Manure with a side of pizza
Anyone thinking about a manure digester might envision a massive unit on a large livestock farm. Matt Steiman, assistant farm manager at Dickinson College, explained that isn’t the case at the Carlisle, PA, farm.
“This is a unique system,” Steiman said. “Most digesters are on large farms with 500 cows or more. We’re trying to demonstrate a small digester model like the ones in Europe. We’re in the first year of operation of making power – the power on the farm is free and we’re being paid for the excess.”
The project is the result of Steiman’s hard work and other factors coming together, including a neighboring dairy farm that needed a solution for manure management and additional animal housing. After several years of planning, all involved are reaping the benefits.
Digester projects are notoriously pricey. The price tag for the Dickinson project was $2.1 million, with the majority funded by outside grants and awards. Funding included an NRCS EQIP grant, a Cumberland County Conservation Excellence grant, an EPA grant and a Pennsylvania Resource Enhancement and Protection (REAP) grant.
While much of the funding came from grants, Steiman found himself immersed in the regulatory side.
“We got a composting permit, a permit for co-digestion of food waste with livestock manure and a National Pollutant Discharge Elimination System (NPDES) permit to develop the site because it’s over one acre,” Steiman said. “We also have a water quality management plan and a building permit from the township.”
The project is small enough to not require air quality sampling, so there’s an air quality waiver for the engine.
The farm’s location – less than one mile from a small town – means there’s extra attention regarding odor. It’s also close to the Yellow Breeches Creek, a high-quality trout stream that feeds the Susquehanna River, making the project eligible for USDA and state funding. The digester project is bound to a conditional use agreement with the township that there will not be an odor problem. (Steiman noted that if there was an odor issue, he would know about it from neighbors.)
A new free-stall barn houses between 100 and 150 heifers and dry cows throughout the year. The barn, adjacent feeding pad, methane digester and other components are situated on land that had been a pasture.
“We started from scratch,” Steiman said, discussing the three-year process. “We converted the pasture to everything here including all the land development for cattle housing, digester and associated equipment.”
Manure in the free-stall barn is scraped into pits at the far end. Each of the two 10,000-gallon tanks is 14 feet deep and considered confined spaces that require an advanced safety protocol for entry.
“When the dairy farmer scrapes, we put liquid in the tank with the mixer running so it liquefies immediately,” Steiman said. “The dairy farm pushes the manure while the mixer is running. We learned the hard way that if we put dry manure in the tank then add liquid later, it’s hard to liquefy.”
Food waste from several sources is an important component of the digester mix. About 2,000 Dickinson students produce about 1,000 lbs. of food waste each day. Food waste also comes from several nearby restaurants, a hospital, a commercial bakery, a brewery and a food bank, making about two tons per day of food waste. Adding this to the digester saves the college money because it doesn’t go to a landfill.
Since the digester requires a certain liquid portion, the food waste tank is supplemented with either water or dairy whey from a large nearby dairy plant. Whey helps as a carbon source – it keeps the microbes happy and making gas.
“Cow manure is a great substrate for making gas but it’s already been through a cow once,” Steiman said. “Cows digest the easy stuff and poop out the stuff that’s harder to digest. It still has gas potential, but on a pound-for-pound basis, food waste yields more energy than cow manure. There’s about two to five more times energy in a pound of food waste than there is in a pound of cow manure.”
The cows supply about 2,000 gallons of microbially rich manure daily. The addition of food waste doubles the energy output and today, the digester is at nearly full power. Without food waste more manure would be needed.
The blend begins with about 7,000 gallons daily. About every two hours, 300 to 400 gallons of food waste is pumped to the digester.
“The digester pumps back to another tank 300 to 400 gallons of digestate after digestion,” Steiman said. “It’s been digested by the ‘beast’ and it comes back to be made into bedding. There’s still a lot of fiber in the liquid that we can press out and use for the cow bedding.”
The fully automated system is equipped with webcams and sensors for remote management. The methane-fueled engine is currently producing about 40 kilowatts or almost 1,000 kilowatt hours per day.
As is the case with many projects, the college farm digester has had a significant learning curve. Steiman took in some 20-ton loads that didn’t work out, but waste from an apple processing facility worked well. The whey has been an easy add because it’s delivered in a tanker and doesn’t require any extra handling.
Dr. Daniel Ciolkosz, assistant professor and research associate, Penn State, listed some potential digester roadblocks from nearby communities. “There’s concern about bringing food waste onto the farm,” Ciolkosz said.
“It’s a collective idea that farms are a source of material, not a destination for material. There are worries that contaminants or too many nutrients end up back on the farm. Farmers are aware of this, and Extension is trying to help farmers understand the potential implications and make sure farms remain balanced from a nutrient perspective.
“There is a lot of push from companies that develop digester projects because of the high amount of biogas from food waste, and they will approach farmers to let them bring in food waste if farmers let them use the farm’s manure. These outsiders may not care about the land or water quality the same way those do who have been here for decades.”
by Sally Colby
