As
part of the project, the team used a novel bacterial strain to upgrade the
biogas in a reactor, converting carbon dioxide with hydrogen into methane or
renewable natural gas. Credit: WSU
A
pilot study of a new method for treating sewage sludge from a wastewater
treatment plant efficiently created renewable natural gas while reducing the
cost of the treatment. The work, reported in the Chemical Engineering Journal, could help communities sustainably clean up
waste while getting 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 reduced the final disposal cost
by nearly 50%. Renewable natural gas could be used in the same way as
fossil-fuel based natural gas for a wide variety of uses, including for
electricity generation, home heating, or for transportation without the same
climate effect as fossil fuels.
"This technology basically converts
up to 80% of the sewage sludge into something valuable," said Birgitte
Ahring, corresponding author on the paper and a professor in WSU's Bioproducts,
Sciences, and Engineering Laboratory and the Gene and Linda Voiland School of
Chemical Engineering and Bioengineering. "If we can replicate this work on
other organic materials, we'll have a waste treatment technology that is
world-class when it comes to efficiency."
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. The biogas composed of
carbon dioxide and methane has limited use, and the leftover sludge, called
biosolids, most often ends up in landfills.
For their study, the WSU research 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.
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. "It doesn't
need organic additives or a lot of nursing. It does well with water and a
vitamin pill."
The researchers are working with WSU's
Office of Innovation and Entrepreneurship and have patented the bacterial
strain. They 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. "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 bioeconomy."
In addition to Ahring, the team on the
project included researchers from Pacific Northwest National Laboratory, and
Clean-Vantage LLC, a Richland-based clean technology start-up company.
Source: Wastewater sludge treatment boosts renewable natural gas 200% and halves disposal costs
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