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‘Wave of the future’? New method better breaks down sewage into methane for electricity

By Eric Rosane, Tri-City Herald
Published: December 5, 2022, 6:00am

Creating their own cheap, renewable energy could soon be a reality for cities thanks to new research from Washington State University Tri-Cities.

Researchers at the Richland campus are working to streamline and scale up a bioengineering process to produce renewable natural gas out of treated human feces.

The process will help cities, counties and smaller governments reduce the amount of carbon dioxide they produce by powering wastewater treatment facilities and city-owned electric vehicles with the natural gas — in this case, methane — that’s produced.

“These small communities, they want to do something to reduce their carbon footprint, and they can do it in a simple way,” said Birgitte Ahring, a professor of biological systems engineering and chemical engineering at WSU Tri-Cities. “If it takes off for those that want to commercialize it, it’ll be big business.”

What Ahring’s research so far has found is that by adding an extra step in sewage sludge treatment — by pretreating the material with oxygen-containing, high-pressure steam to make it easier to biodegrade — they were able to convert more than 85% of the organic material into biogas methane, which can be used to produce electricity.

It was shown to be extremely effective. The normal yield of methane from sewage sludge without the extra step is less than 50%. Initial tests show they produced 98% more methane overall compared to current practices.

Ahring said adding this extra step could be a game changer for smaller U.S. cities.

WSU researchers have previously studied this pretreatment process on straw and wood materials, but Ahring said she wasn’t sure at first how it would work on sewage.

“This can be applicable, and something we could begin to explore, in Washington state. Not wasting waste, but using its potential instead has major advantages,” Ahring said in a statement.

Those trial runs are already taking place at the City of Walla Walla, which is looking to install a bioreactor at its wastewater treatment facility as part of a number of other upgrades.

After those upgrades are complete, and the extra pretreatment step is added, the city’s facility could be producing enough electricity to power itself and reduce nearly all its greenhouse gas emissions. The studies for this work began at Walla Walla Community College.

Ahring and her research team are working with Richland-based Clean-Vantage, a clean technology start-up company, as well as the Pacific Northwest National Laboratory, which is conducting an economic analysis of the new process.

Her work is being funded by a $2.5 million grant from the U.S. Department of Energy.

Sludge to biogas

About 17,000 wastewater treatment facilities operate around the United States, most of them by local governments.

They work to treat about 34 billion gallons of wastewater every day, separating the substance into reclaimed water and biosolids.

That also takes a lot of electricity to do — about 30 terawatt hours per year, or about $2 billion in annual electric costs. Wastewater treatment facilities make up the largest amounts of electricity used in a community — and It’s often mostly fossil fuels powering those systems.

“If they could make their own electricity or, for some of the large plants, make renewable natural gas and add it to the natural gas grid, then they can reduce the use of fossil fuels. Here we are beginning to move into the idea of a circular economy,” Ahring said.

After processing, the cleaned water is usually pumped back into reservoirs or rivers. But what happens to the biosolids varies — some facilities try to find a reuse scenario, but oftentimes will hand them off to a third party.

“Approximately half of them try to get rid of it. Nobody wants that,” Ahring said.

Mineral-rich biosolids under stringent pollutant and pathogen requirements can also be used in land application at land fills, forestries or for agricultural purposes.

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Walla Walla has been providing WSU Tri-Cities with the necessary sludge — about “20 to 30 buckets a month” — to do its research.

Frank Nicholson, the City of Walla Walla’s utility engineer, said using the steam-treatment method ultimately results in less biosolids byproduct and more electricity potential for them.

“We’re excited about it,” he said. “We’re already starting the design of our new digester project and we’d like to incorporate this technology into it. The reason we’re excited about it is because it would produce more methane gas.”

The microturbines that would produce electricity from the renewable methane gas could also be used to power the city’s small fleet of electric vehicles. The high price of renewable energy credits is also making this process more appealing to smaller-to-mid-sized cities like Walla Walla.

But adding bioreactors to wastewater treatment facilities is expensive, Nicholson said.

His city will need about $5 million in funding for a new digester, microturbines and a bioreactor system. Walla Walla is home to one of the state’s oldest class A reclaimed water facilities in the state. The city plans to apply for federal grants to build the system within the next two years.

“It’s definitely the wave of the future and we enjoy working with WSU,” he said.