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Efficient Conversion of Sewage to Biogas with New New Waste Treatment Method

The efficient conversion of remaining sludge to biogas is now made possible with a new method to treat sewage. It is an advance that could support communities in reducing their waste while helping the environment.

In front of the sewage sludge biogas reactor in Walla Walla. Image Credit: Washington State University.

A new study performed at Washington State University has tested a sewage waste pretreatment technology. This adds an additional step to normal treatments, utilizing oxygen-containing high-pressure steam to collapse sewage sludge.

The study has been reported in the Waste Management journal.

The researchers discovered that they were able to change over 85% of organic material to biogas, which can be utilized to generate electricity or be upgraded to renewable natural gas (RNG) for local use or use in the natural gas grid.

The addition of the new pretreatment step enhances the anaerobic change of sewage sludge at the wastewater treatment facility, improving upon a conversion rate below 50%. They generated 98% more methane on the whole than in present practices.

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.

Birgitte Ahring, Study Lead and Professor, Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University

Nearly half of the wastewater treatment plants present in the US make use of anaerobic digestion to decrease sewage waste. However, this process, in which microbes break down the waste, is ineffective. The remaining sludge, known as biosolids, usually ends up in landfills.

Also, wastewater treatment facilities utilize enormous amounts of electricity to cleanse municipal wastewater. Often, they are known to be the biggest users of electricity in small communities.

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.

Birgitte Ahring, Study Lead and Professor, Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University

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

In their study, the WSU research team processed the sludge at high pressure and temperature, with oxygen added prior to the anaerobic digestion process. The small amount of oxygen under high-pressure conditions serves as a catalyst that breaks apart the polymers in the material.

The scientists of WSU have been studying this pretreatment process for many years, utilizing it to break down woody materials and straw. They were unsure whether the process would function with the diverse composition of sewage sludge, like proteins and lipids, but were surprised by their findings.

This is not a very high-tech solution. 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.

Birgitte Ahring, Study Lead and Professor, Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University

The technology could be especially beneficial for smaller communities, many of which are driven to decrease their waste and climate impact, added Ahring.

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 delivering a techno-economic examination of the new process.

Currently, the scientists are expanding the work in their pilot facility situated at WSU Tri-cities to further illustrate the process. They are also studying how to convert the biogas to more suitable renewable natural gases with a new bioprocess.

If biogas could be utilized to produce electricity, generating renewable natural gas can enable rural communities to make fuel for municipal vehicles.

This study was financially supported by the US Department of Energy.

Journal Reference:

Dutta, N., et al. (2022) Improved valorization of sewage sludge in the circular economy by anaerobic digestion: Impact of an innovative pretreatment technology. Waste Management.


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