Novel waste treatment converts sewage to biogas

A new method to treat sewage can efficiently convert leftover sludge to biogas, an advance that could help communities lower their waste treatment costs while helping the environment. 

A new method to treat sewage can efficiently convert leftover sludge to biogas, an advance that could help communities lower waste treatment costs while helping the environment.

Reporting in the journal Waste Management, a Washington State University research team tested 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 they could convert more than 85 per cent of the organic material to biogas. It can be used to produce electricity or upgraded to renewable natural gas (RNG) for the natural gas grid or local use.

Adding the new pretreatment step improves the anaerobic conversion of sewage sludge at the wastewater treatment facility from the current less-than-50% conversion rate. They produced 98 per cent 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.”

Sewage sludge is not a sought-after product. About half of the wastewater treatment plants in the U.S. use anaerobic digestion to reduce this waste. The process by which microbes break down the waste is inefficient. The leftover sludge, called biosolids, generally ends up in landfills.

Sludge to biogas has enormous potential

Wastewater treatment facilities also use large amounts of electricity to clean up municipal wastewater. They are often the largest user of electricity in a small community.

“If they could make their electricity or 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, 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 that breaks down the material’s polymers.

The WSU researchers have studied this pretreatment process for several years, using it to break down straw and woody materials. They weren’t sure the process would work with the different compositions of sewage sludge. However, they were positively surprised.

“This is not a very high-tech solution,” Ahring said. “It’s 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.”

She added that the technology could benefit smaller communities. Many of these communities are motivated to reduce waste and its 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 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. Producing renewable natural gas could allow rural communities to make local transportation fuel for their municipal vehicles.

Related Articles:

Send this to a friend