Re-engineered Dane Co. manure digester model for RNG production

Jan Shepel
Partners in a Middleton manure digester offered tours of the plant, including these large tanks where manure is heated and methane is collected. The plant has recently been retrofitted to produce renewable natural gas rather than electricity.

MIDDLETON – A new era for manure digesters was unveiled to the public last week at a large facility near Middleton. Dane County partnered with EnTech Solutions, Faith Technologies and Northern Biogas to repurpose a digester there to produce renewable natural gas.

The gas has been shipping out of the facility since January, but the partners decided to hold an event to highlight the sustainability and unique nature of the plant around Earth Day.

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The digester was originally designed to take manure from several nearby large dairy farms, add food waste, digest it all and then collect the gases from that waste stream in three large tanks with domes covering them. In the original concept, that gas was then run through two large motors to produce electricity which was sent on to the grid.

Today, the system has been retrofitted to eliminate the food waste from the process and instead of producing electricity, the facility was repurposed to produce compressed natural gas. Large truck-mounted tanks are used to take that natural gas and input it to pipelines through an access point at the Dane County Landfill site, where the county is also making natural gas.

The roof of the large solids building is covered with solar panels and two other solar fields are part of the property. On a sunny day, those solar grids produce nearly 2.8 megawatts of electricity.

The partners in the project held a ribbon-cutting event to introduce the concept to the public and to allow tours of the facility. Scott Romanesko, president of EnTech Solutions, said the changes that have been made at the Middleton site make it cleaner environmentally and the addition of those large arrays of solar panels make the whole process more sustainable.

“The more renewable energy that is used to product RNG (renewable natural gas), the more valuable the RNG is in the marketplace,” he said.

When Dane County partnered to build this facility, one of the goals was to keep phosphorus out of the Madison chain of lakes in an effort to reduce the algal blooms that diminish the recreational value of the lakes. Keeping that phosphorus out of the lakes is still part of the process. The solid waste at the end of the digestion process goes back to the farms who supply manure, so it can be used as fertilizer.

“This is a holistic, sustainable project,” said Romansko.

The facility incorporated a nutrient concentration system that returns clean water to the region’s Yahara Watershed and reduces phosphorus runoff to nearby streams and lakes. In 2021, over 27 million gallons of manure was processed by the biodigester, removing more than 57,000 pounds of phosphorus from the watershed.

The digester was originally built in 2014 under a partnership between Dane County and Gunderson Lutheran. The Gunderson health care company’s interest was to reduce their carbon footprint through participation in sustainability projects like the digester.

County partnership critical

Now that the facility produces RNG, the partnership with Dane County has allowed the gas to be transported to the county’s landfill on the east side of Madison to get the gas into the interstate transmission pipeline. The RNG is eventually marketed to the transportation sector in California.

Experts at the ribbon-cutting event said the Middleton digester’s RNG is indistinguishable from the other forms of natural gas in the pipeline. They noted that RNG from cow manure can reduce greenhouse gas emissions (GHG) up to 400 percent when used to replace traditional fossil fuels.

They added that not all digesters would be able to get this kind of RNG into the system of pipelines because not all would have access to a gas inlet like the one made available by Dane County.

These three tank trucks are used by a new concept at the Middleton digester to haul renewable natural gas (RNG) to a pipeline inlet at the Dane County landfill, which is also producing RNG.

In the future, to make the facility even more sustainable, Romansko said they would like to transport the gas to the other side of town using electric vehicles. He praised the involvement of Dane County, the local farmers who supply manure – one through a direct manure pipeline to the digester and others by trucking it in – and Northern Biogas to make the facility a success.

“It’s a nice circular sustainable model,” he added. “We think this facility can be used in the future to show farmers what a project like this can do.”

From 90,000 to 100,000 gallons of dairy manure comes into the digester every day. Some is trucked from 10 or 11 miles away. The experts operating the digester said the intake comes from “4,200 wet cow units” from the four farms supplying the manure.

During the open house, Dane County Executive Joe Parisi said the solutions being offered by the new redesign of the Middleton digester bring with them a new “hope and excitement for the future.”

Phosphorus goals

Ten years ago when the concept was being developed, water quality was the number-one issue, Parisi said. “Today it has evolved into so much more. What’s important about today is that the solutions exist – reducing the phosphorus going into our lakes, producing clean solar energy, the partnerships at work here and the production of RNG at the Dane County landfill.”

When the county moved to produce RNG at the landfill, an addition was put onto the facility there to allow the trucking in of additional gas like that produced at the Middleton digester. “We want to share this technology with the rest of the state, the rest of the country and the rest of the world,” Parisi said.

“It isn’t that risky. There is a blueprint now. What’s good for family farms can be what’s good for the community.”

Two or three truckloads of RNG leave the Middleton plant each day bound for the Dane County landfill gas valve.

Mike Jansen, is CEO and Chair of the Board for Faith Technologies Inc., (FTI) another partner in the project. He’s been with the company 38 years and it is his company’s 50th anniversary. They became a partner in this project after the vision to transform this facility was articulated three years ago. The proof of concept took 12 months, he said, because this is the first of its kind.

Solar panels – on top of buildings and in fields like this one – produce renewable electricity at the Middleton digester facility, making the production of renewable natural gas even more “sustainable” and sought-after in the marketplace.

“FTI’s purpose is to find world-class opportunities and this is one of those opportunities,” Jansen said. “We have learned a lot and we continue to optimize this facility. This is real worldwide challenge. We expect to do many more facilities like this.” EnTech Solutions is a division of FTI.

Transformation of the facility

Before changes were made to the digester facility – which includes a large lagoon at one side of the property, a huge shed for solids (which now is covered with solar panels) and three million-gallon fermentation tanks – Romanesco said it was losing money every month.

But as changes were pursued, the technology was expensive but has gotten better and less expensive over time, he added.

The manure that is piped underground into the facility or trucked in, is moved to the three fermentation tanks where the manure is heated to 100 degrees which is the perfect environment for anaerobic bacteria, Romansco explained. The manure is also agitated and the methane that is generated in the three tanks is captured in the domes that cover each tank.

Carbon dioxide and sulfur are removed in the process and the gas that is produced is tested continuously for quality. Romansco said it is their goal to bring in more farms to this system and build one additional tank.

The gas is compressed at the facility in order to place it in tanks to move it, he explained. Once it reaches its destination it's “decompressed” to match the natural gas which it shares the pipeline with.

Romanesco said this facility proves that such a process is feasible. “We know this is a replicable process that we can apply to help other biodigesters achieve their environmental goals.”

Northern Biogas President Dave Fitch said his company saw the Middleton project as a great opportunity to leverage its 18-year history working with anaerobic digesters and nutrient management to support EnTech Solutions in their mission to create “sustainable, reliable energy solutions.”