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Biodigesters On A Small(er) Scale

Researchers at Ohio State University (OSU) recently displayed their small-scale, biodigester technology at the Farm Science Review, an event held in London, Ohio, at the end of September. The idea of generating an unlimited supply of energy on the farm attracted a lot of interest. The experimental system converts cattle manure from the university’s research dairy farm into methane that can fuel a variety of on-farm systems, such as heating water for washing inside the barn.

While the digester concept isn’t new, looking into the viability of small-scale systems and adapting them to the needs of the average North American farmer is. “The digesters here, in the United States, are pretty large and mechanically complex,” says Jay Martin, an associate professor at OSU who is leading the research.

“That technology is only suited for dairy farms that have about 1,000 cows,” he says. “But the average-size dairy in the U.S. only has 100 or 150 cows, so there was a big need for this. Talking with some of the farmers, there’s a lot of interest but most get discouraged because they find there is no technology that meets the needs of average-size farms.”


Martin hopes to change all that by building on his experience with small-scale digesters used in warmer countries, such as Costa Rica. “With that system, a family with four or five cows is able to generate all the gas they need to cook their meals. Some even use it to heat water for showers and other things,” he says. “Our goal was to take the technology that exists and adapt it to something that will work here.”

The experimental-scale digester installed on the university’s farm just began working in the spring of 2010. Even with a relatively small 300-gallon capacity, Martin says the amount of methane produced has been good. But because the microbes that produce methane are affected by temperature, researchers want to see how it performs this winter before declaring it a success. “The key thing will be if we can get good numbers through the winter,” he says. “I think we can, based on the colleagues I’ve talked to in China and the designs in use there.”


If the digester performs as expected, Martin believes an average-size dairy could produce enough energy to heat all the wash water used to clean barns for an initial investment of $10,000 to $20,000. And unlike large digesters, the university’s prototype is based on a very simple design, so farmers could set up the system and operate it themselves.

“What we’d like to do is come up with a kit,” he says. “We’d like to work with a manufacturer that can build the digestion tank. Our goal is to make it something a farmer can install himself and keep costs down. We also want to design something that won’t take a lot of man-hours to maintain.”

The amount of methane produced by any digester depends on how much carbon goes in. “It’s that carbon that is actually turned into methane by the microbes inside the digester,” says Martin. “The most important variables are: the amount of carbon that comes out of the cow, which depends on what type of cow and what it’s eating; how the manure is collected; and the temperature of the digester. So, there can be a large variation in the amount of gas produced.”

And while most digesters are used in dairies, and OSU’s research has focused on that segment as well, Martin says any type of operation where livestock manure is concentrated, such as a feedlot, could use a digester. “Anywhere you can collect the manure, you can use a digester.”


Establishing the proper type of microbes in the digestion tank is an important first step when setting up a system. Martin and the students working on the project had expected their digester to grow its own population from only a small seed stock, but that met with only limited success.

“We tried to start it up with a very small amount of inoculant, but we found out that didn’t work very well. Later on we added a lot more from an existing system and that worked,” he says. “You can get it started by taking raw manure, but it takes a while and is a lot more temperamental.”

Establishing the correct microbial population is easily done, though, by just adding some effluent from another digester. That effluent (the remaining manure left over after digestion has occurred) also makes a very high-value fertilizer, because the organic nutrients have already been mineralized and are immediately plant available, unlike those in raw manure.

The most common use for large-scale digesters is to use methane to run a generator and produce electricity, but Martin is looking for a more direct way to use the gas from the OSU system, which also makes for a less complicated and cheaper system.

“That’s why I keep referring to heating water,” he says. It’s the easiest way to use the fuel produced. If the methane is to be used as fuel for an engine it has to be taken from low-pressure containment in the digester to a high-pressure storage system. “When you start adding in pressurizing the gas, cleaning it, and the generator itself, the system cost rises pretty rapidly.”

Martin believes development of a do-it-yourself kit is likely a couple of years away. The experimental model will be evaluated after a full year in use. “If we get good data we’ve already identified a few farmers to implement farm-scale tests. Then we’ll monitor those. After that we’ll get some kits built.”


Just getting cheap energy isn’t the only advantage a digester like OSU’s can offer. “There are some other benefits. One of them is the reduction in greenhouse gases,” Martin says. Burning the methane and converting it to CO2 is much less harmful to the atmosphere and mitigates the much-publicized gas contribution from livestock manure. “Methane is a greenhouse gas that is 21 to 25 times worse than CO2,” he says.

And farmers could reap an extra financial advantage for their trouble. Martin notes farmers using large-scale digesters in the U.S. are already getting payments from carbon credit exchanges.

“Smaller farmers could potentially get those payments as well,” he says. And even those with systems too small to qualify on their own may meet the minimum threshold if they band together. “What I see is a co-op of four or five smaller farmers in a given region that go together and as one unit apply to the carbon exchange to get those payments.”


About the author


Scott Garvey

Scott Garvey is a freelance writer and video producer. He is also the former machinery editor at Grainews.



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