The downsides of dairy digesters

Excluding the decline in herd populations—which has been driven by growing international competition and rising costs—the vast majority of California’s estimated methane reductions come from the use of what are known as anaerobic digesters. This technology entails covering the slurry lagoons to prevent methane from leaking into the air and then piping the biogas into separate vessels, where it’s cleaned and converted into natural gas. Under California’s Low Carbon Fuel Standard program, dairies that use digesters to produce gas delivered into pipelines can earn credits and sell them to petroleum refineries and other major polluters, as a means of helping those companies meet their own emissions reduction requirements. The gas can then fuel power plants, produce hydrogen, or power natural-gas vehicles. These uses still release carbon dioxide, but the state considers it a climate win because it avoids the release of methane, which traps even more heat.

The rich revenue stream from California’s program has spurred hundreds of US farms to install anaerobic digesters over the last decade. Since 2020, it has produced more than $1 billion for farms, Cal Poly researchers noted in a paper last year. But there are a variety of concerns about this approach. The first is that it’s viable only for farms with about 2,000 cattle or more, because the equipment is very expensive to install, says Frank Mitloehner, a professor and chair of the Department of Animal Science at the University of California, Davis. “For the lion’s share of dairies, digesters will not be a solution,” he says.

Since the manure is often still spread across fields, digesters also do little to address the water pollution problems—and can even exacerbate them because of some of the chemistry that occurs during that process.