$24.3 million AD project planned for WWTP in Duluth, Minnesota

Over a decade ago, anaerobic digestion (AD) was brought to the Western Lake Superior Sanitary District in Duluth, Minnesota, to assist in dealing with the resulting sludge from the waste water treatment plant (WWTP) process. Prior to 2001, when AD was installed at the facility, the solids were burned in an incinerator. WLSSD decided they either had to invest money into improving the incineration process or try something different. “The something different is digestion,” said Karen Anderson, director of community relations with WLSSD.

The facility has four 1-million-gallon digester tanks, which since their installation, have produced biogas to heat the plant’s boilers in the winter and the AD process itself. The resulting solids from the process are marketed as fertilizer under the name Field Green. However, the digesters are currently underutilized. WLSSD plans to combat this shortfall by making their current operation more efficient and generate more gas. The company has enough capital funds and a plan to help work towards production capacity and the installation of a new heating system. The $24.3 million project will be implemented in three phases over the next 5 years, beginning in 2015.

Marianne Bohren, executive director with WLSSD, believes now is the time to move forward with the project. “One of the reasons is the cost of energy in its entirety, electricity in particular, has increased dramatically,” Bohren said.

Bohren recognizes that in northern Minnesota electricity is fairly inexpensive compared to the rest of the country, but said the price has still increased by 57 percent and takes up 30 percent of the company’s overall waste water operating budget, amounting to around $3 million. Bohren hopes purchased electricity can be cut in half after the company completes the project. “It’s time for us to start looking at utilizing this wonderful opportunity we have to generate some of our own electrical needs,” Bohren said.

Self-sufficiency is WLSSD’s goal over the next few years. The company has intentions for 2020 and beyond, if biogas can be produced beyond the facility’s electrical needs, to explore the possibility of scrubbing and compressing the gas and utilizing it internally to power natural gas fueled vehicles.

Before those ideas are entertained the company plans to move forward with the initial three phases. “The first phase is getting rid of the steam boilers and replacing them with modular hot water boilers,” said Nathan Hartman, an engineer with WLSSD.

According to Hartman, biogas can be burned in half of the hot water boilers, and the other half will use natural gas as fuel. The first phase will also include a biofilter to scrub the gas of hydrogen sulfide, so it is easier on the equipment. This is slated to take up $11.8 million in the 2015 budget, and is expected to take all of 2015 and 2016 to complete.

The second phase in 2017 is the installation of two 825 kW engine generators and the rest of the gas conditioning equipment, which includes refrigeration and compression of the gas to get all the moisture out, as well as equipment for siloxane removal, according to Hartman.

The third phase in 2018 and 2019 explores how the digesters can reach capacity. “In the third phase we will add a receiving station for high strength waste to add to the digester, and then max out the engine generators themselves—try to make as much electricity as we can,” Hartman said.

Beyond wastewater treatment, WLSSD does solid waste management where they have an onsite organics program. The program takes waste from schools, hospitals, restaurants and other operations and composts the material. “We would have the opportunity to take that food waste and put it directly into the digesters to utilize it in a different way,” Bohren said. “The third phase will really be about studying what additional wastes can be added and building an introduction system to bring those into the AD process.”

The process currently produces between 300 to 350 cfm of biogas, which ramps up and down based on the feed rates to the digester, Hartman said. “We’re assuming our digester capacity right now could produce twice the amount of gas,” Bohren said.

The company presented the project to the Minnesota Department of Commerce’s energy division last week, and WLSSD’s board will be approving the company’s operating and capital budget for 2015 on Sept. 22.