When you travel through Vermont, tree-lined farms and hills dominate the landscape. Throughout this patchwork lies hundreds of producers that, until now, have been cut off from the advancing world of manure digestion, simply because people have said their operations are just too small. With the backing of a renewable energy company, Avatar, and a desire to prove everyone wrong, Dr. Guy Roberts is about to change the face of anaerobic digestion and its implications for smaller operations.

“My interest into digesters was piqued when I attended a conference in Syracuse, New York,” Guy said. “There were five farmers who were asked why they built digesters and the first answer from each one of them was for odor control. That is when I realized that farmers are under all kinds of pressures and it is kind of ridiculous to concentrate on only biogas for energy. There are all kinds of benefits for farmers, from odor control to gas for heat and electricity. Nutrient management and air quality also come into play with digesters, so finding a way for every farmer to take advantage of this kind of technology became my focus.”

But getting from an idea to a working model and from a model to a full-scale operating system has proven to be quite a challenge. But, as Guy explained, the journey has been enlightening and helpful in creating the final product.

“I have met with several people familiar with digester technology and most of them have said that a small-scale digester isn’t feasible. They continue to go back to the need to produce electricity, forgetting there are so many benefits to a digester beyond electrical output. Something didn’t strike me about the whole economy of scale argument that kept coming up in the conversations.

“As I kept looking into current technologies, it finally came to me that the real economy in creating digesters was to come up with a model that would work for just about any situation” Guy added. “We needed to come up with a sort of kit that would work into any situation, any size. The economy didn’t come in making it bigger, but rather making it fit into the system in place – something smaller, leaner and easier for everyone involved to work with.”


Guy’s desire to make a digester that not only worked, but was easy to work with was also at the forefront of his planning. He realized that unless he could create a process and products that works for the farm, you really haven’t helped anyone.

“Another reason for developing this model was to find a way to make it simple and easy to work,” Guy said. “Producers are already tasked beyond what they can do in a day without adding the hassles of digester maintenance on top of it. It had to be easy to put together, easy to work and easy to maintain.”

With those desires has come the need to completely rethink traditional digester design. The final design is something totally new, yet captures the main ideas needed for a good digester to work.

“The first thing that struck me was to make the digester above-ground. I had seen too many examples that showed in-ground digesters were too prone to problems that required excessive time to clean and maintain. They also had the tendency to develop leaks that went too long undetected. Getting everything above-ground would give people a first-hand account of what was going on all the time, not to mention the ease of keeping it maintained.”

“The hardest part of the design was the control system and the elevated tails to keep the pressure for gas production and consistent gas flow,” Guy added. “The choice of materials was also a learning curve for us, but we have found a mix of materials that look to handle the continual processing of manure. While it has been a long road to go down, what we have seen has been very promising and working well for us.”

Although this digester system produces methane as other systems do, it is the treatment and use of the effluent that starts to set this system apart from the others.

“One of the biggest discoveries in looking at the bacteria within the system and the effluent was what happens to the nitrogen once methane has been produced,” Guy added. “We realized that through the digestion process, a lot of nitrogen was being converted over to ammonia. This ammonia was quickly dissipating from the effluent, resulting in a tremendous loss of nitrogen as a fertilizer while adding volatile compounds back to the atmosphere. Neither result was something we wanted to see happening.

“We have tested and are now implementing a simple aerobic process after digestion that allows us to take the effluent and run it through a bacteria-sustaining medium,” he said. “This allows the microbes to convert ammonia to nitrate, allowing the nitrogen to remain in the system and not escape to the atmosphere. I am not aware of any digestion system out there that allows for this type of nitrogen control from beginning to end.

“Most recently, we have been looking at phosphorus removal systems that could easily be added after the nitrification process,” he continues. “In essence, with both systems added to the back of our digester, producers would have a very effective tool in knowing what type of effluent they had to return to their fields and be able to make adjustments according to their needs. Since the nitrogen is in a nitrate form, there is the potential for runoff from the farm. But it will also allow producers to apply nitrogen throughout the growing season, giving them the opportunity to match crop needs and growth with a readily accessible nutrient source. Any excess nutrients could be sold off to other farmers or homeowners. It just makes the system, as a whole, a much better management tool.”

Because of its above-ground, tubular design, the digester is scaleable to nearly any farm with 100 or more cows. This innovation, not considered before in digester designs, is something that can truly make a difference for farms across the country. When asked about the future and how soon this system would be available, Guy responded: “We are seeing it now, today. The system is in and working well. We have in place what we need to begin placing these systems on dairies throughout the U.S. I am excited with what I have seen, what the possibilities are and that what I had hoped for is becoming a reality in this building. It is only up from here.” ANM