Novel tech can turn sewage into biogas

In November 2022, we learned about a new method to treat sewage that can efficiently convert leftover sludge to biogas. As a result, it could help communities lower their waste treatment costs while helping the environment.

The information came from the journal Waste Management, where a Washington State University research team explored a pretreatment technology, adding an extra step to typical treatments and using oxygen-containing high-pressure steam to break down sewage sludge. They found that this helped convert more than 85% of the organic material to biogas, which can be used to produce electricity or upgraded to renewable natural gas (RNG) for the natural gas grid or local use.

That new pretreatment step improves the anaerobic conversion of sewage sludge at the wastewater treatment facility from the current conversion rate (less than 50%) — they managed to produce 98% more methane overall compared to current practice.

“It was shown to be extremely efficient, and that’s very exciting,” said Birgitte Ahring, professor at the Gene and Linda Voiland School of Chemical Engineering and Bioengineering, who led the work. “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.”

It is estimated that about half of the wastewater treatment plants in the U.S. use anaerobic digestion to reduce sewage sludge. Still, the process — in which microbes break down the waste — is inefficient. The leftover sludge, called biosolids, generally ends up in landfills.

Also, wastewater treatment facilities use large amounts of electricity to clean up municipal wastewater. In small communities, they are often the largest user of electricity.

“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 the circular economy,” said Ahring, who is also a faculty member in the Bioproducts, Sciences, and Engineering Laboratory at WSU Tri-cities.

For their study, the WSU research team treated the sludge at high temperature and pressure with oxygen added before the anaerobic digestion process. The small amount of oxygen under high-pressure conditions acts as a catalyst, breaking down the polymers in the material.

The WSU researchers have been studying this pretreatment process for several years, using it to break down straw and woody materials — and were surprised to learn that it can also be applied to the different compositions of sewage sludge, such as lipids and proteins.

“This is not a very high-tech solution,” Ahring added. “It’s actually a solution that can be useful even at a small scale. The efficiency has to be high, or else you cannot warrant adding the extra costs to the process.”

As mentioned, the technology could be particularly helpful for smaller communities, many of which are motivated to reduce waste and their climate impact.

The WSU team is working with Clean-Vantage, a Richland-based clean technology start-up company active in the pretreatment area, as well as with Pacific Northwest National Laboratory (PNNL), which is doing a techno-economic analysis of the new process.

The researchers are now scaling up the work in their pilot facility located at WSU Tri-cities to demonstrate the process further. They are also studying how to efficiently convert biogas to more valuable renewable natural gas by a new bioprocess. This, in turn, could allow rural communities to produce local transportation fuel for fueling their municipal vehicles.

The work was funded by the U.S. Department of Energy.