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Like A Cow's Stomach Magnified, Methane Digesters Make Energy, Reduce Waste

In a few years all Vermonters will be required to keep food waste out of landfills, but for some institutions that create a lot of food scraps, those requirements kick in a lot sooner. While some of that food waste will be composted, under a new pilot project some of it will be headed to anaerobic digesters that create energy, including the one at Vermont Technical College in Randolph.

Chris Dutton, director of the Institute for Applied Agriculture and Food Systems at Vermont Technical College, said the digester is "essentially a cow's stomach magnified by about  100 or 200 or 300. It's a big tank that's kept to the same temperature as a cow, about 101 degrees that stirs liquids round and round so bacteria can grow and digest -- that's why it's called a digester -- carbon-based materials into methane, which then can be piped into a generator and the gas explodes, turns pistons, and turns the generator which creates electricity while making heat as well."

It's similar to composting, Dutton said. For compost, you need oxygen and aerobic bacteria. When that bacteria is exposed to oxygen and carbon based materials, it breaks down, making a good soil additive.
The bacteria used in methane digestion don't need oxygen to break down the carbon-based materials, and the gases are captured to make electricity in a generator.

The Vermont Tech digester has been running for about a year, and it's been a learning process. The digester started making electricity in early June. "We're hoping to get the thing ramped up to 375 kilowatts for about 90 percent of the time," or 23 hours per day, Dutton said. "It's enough to power the whole campus with electricity and heat well one or two buildings."

The digester is fed with manure from the school's farm and one neighboring farm, together around 150 cows.

"That manure's actually not that good a feedstock for the digester in terms of its carbon quality, but what it has, because it's coming from cows, it has the bacteria that we want to use to continually populate our digestion process." Dutton said they can take carbon from anywhere. They can add grass, food waste, pre-consumer food byproducts, brewery waste.

A large composting company, called Grow Compost, has won a $131,500 Clean Energy Development Fund grant to start hauling food scraps from businesses and institutions to Vermont Tech's digester. Currently, the school's permit only allows them to use 1 percent food waste in their digester. They hope to get a permit this spring to increase that amount.

The restrictions come from concerns over the spread of bacteria that exist in food waste, and it needs to show that those bacteria break down. "I'm quite confident that all those things go away," he said.

"We're really excited to be working with Grow Compost because we can sort and separate materials based on their ability to break down in the right system," Dutton said.

Each day the digester is fed with 8,000 gallons of manure on farm feed stock, and 7,000 gallons of other things. "We like liquid stuff. This is what's convenient, those are hard to compost because they run across the soil and they can't be composted as well. So if we put them in a tank and convert them we can be more successful and better to the environment," he said.

Vermont has a number of methane digesters, primarily at large farms. Casella Waste Systems also won a $139,000 grant to take food scraps to a digester at Blue Spruce Farm in Bridport.

Dutton said he hopes to see the technology spread, as it's a good way to manage food waste and it's good for farms. "It essentially brings in fertilizer to the farm rather than us having to burn oil to create fertilizer to go to those farms," he said.

And there's post-digestion technology that's also exciting. Besides the carbon, there's a tea-colored liquid that's created. "That contains phosphorus, potassium and nitrogen, and we run that through a separator and we can get more than 50 percent of the phosphorus to come out in what's called the solids, and in doing so we can keep the phosphorus off the ground and out of the lake," he explained. "Because we've worked to create this very study-able, very controllable stream, I think the opportunities are kind of endless."

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