SIOUX CITY -- For years, a potential resource at Sioux City's wastewater treatment plant has literally been going up in flames. 

Like many facilities around the country, the treatment plant uses a flare to burn the methane gas created as a byproduct during the anaerobic digestion process the city uses to break down raw sludge.

"It burns this fuel off so that it’s basically environmentally acceptable," city utilities director Mark Simms said. "That’s very common throughout the U.S."


Sioux City leaders are now hoping to harness the potential of this gas through a project that will allow it to be cleaned and re-purposed. A $9.1 million project set to break ground this August will install new equipment to capture, clean and compress the gas and turn it into renewable fuel to be sold on the national market for use in vehicles.

Excess natural gas is being burnt at Sioux City Wastewater Treatment Plant in Sioux City. Rather than continue to burn it off, the city plans to capture, clean and compress the gas in order to sell it on the market as renewable fuel.  Justin Wan, Sioux City Journal

It's a process the city anticipates will generate significant revenue, as well as reduce air emissions and improve odor control at the facility. Once the project is complete next summer, the revenue from the sale of the renewable fuel is expected to pay for itself in two to five years, Simms said. He said conservative estimates show it could generate $4 million per year for the city.

"The expected revenue may offset the need for some future rate increases," he said. 

The Sioux City Council gave the original go-ahead for the project in January 2017, allowing the city to enter the planning phase. Ground will be broken later this summer and, if all proceeds as scheduled, work will finish by June 2019.

At that time, Simms said, Sioux City will likely be the first wastewater treatment facility in the state -- and one of only five in the country -- to have such a program. 

"With this cutting-edge equipment and technology, as well as changes in some of our treatment processes, the city could have the greenest renewable natural gas project to date in the United States," he said.

The city has contracted with Bartlett and West, an engineering firm based in West Des Moines, for engineering and planning services for the project. 

Philip Gates, a senior project manager with the firm, said the technology the city will use has been established in European countries for around 20 years. 

"It's a proven technology but just hasn't been introduced here because we have cheaper fuel rates and electricity is cheaper here than what it is in Europe," he said. 

The city has purchased equipment including a chemical scrubber, moisture removal system and carbon dioxide removal system through a $2.25 million contract with DMT Clear Gas Solutions of Tualatin, Oregon. Gates said he believes the project will break ground this year. 

A digester sits at the city of Sioux City's wastewater treatment plant. The city has cited the Big Ox Energy plant for multiple violations of its permit to discharge sewage into the regional treatment plant. Justin Wan, Sioux City Journal

The city currently produces approximately 300,000 cubic feet of methane gas per day and could potentially create more in the future, Simms said. Once the equipment is online, that is expected to create renewable fuel equal to 2,631 gallons of gasoline per day.

Through the cleaning, Gates said the city intends to also capture carbon dioxide -- one of the contaminants in the gas -- and reuse it in the wastewater treatment process as a chemical additive, which could potentially reduce the use of other chemicals. 

Gates said the city is looking at meeting the requirements of California's low-carbon fuel standard, which would allow it to sell the gas to markets in that state.

Since the nationwide market for national gas is volatile, the city is still considering whether it will ride with the ups or downs of the market or lock itself into a fixed long-term price, which would be reduced from the current rate, Gates said.

The wastewater treatment plant will be hiring two additional employees that will assist with the project. The city is also considering whether it could use some of the fuel it creates in its own vehicle fleet. 

This project is the latest move by the city to reduce the environmental impact of its wastewater treatment process and lessen the impact on ratepayers. 

In late 2016, the city entered a five-year land application agreement with BigAg Organic LLC, a Larchwood, Iowa, company, to allow biosolids produced during the treatment project to be used as fertilizer on approved public and private land. The city generates approximately 35,000 wet tons of the product per year. 

The biosolids had previously been deposited in the landfill by Gill Hauling Inc., the private contractor that hauls the city's garbage and recycling. The city is expected to save more than $2 million over the first five years through the switch.