Are manure digesters a real solution to dairy farm emissions?

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

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

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

Agriculture is responsible for about a third of human-caused emissions. In the United States, about a third of that comes from cow belches, but another 14 percent comes from manure. Industrial dairy farms must continually scrape and flush colossal quantities of manure out of vast barns full of cows and into lagoons.

The first commercial-scale digesters to cover these lagoons or replace them with reservoirs appeared in the 1970s. Today, more than 17,000 digesters have been installed in the European Union, mostly on farms, and the United States and the United Kingdom each have around 400. China has millions of them, but most of them are brick digesters installed on small farms.

When organic matter is stored in the absence of oxygen, anaerobic microbes break it down and emit carbon dioxide as well as methane. This happens when sewage is retained in sewage treatment plants or manure is washed into lagoons and slurry pits.

But if the waste is covered in plastic or put in a closed tank, the CO2 and methane from the biogas can be captured via a pipe. This digester is typically heated to accelerate the production of biogas, which can be burned to produce heat or electricity, purified into natural gas, or compressed into a vehicle fuel. Although CO2 is still emitted, methane, an even more potent greenhouse gas, is not. The digested manure is then used as fertilizer and animal bedding.

Manure that has passed through a digester emits 91 percent less methane during storage. But the bigger picture is more complex, according to a new study analyzing methane plumes from 98 California dairies. The state, which has 1.7 million dairy cows on factory farms, more than anywhere else in the United States, has awarded $389 million in grants to build digesters over the past decade, its largest initiative against methane.

Installing a digester reduced point source methane emissions from 91 kilograms per hour to 68 kg/h on average, reducing emissions in two-thirds of dairies. But average emissions briefly increased while the digesters were being built. Although the reason is unclear, it is possible that the slurry had to be diverted, which would have agitated it and caused emissions.

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

“Given the rate of leaks in these very large cases, it’s absolutely a cautionary tale about how something can come from a single-prime emitter solution,” said Alyssa Valdez of the University of California, Riverside, one of the study’s authors.

But a California program that notified farms of leaks in 2023 was able to repair 20% of them, and most research suggests digesters can still reduce manure emissions by about half.

“If you operate a digester and you lose gas, you lose money, so the onus is on digester operators to minimize emissions,” says Angela Bywater of the University of Surrey, UK.

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

The big question is to what extent governments should promote digesters. California’s support appears to be encouraging factory farms to become even bigger. Its Low Carbon Fuel Standard, a program aimed at reducing automobile emissions, issues salable credits for biogas produced from digesters. A preliminary study found that receiving such incentives increased dairy herd size by an average of 860 cows.

“Taxpayer money is used to inflate the value of manure so that it begins to compete with the value of milk, which creates a perverse incentive structure,” says Brent Kim of Johns Hopkins University in Maryland. “We could and should consider more proven and effective approaches to mitigating climate change that do not support an industry with a body of literature documenting harm. »