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Novel waste treatment efficiently converts sewage to biogas

Novel waste treatment efficiently converts sewage to biogas
In front of the sewage sludge biogas reactor in Walla Walla (photo courtesy WSU).

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.

In the United States (US), a Washington State University (WSU) 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 were able to convert more than 85 percent 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 for local use.

The pretreatment is described in a paper entitled “Improved valorization of sewage sludge in the circular economy by anaerobic digestion: Impact of an innovative pretreatment technology” and published in the journal Waste Management.

Increased conversion rate and methane yield

Adding the new pretreatment step improves the anaerobic conversion of sewage sludge at the wastewater treatment facility from the current less-than-50 percent conversion rate, and they produced 98 percent more methane overall compared to the current practice.

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

Sewage sludge is not a sought-after product. About half of the wastewater treatment plants (WWTP) in the United States use anaerobic digestion (AD) to reduce this waste, but the process, by which microbes break down the waste, is inefficient.

The leftover sludge, called biosolids, generally ends up in landfills.

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 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 Professor Ahring, who is also a faculty member in the Bioproducts, Sciences, and Engineering Laboratory at WSU Tri-cities.

Cellulosic pretreatment tech

For their study, which was funded by the US Department of Energy (DOE), the WSU research team treated the sludge at high temperature and pressure with oxygen added before the anaerobic digestion (AD) process.

The small amount of oxygen under high-pressure conditions acts as a catalyst that breaks 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. They weren’t sure the process would work with the different compositions of sewage sludge, such as lipids and proteins, but were positively surprised.

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

The technology could be particularly helpful for smaller communities, many of which are motivated to reduce waste and their climate impact, she added.

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 further demonstrate the process. They are also studying how to efficiently convert biogas to more valuable RNG by a new bioprocess.

While biogas can be used to produce electricity, producing RNG could allow rural communities to produce local transportation fuel for fueling their municipal vehicles.

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