintended consequences for the climate and human health.

“Is this money more effective at reducing the climate than other strategies like building solar panels?” he says Rebecca Larson at the University of Wisconsin-Madison. “That’s something that should be explored … but for livestock, it’s one of the most powerful mitigation measures we have.”

Agriculture charges about one-third of human-caused emissions. In the US, about a third of that comes from cow burps, but another 14 percent comes from manure. Industrial dairy farms must constantly scrape and flush colossal amounts of manure from vast barns full of cows into lagoons.

The first commercial-scale digesters to cover these lagoons or replace them with tanks appeared in the 1970s. More than 17,000 hoods have now been installed in the European Union, mostly on farms and OUR and United Kingdom each has about 400. China has millions but these are mostly brick hoods on small farms.

When organic matter is stored without oxygen, anaerobic microbes break it down and release carbon dioxide as well as methane. This occurs when wastewater is held up in sewage treatment plants or manure is washed into lagoons and sumps.

However, if the waste is covered with plastic or placed in a closed tank, CO2 and methane biogas can be captured through the pipeline. This digester is typically heated to accelerate the production of biogas, which can be burned for heat or electricity, cleaned for natural gas, or compressed for vehicle fuel. While CO2 is still emitted, the even more powerful greenhouse gas methane is not. The digested manure is then used as fertilizer and bedding for animals.

Manure that has passed through the digester emits 91 percent less methane during storage. But the big picture is more complex, according to a new study that analyzed methane plumes from 98 dairies in California. State which has 1.7 million dairy cows on farmsmore than anywhere else in the U.S., it has awarded $389 million in grants over the past decade to build fume hoods, its largest methane initiative.

The installation of the fermenter reduced point source methane emissions from an average of 91 kilograms per hour to 68 kg/h, reducing emissions in two-thirds of the dairies. However, average emissions rose briefly during the construction of the digesters. Although the reason was not clear, one possibility is that the slurry had to be diverted, churning it up and causing the emissions.

Because digesters are heated, they produce methane faster than open lagoons, and leaks from them can in some cases lead to even higher methane emissions than before. Some leaks exceeded 1,000 kg per hour, the study found.

“At the rate they’re leaking in these very large cases, it’s absolutely a cautionary tale of how something can come from a solution to a major emitter,” he says. Alyssa Valdez at the University of California, Riverside, one of the authors of the study.

However, a California program that notified farms of leaks in 2023 resulted in 20 percent of them being repaired, and most research suggests digesters can still reduce manure emissions by about half.

“If you’re running a fume hood and you’re losing gas, you’re losing money, so that drives fume hood operators to minimize emissions,” he says Angela Bywater at the University of Surrey in Great Britain.

Digesters also accelerate the formation of ammonia from manure, but this raises concerns about “pollution mistaking” methane for ammonia. And if biogas cannot be sold and is burned, it can produce hydrogen sulfide in some cases.

The big question is how many governments should support hoods. California’s support for them seems to be encouraging factory farms to get even bigger. Its Low Carbon Fuel Standard, a program to reduce automotive emissions, issues marketable credits for biogas produced from hoods. ON preprint a study found that incentives like these increased dairy herd size by an average of 860 cows.

“Taxpayer dollars are being used to inflate the value of manure so it competes with the value of milk, and that creates a perverse incentive structure,” he says. Brent Kim at Johns Hopkins University in Maryland. “We could and should be looking for more proven and effective approaches to mitigating climate change that are not supported by an industry with a wealth of literature documenting harm.”

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