Updated: MPCA releases environmental assessment worksheet for proposed Dem-Con anaerobic digestion facility

The Minnesota Pollution Control Agency (MPCA) has released an environmental assessment worksheet (EAW) for a new facility proposed by Dem-Con Cos. LLC that will process organic materials through anaerobic digesters at its environmental campus in Shakopee, Minnesota.

The proposed facility includes two anaerobic digesters that will process up to 75,419 tons per year of organic materials, including yard waste and food scraps, into natural gas and biochar, creating an alternative to landfilling.

Dem-Con President Bill Keegan says the project is currently in its permitting phase and is expected to begin construction in 2025, with renewable natural gas (RNG) and biochar production slated for 2027. The estimated lifespan of the facility is 30 years.

The EAW process reviewed potential impacts on air, soil, groundwater, surface water and other potential environmental impacts.

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Dem-Con began planning the project approximately five years ago as an opportunity to provide food waste recycling services to Minnesotans. The Shakopee, Minnesota-based company has partnered with waste-to-energy solutions provider Hitachi Zosen Inova on the project, forming the Dem-Con HZI Bioenergy LLC joint venture. 

“The reason we're doing this project, one, [is because] there was a need,” Keegan says. “We're lacking infrastructure to process organics.”

The state of Minnesota has goals in place pertaining to waste reduction, recycling and renewable energy, including 75 percent recycling and 15 percent waste reduction by 2030, 100 percent clean energy by 2040 and to achieve zero emissions by 2050.

“The opportunity we saw was helping the state meet those goals and providing some of this infrastructure for that,” Keegan says. “The facility is a carbon-negative facility.”

An independent consultant worked with Dem-Con to determine the facility's carbon intensity, or CI, score. According to Keegan, the consultant determined the facility’s carbon intensity score was negative 62 and would reduce greenhouse gas emissions by 30,000 tons of CO₂ per year.

“It’s the equivalent of removing 6,147 vehicles from the road per year,” he adds.

In addition to the digesters, Dem-Con plans to construct an enclosed flare, separate buildings for operations and offices, a digester storage bunker, liquid digestate dispatch/biogas holder, biochar storage area, truck scale and stormwater management system that includes a pond and infiltration basin.

According to MPCA, most air emissions will come from producing biochar. The EAW is coupled with an air emission permit to assess the potential for significant environmental effects. MPCA will release an upcoming draft air permit for the facility. 

The project will produce an estimated 8,600 tons of biochar per year, which will be sold to a third party for use as a soil amendment or for remediation purposes. In addition, the project will use 3.5 million gallons of groundwater annually with the potential for using 8.8 million gallons.

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While per- and polyfluoroalkyl substances (PFAS) may be present in materials received by the facility, MPCA says the processes at the facility will not add PFAS to the end products. In fact, Keegan says the facility’s processes serve to treat PFAS.

“One of the reasons we’re doing what we’re doing is to treat PFAS,” he says. “This project has a biochar process, which is a high temperature. … That high temperature is well documented… to [treat] PFAS.”

The project will produce RNG that will be compressed and sent to a pipeline at the proposed gas interface station. MPCA says this pipeline will be reviewed in an EAW at a later date.

A permitting pathway does not currently exist for pyrolysis projects such as this one, and Keegan says the company had to pursue a municipal solid waste (MSW) combustor permit to begin construction, although it does not accurately represent operations.

“We’re proposing something that’s novel. It hasn’t been done before, and therefore there’s no permitting pathway for this,” he says. “We’re permitting this the same way as if you’re burning garbage, [which] we’re not.”

The air permit authorizes construction and operation of the facility and requires Dem-Con to install and maintain several controls to reduce air emissions, including the following controls on the biochar process:

• activated carbon injection for mercury and dioxins/furans;
• a fabric filter baghouse for particulate matter and heavy metals; and
• a liquid scrubber for acid gases, particulate matter, heavy metals and dioxins/furans.

Keegan says the project is expected to produce less emissions than that of an MSW incinerator, but that the emissions controls are necessary to abide by the permit.

“We’re being extremely conservative and permitting this with all these emission controls to make sure that we’re protective of human health and the environment.”

When more appropriate permitting is available, Keegan says Dem-Con may attempt to adjust the air permit.

The proposed facility will be located south of the existing Dem-Con landfill in Shakopee and will accept materials from Ramsey and Washington counties and other third-party sources.

Existing permits are tied to a landfill expansion for stormwater management from February 2023 and on-site mining from July 2020. Dem-Con says it will begin construction on the facility after mining operations are completed, which the company expects to conclude in the fall of 2024 with construction of the project to follow reclamation activity.

MPCA encourages the public to comment on the EAW and air permit.

The EAW was opened for public comment July 16 and will remain open for 30 days. The air permit will be open for comment later this month.

MPCA also will host a public meeting for community members Aug. 5 to discuss the project in full detail.

*This article was updated July 25, 2024, with comments from Dem-Con