A silver lining in sewer sludge: volatile fatty acids
WashU researchers find ways to more efficiently reclaim useful feedstock from waste
Many sewage treatment plants are equipped to process waste using anaerobic digestion, in which the sewage sludge is held in an oxygen free chamber to ferment and break down. As part of that degradation, biogas such as methane can be reclaimed from that sludge. But the process can be expensive since it requires electric generators to convert the biogas to a more useful form of the fuel such as electricity and such conversion may not be economically viable when biogas production is at a small scale.
Jason He, the Laura and William Jens Professor of Energy, Environmental & Chemical Engineering at Washington University in St. Louis wanted to find more efficient ways to get riches out of the rubbish.
“Can we recover something potentially of higher value than biogas,” he said.
He and his team looked at short chain volatile fatty acids (VFA), a common intermediate compound for many materials, including bioplastics.
VFAs can also serve as the feedstock to clean the sewage at lower cost. Treatment facilities could use VFAs from their own sewage to serve as a carbon source for the nitrogen converters, and biological phosphorus removers, He said.
Along with reducing the expenses of sewage cleanup, VFAs can be reclaimed in liquid form and potentially sold for use in manufacturing and agricultural processes to further recoup costs of sewage treatment.
In work published in Water Research, the team shows how using hydrogen peroxide on the sewage will inhibit methanogenesis and send sewage down the path of VFA production instead.
Hydrogen peroxide is a low-cost addition that leads to more than 30 times the VFA production in sludge compared to controls. Even further efficiencies were found when He’s PhD student and the paper’s first author Jiasi Sun noticed how light exposure further improved the efficiency of the process.
Sun saw two identical reactors were producing differing results. She thought at first it was a measurement error but then realized the one reactor was closer to a light source.
“I realized that light was accelerating the breakdown of hydrogen peroxide into reactive species. That small observation completely changed how I understood the process of reclaiming VFAs,” she said.
The bit of scientific serendipity is sending them down a new research rabbit hole: bringing light back to the dark fermentation tanks.
“With light, the hydrogen peroxide dosage can be greatly reduced,” added He.
For now, they are exploring the addition of LED lights to the reactors and tinkering with reactor design to push the efficiencies even farther.
“It’s all coming from these unexpected results.”
Sun J, He Z. Light stimulated H2O2 inhibition on methanogenesis during anaerobic digestion towards enhanced VFAs production. Water Research, Volume 286.
https://doi.org/10.1016/j.watres.2025.124229
This work was financially supported by the U.S. National Science Foundation (award # 2150613).
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