- A pilot project has turned sewage sludge into renewable natural gas measuring 99% pure methane.
- The process could triple the energy output of standard wastewater treatment systems.
- Researchers say the technology could help cities cut waste while producing cleaner energy.
Sewage sludge has been turned into 99% pure methane in a pilot project that researchers say could help cities make more energy from one of their dirtiest waste.
The Washington State University team used a new treatment process to break down sludge from a wastewater facility in Walla Walla, Washington, before converting much of it into renewable natural gas.
The pilot produced nearly 50% more methane than untreated sludge and tripled the overall energy output compared with the standard wastewater treatment process.
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The work, published in the Chemical Engineering Journal, could offer wastewater plants a way to reduce disposal costs while producing a cleaner fuel from material that is usually expensive to manage.
The process, called Advanced Pretreatment and Anaerobic Digestion, or APAD, begins by treating sewage sludge with high heat, high pressure, and carefully controlled oxygen.
The sludge is heated to 175C before being rapidly depressurized, which helps break apart tough organic material, making it easier for microbes to digest.
In standard wastewater treatment, about half of the material going into anaerobic digesters can still come out as biosolids, which then have to be disposed of. In the U.S., about 2 million dry tons of those biosolids are sent to landfills each year.
But Professor Birgitte Ahring, who led the WSU study, said the new process treats sewage sludge as a resource rather than a burden.
“This technology basically converts up to 80% of the sewage sludge into something valuable,” said Ahring, a professor in WSU’s Bioproducts, Sciences, and Engineering Laboratory and the Gene and Linda Voiland School of Chemical Engineering and Bioengineering.
In the pilot, pretreated sludge produced an average of 39.7 liters of methane per day. Untreated sludge produced 26.7 liters per day.
Conventional digestion converted just 37.3% of the sludge sample’s carbon into useful gas. With WSU’s pretreatment step, that rose to 61.6%. And when researchers added a second biological stage to upgrade the gas, the overall carbon conversion reached 83%.
The economic results were mixed but promising, with the pretreatment-only version reducing sludge treatment costs from $494 to $253 per dry ton, largely because it left behind far less waste to dispose of.
The full APAD system, which included the gas-upgrading stage, cost $530 per dry ton. Researchers said that was mainly because hydrogen remains expensive.
The climate figures followed a similar pattern. The full process reduced net greenhouse gas emissions from 490 kilograms of carbon dioxide equivalent per ton of treated sludge to 220 kilograms.
And the pretreatment-only version performed even better, reaching minus 160 kilograms per ton when the benefits of avoided natural gas use and fertilizer offsets were included.
The sludge used in the pilot came from a small wastewater treatment facility in Walla Walla that processes 4.5 tons of sewage sludge a day.
The research team included scientists from WSU, Pacific Northwest National Laboratory, and Clean-Vantage LLC, a clean technology startup based in Richland, Washington.
The bacterial strain used in the gas-upgrading stage has been patented, and the group is now working with WSU’s Office of Innovation and Entrepreneurship and an industrial partner on larger-scale development.
