“Big Bertha”

VTC generates research results along with electricity, heat, and fertilizer.

Vermont Tech Community Anaerobic Digester or VTCAD

Written By

Tatiana Schreiber

Written on

May 15 , 2017

Anaerobic digesters (ADs) have been sprouting up on Vermont landfills and farms over the past 10–15 years, with a few even older. In an AD, microbes that can function without oxygen break down organic materials such as animal manure and food wastes, producing “biogas” in the process. Biogas is made up primarily of methane and carbon dioxide. The process also generates heat and creates the “digestate,” which can be separated into a liquid material for use as a fertilizer and dry material that can replace sawdust for animal bedding. In a well-functioning digester, the nutrients from manure are retained in the digestate and can continue to nourish crops. The methane can be used to produce electricity.

The one dubbed “Big Bertha” at Vermont Technical College (VTC) is certainly big—it’s made up of two huge green metal silos (one for an initial aerobic process, the second for the anaerobic digestion), additional buildings that house the feedstock (the organic materials that go in), the separating machine, and the giant engine that generates the electricity. There’s also a storage tank for the solid digestate and a large pond that holds up to three million gallons of liquid digestate intended for use as fertilizer. After more than five years and 4.5 million dollars getting to this point, VTC operators are still working out the kinks of a very complex facility. The last component, using the waste heat to warm four campus buildings, needs even more funding to be completed.

Project Manager Mary O’Leary, a civil engineer and VTC faculty member, says everyone involved is constantly learning. “We’re learning to develop the best recipe to keep all the ‘methanogens’ [microbes that produce methane] happy and functioning,” she says, pointing out that Big Bertha is a “two-stage complete mix digester,” the only one in the state so far, which means it can take in both agricultural waste and food waste (like what’s left over from beer processing), and the exact amounts of each component need to be carefully adjusted.

Because Big Bertha (whose official name is the Vermont Tech Community Anaerobic Digester or VTCAD) is completely integrated into VTC’s educational mission, all of this learning is reflected in data that is available to the public, primarily via a website: vtc.edu/meet-vtc/anaerobic-digester. There one can read an interim report from 2015 that details all stages of the project’s development and the trials and tribulations along the way, such as odor problems, clogged pipes, the dissolution of one of the principle construction companies involved, and the challenges of permitting a facility that had no prior precedent in Vermont. In addition to sharing its research, VTC is also committed to training its students in anaerobic digester technology. There’s an apprenticeship program and several courses that are part of the college’s bachelor’s degree in renewable energy and agriculture.

On the Farm

VTC Field Foreman Charlie Dana has been farming for most of his 70 years. He was a bit wary of using the AD digestate as bedding in the farm’s free stall barn because it’s derived partly from manure. “I didn’t think we ought to bed our cows on manure,” he said. “I was afraid of environmental mastitis, but it hasn’t happened.” In fact, they’ve seen a decrease in mastitis, perhaps because, since the bedding is free, it can be laid in at more depth than the more expensive sawdust or sand, and as soon as it gets soiled, it is raked out and new bedding is added. The soiled material goes back to the digester and eventually gets broken down enough to become liquid. The bedding material is soft and moist, smells pleasant, and looks like a grayish, crumbly dirt. “As soon as we put it in there the cows went in and lay down,” says Dana. “They’re very comfortable on it.”

The liquid digestate is spread on the farm’s fields and that of a neighboring farm. Here too, Dana has been pleased with the results. “It soaks into the ground immediately,” he says. “After we spread it we can walk over the field in 15 minutes without getting manure on our feet…it’s getting to plant roots more quickly, and I think we’re losing less nutrients to evaporation.” There’s less odor than with liquid manure, and the ratio of nitrogen to phosphorus is higher, so more can be used without risk of phosphorus run-off. Yields have increased, although the exact amount hasn’t yet been calculated. Dana says last year hay was cut 30 days after each round of fertilizing. “Our silos were full, our barns were full, and it makes our bottom line look better,” Dana says. “It makes the people over on the campus like us better too.”

Showing financial viability, is, of course, one of the goals of the VTCAD. The initial funding was secured from the U.S. Department of Energy and a bond from Vermont State Colleges. Additional funding through the Clean Energy Development Fund (made available as part of the closure of the Vermont Yankee nuclear power plant) helps with collection and management of food wastes for Big Bertha. These funding sources and others through the USDA and EPA are likely to be limited under the new administration in Washington, so if community-scale digesters like this one are to be replicated, they must demonstrate that they can generate income in the long run.

Project Manager O’Leary notes that the VTCAD runs 24/7 and produces 370 kWhs per hour. VTC has a 20-year contract with Green Mountain Power guaranteeing a rate of 12–14 cents per kWhs, so the plant can earn $25,000 to $30,000 a month in electricity revenue. There’s the cost savings on animal bedding and eventually, when the heating system is functioning, money will be saved there also. It’s unlikely, however, that a large AD like Big Bertha could be replicated on smaller farms. Smaller co-digesting ADs can be built, and many are operating in Europe, but so far the appropriate technology is not readily available in the U.S. according to O’Leary.

Collecting Feedstock

Some of the benefits of a community-scale AD go beyond financial, however. Vermont has a goal of achieving 90 percent renewable energy by 2050. In addition, Vermont’s Universal Recycling Law (Act 148) will ban food scraps from landfills as of July 1, 2020. As of this July, food scrap generators of 18 tons per year or more must divert material to a certified facility within 20 miles. Big Bertha is playing a role in achieving that goal by taking food processing waste from a brewery and from Ben & Jerry’s ice cream production. Glycerol from biodiesel production is also part of the mix. Grow Compost of Moretown is handling the food waste collection and the process of “pulping” it for Bertha’s use. The biggest problem, according to Grow Compost’s Lisa Ransom, is plastic. “A lot of biodegradable service ware that is being ‘green-marketed’ is full of plasticizers,” she says. “Our goal is to collect food waste from the food system and put that back in the soil to grow more food. We work hard to help people understand that plastics, and even food stickers, are a hazard to our food system.”

ADs vs. Composting and other questions...

To those whose focus is soil health, it may seem incongruous to haul food waste and animal waste to a facility like Big Bertha rather than compost it for direct use on farms. Compost, as compared to liquid digestate, can increase soil organic matter, which is critical to improving water retention and decreasing erosion and nutrient run-off. On the other hand, nutrients, especially nitrogen, can be lost in the composting process, and Grow Compost’s Ransom points out that “There is way more material than we are able to compost.” The goal, Ransom says, is first and foremost to reduce food waste, next to get food waste to animals (such as chicken feed), next to compost it, and only after that to take it to digesters. “It’s going to require every single partner, including ADs in every corner of the state…We need farms, composters, food shelves, and digesters throughout the state, so we’re not hauling stuff across miles and miles.” In the meantime, Ransom is hopeful: “We see Vermont going in the right direction, toward a more sustainable future, and we’re excited to be part of this work…we have a roadmap and we have to figure it out as we go.”

Big Bertha was the first digester to be permitted in Vermont as a solid waste facility that collects food waste, and it has led to new permitting systems and reporting requirements. Many questions still must be worked out regarding whether facilities like this will be able to collect “tipping fees” from food waste haulers, what rates power companies will be willing to pay for electricity from co-digesting facilities in the future, and which regulations are needed if facilities want to sell digestate as a fertilizer. But one thing is clear: Big Bertha is slowly but surely generating answers to these questions and more, and that may be as important a product as any other.

About the Author

Tatiana Schreiber

Tatiana Schreiber

Tatiana Schreiber is a research associate at Rich Earth Institute in Brattleboro, and also consults on complementary plants for growing with solar arrays. She teaches ecological agriculture at local colleges and grows heirloom and unusual garden seedlings including medicinal plants at Sowing Peace Farm in Westminster West.

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