A pilot study employing a new method for treating sewage sludge efficiently created renewable natural gas while slashing in half the cost of the treatment.
The Washington State University team described the process this week in Chemical Engineering Journal, touting it as a way to help communities sustainably clean up their waste while providing renewable natural gas for their energy needs.
When the researchers pretreated sludge collected from a nearby wastewater facility, they produced 200% more renewable natural gas compared to current practices—and cut the cost of disposal by nearly 50%.
“This technology basically converts up to 80% of the sewage sludge into something valuable,” said Professor Birgitte Ahring of WSU’s School of Chemical Engineering and Bioengineering, and one of the authors of the paper.
The renewable gas can be used in the same way as fossil-fuel based natural gas—for electricity generation, home heating, or transportation—all without the heavy climate imprint left by fossil fuels.
Addressing the giant energy drain from current methods of processing waste
Wastewater treatment facilities use large amounts of electricity to clean up municipal wastewater, making up between 3% and 4% of the total electricity demand in the U.S.
They are often the largest user of electricity in a small community. Their treatment processes also contribute to global warming, adding about 21 million metric tons of greenhouse gases to the atmosphere annually.
About half of the approximately 15,000 wastewater treatment plants in the U.S. use anaerobic digestion to reduce sewage waste and make biogas, but the process, in which microbes break down the waste, is inefficient and struggles to break down all the complex molecules in the sludge.
Additionally, the biogas composed of carbon dioxide and methane has limited use—while the leftover sludge, called biosolids, most often ends up in landfills.
For their study (funded by the U.S. Department of Energy Bioenergy Technologies Office), the WSU team added a pretreatment step, treating 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 to break down the long polymer chains in the material.
The researchers showed that their pretreatment resulted in reduced cost to treat the sewage from $494 to $253 per ton of dry solids.
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The team then used a novel bacterial strain that they discovered and isolated to upgrade the biogas, converting carbon dioxide with hydrogen into methane or renewable natural gas. The researchers analyzed and verified the renewable gas, showing that it was 99% pure methane.
“This (bacterial strain) bug doesn’t need anything—it is a workhorse,” said Ahring in a news release. “It doesn’t need organic additives or a lot of nursing. It does well with water and a vitamin pill.”
With help from WSU’s Office of Innovation and Entrepreneurship the researchers have patented the bacterial strain, and are now working with an industrial partner to develop a larger scale project.
“This approach not only enhances carbon conversion efficiency and methane yield but also enables direct production of pipeline-quality renewable natural gas with minimal CO2 content — addressing two major limitations of existing sludge-to-energy systems into a single, scalable methodology,” said Ahring.
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“By successfully bridging advanced pretreatment with biological biogas upgrading, this work provides a new, integrated paradigm for sustainable sludge treatment maximizing energy recovery while contributing to the circular bio-economy.”
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