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Western Lake Superior Sanitary District's new $16M oxygen facility treats wastewater using less energy

The two white towers on the WLSSD campus in Duluth were taken down Friday.

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The operators of a crane and an excavator work together to lower a 68-foot-tall, 30-ton oxygen tank on its side at the Western Lake Superior Sanitary District wastewater treatment plant Friday. The tank and a second one are no longer needed as WLSSD recently finished installing a new oxygen production facility used during the process of treating wastewater. The new facility requires less electricity to operate. (Steve Kuchera / skuchera@duluthnews.com)

A new and more energy-efficient oxygen facility used to treat wastewater is replacing the two white towers located in front of the Western Lake Superior Sanitary District in Duluth.

In the first two months the new oxygen production facility has been online, it’s used 19.5% less electricity on average than the 41-year-old facility used, but Marianne Bohren, executive director of WLSSD, anticipates that number will increase to 25% as the operation becomes more efficient.

“Can we say that putting this plan in place is going to reduce people's flow or people's rate? We probably can't say that,” Bohren said. “But I think what we can say is putting this in helps us stabilize our rates and keep them low.”

Electricity rates have increased more than 72% between 2007 and 2019 and it’s one of WLSSD’s largest non-payroll expenses, coming in at more than $3 million annually. Bohren said the new $16 million facility, along with other projects in the works, helps move the plant toward its goal of energy sufficiency.

WLSSD paid for the project using low-interest loans through the Minnesota Public Facilities Authority, meaning the loan — with an interest rate around 1% — is built into the rates users pay, Bohren said.

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"In all of the investments that we make, we're really looking at, 'What can we use to improve the reliability of the process? What can we use to meet all of our state and federal regulatory requirements? And ... how can we do it with the most energy efficiency possible?'" Bohren said. "I think this project will serve this region very well for decades to come."

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Two workers reposition cables at the 11-foot-diameter base of a partially lowered oxygen tank Friday. (Steve Kuchera / skuchera@duluthnews.com)

What do oxygen facilities do, anyway?

Oxygen is necessary in the treatment of wastewater because it creates an ideal environment for helpful bacteria that feed on organic matter in the wastewater, while preventing other bacteria from forming. A facility that produces oxygen is a faster version of what happens in a river, Bohren said.

“When folks were discharging directly into a river, a river would — given enough time and not being overloaded — treat waste with biological activity given enough oxygen,” Bohren said.

The landlocked facility has to be able to do what a river does, but with a much smaller footprint and at a much faster rate, which is where the need to produce oxygen on site comes into play.

The old oxygen facility has been in use since the plant was built more than 40 years ago. It used a cryogenic process that refrigerated air to a temperature that causes oxygen in the air to condense out, said Al Parrella, manager of operations and maintenance at WLSSD.

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An excavator operator demolishes an old oxygen tank at WLSSD Friday. (Steve Kuchera / skuchera@duluthnews.com)

“It was a very reliable system, but it used a lot more energy per unit of oxygen because you were compressing air to a higher temperature," he said.

The new oxygen facility uses an ambient temperature process that screens out the nitrogen that’s in the air so only the oxygen passes through,” Parrella said. “It took a lot of things to keep this sustained.”

Parrella said the new process is also safer in that liquefied gases are not a part of the production process.

The new facility allows for “turn-down capabilities,” which means WLSSD is able to match the amount of oxygen it produces to the demand.

“It’s very desirable to be able to match the oxygen flow to the needs of the biological process,” Parrella said. “It was more difficult to do that with the cryogenic system.”

The new facility sits at the back of the plant next to the St. Louis River.

In its efforts to increase self sustainability and energy efficiency, WLSSD is looking ahead to an investment in generators that would harness energy from wastewater. WLSSD has been requesting that 50% of that project be paid for through a state bonding bill.

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Workers watch as the first of two 68-foot-tall, 11-foot-diameter, 30-ton WLSSD oxygen tanks is lowered onto its side Friday. (Steve Kuchera / skuchera@duluthnews.com)

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