Researchers Discover Waste Treatment Process to Turn Sewage Into Biogas

Researchers from Washington State University have found a new method of waste treatment that can turn sewage sludge into biogas, which can be used for producing electricity or making renewable natural gas (RNG) for community use.

The research, expected to publish in the journal Waste Management this December, tested a waste pretreatment process that applied high-pressured steam with oxygen on the sewage sludge. This added step using what is called the Advanced Wet Oxidation & Steam Explosion process (AWOEx) led to a conversion of over 85% of the sludge into biogas.

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

The oxygen pretreatment and improved anaerobic digestion on the sewage sludge improved the methane yield by 98%, making a more efficient process that could make waste more useful and more in line with a circular economy, according to the study. 

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

Although some wastewater treatment plants do already use anaerobic digestion on sewage sludge, most of the leftover sludge is sent to landfills. Not all of it breaks down or is useful after treatment. But the researchers, who applied oxygen, high heat, and pressurized steam to the sludge in a pretreatment, were able to better break down polymers in the sludge. The result is a low-tech solution that could help turn this waste into something useful for the wastewater treatment plants themselves and the community at large.

“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,” Ahring said. “Here we are beginning to move into the idea of the circular economy.”

The study research was funded by the U.S. Department of Energy, and the research team is partnering with Clean-Vantage and the Pacific Northwest National Laboratory (PNNL) to scale up the process. The researchers are also looking into ways to convert the biogas into renewable natural gas that could be used for fueling municipal vehicles in smaller and/or rural communities.