The Why Files The Why Files --

Putting decay to work: Making methane from trash

Excellent digestion on the farm?

For years, some farmers have been tempted to use bacteria to digest poop from their cows to produce natural gas, and capture the energy it contains. After all, municipal sewage systems digest oodles of human poop and burn the methane to make electricity for their operations.

But the few farmers who have built manure digesters often found that the complex machinery meant to make methane from massive mounds of manure are monsters to manage, and many meth-makers have been mothballed.

Why bother digesting manure instead of spreading it directly as fertilizer on farmland? Because digesters can:

• Produce methane to power vehicles or electric generators;

• convert manure into a fertilizer that is easier to use;

• reduce energy costs required to spread manure on fields;

• stanch odor problems; and

• reduce the release of methane, a powerful greenhouse gas, into the atmosphere.

That's a nice list of benefits, but a weighty and familiar factor burdens the other side of the equation: payback. One test of the profitability of manure-powered methane-maker has been operating for a year and a half at the Crave Bros. dairy farm, in south-central Wisconsin. The system is owned and operated by Clear Horizons LLC, a subsidiary of a firm that makes wastewater treatment systems for municipalities and industries.

Clear Horizons figures its experience will improve reliability, says general manager Dan Nemke. "A lot of these [farmer-built] systems were poorly integrated. You have a digester system, an engine-generator, pumps and pipe, and a solids separator. … we incorporate all the pieces and put in the control system."

White metal boxes connected to a shed by conveyor belt
Courtesy Clear Horizons Clear Horizons
This manure digester, at the Crave Brothers Dairy Farm near Waterloo, Wis., is a hub of beneficial biological activity.

Tanking up

The center of the system is the anaerobic digester -- think giant swimming pool with slightly murky water -- where countless bacteria convert volatile organic compounds in the manure into methane. (These bacteria, called anaerobes, hate oxygen, and are quite content to live in a world of liquid feces.)

The farm delivers manure from the herd (800 milk cows and counting) to the digester, where bacteria get about 25 days to engorge themselves on the foul brew.

Clear Horizons collects the gas that rises to the top, which is about 55 percent methane, cleans it and pipes it to a piston engine that drives a 230-kilowatt generator connected to the utility grid. This generator, operating full blast, could power about 230 homes.

Clear Horizon built and operates the plant, and earns its money from electricity sales, says Nemke. The farm gets the solid and liquid remains after digestion. The solids may replace straw as bedding for the cows, and the liquid is spread as fertilizer on the cropland.

Mound of a sandy substance inside a storage shed
Courtesy Clear Horizons Clear Horizons
The solids left after bacterial digestion can replace straw as bedding for cows.

Leading by the nose?

Anaerobic digestion has many potential benefits, but for some farmers, especially those facing encroachment by suburbs, stench control may top the list. Digestion produces a "drastic" reduction in odor, Nemke says. "We've seen [smelled?] very minimal odor. In previous years, you would have had a hard time standing near a field while they were spreading manure."

“It stinks less, that’s one of bigger comments I have heard,” agrees Carrie Laboski, an assistant professor of soil science at the University of Wisconsin-Madison. Bacterial digestion, usually using a mix of anaerobic and aerobic bacteria, at sewage districts similarly attenuates the odor of human feces, we should add.

Blonde woman with glasses standing in front of corn
Photo: University of Wisconsin-Madison
Soil scientist Carrie Laboski in a manure-treated corn plot. The probe she holds contains a soil sample.

The ultimate fate of virtually all manure, whether raw or digested, is to be spread as fertilizer on cropland. What does digestion do to the fertilizer value of manure, especially regarding the vital plant nutrients nitrogen and phosphorus?

The answer is a bit obscure.

Nemke says the fertilizer value increases, primarily because organic nitrogen (molecules containing nitrogen bonded to carbon and other elements) is converted to inorganic forms -- either ammonium (NH4), or nitrate (NO3), which are more available to plants. "The farmer is able to utilize the liquid throughout the growing season, and because it's inorganic, the crops can take it up," says Nemke.

Could be, responds Laboski, who studies changes in nitrogen and phosphorus during digestion. She says her “very preliminary” results suggest that anaerobic digestion raises the level of inorganic nitrogen in the liquid, but reduces it in the solids.

Pollution solution?

While both nitrogen and phosphorus are critical plant fertilizers, phosphorus is more tightly linked to over-fertilization of surface waters, and damaging blooms of algae.

Due to concerns about polluting the Mississippi River and Gulf of Mexico, a good number of state governments have limited applications of phosphorus (whether in fertilizer or manure). This can be a big problem for large animal farms, which must (or at least should) haul manure long distances to avoid overloading fields with phosphorus.

However, many uncertainties remain regarding the plant availability of phosphorus, Laboski says. “Theoretically, if you are breaking down organic compounds, you probably will be converting organic phosphorus to inorganic and perhaps increasing the amount of phosphorus that plants can use. But I cannot say for certain, we are working on that now.”

10 black cows with their heads through a fence feeding
Photo: USDA
The opposite end of the cow is the source of milk -- and, ahem, "waste products."

Money matters

Although digestion by itself does not remove any phosphorus, the dry stuff remaining after digestion can be sold as fertilizer, as the Clear Horizon installation in Wisconsin is doing, which reduces the amount of phosphorus sent to farmland. Furthermore, because the solids are lighter than liquid manure, farmers can afford to haul them further from the barn, to fields where the phosphorus level is lower.

The one drawback, to date, appears at the bottom line: there is not much money to be made. "It's about a breakeven process now," admits Nemke. "If the price of electricity goes up, that definitely helps the economics of the situation," but he suggests it may also be possible to reduce costs by establishing a string of digesters to serve large farms in an area.

Many people say you need at least 1,000 cows to justify a bacterial poop-eater


Terry Devitt, editor; Nathan Hebert, project assistant; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

©2021, University of Wisconsin, Board of Regents.