The wonders of waste: How a WSU prof helped turn sewage into 99% pure natural gas while cutting costs in half

Poop has plenty of potential.

But before it can fulfill a new purpose, it has to be purified.

A team of scientists, including one from WSU, have completed a pilot study that shows by pretreating sewer sludge, they can produce 200% more renewable natural gas while simultaneously cutting disposal costs in half.

It's no secret that Americans flush and pour billions of pounds of wastewater down the toilet and sink every single day. Biosolids are the firm product of wastewater treatment facilities, where solid waste is separated from liquid waste through repeated treatment and filtering.

Once there's separation between solids and liquids, a process called anaerobic digestion uses microorganisms to break down biosolids and other waste in oxygen-free tanks into biogas - made mostly of methane and carbon dioxide - which can then be refined into things like renewable natural gas and organic fertilizer. Renewable natural gas, once it's up to industry standards, can then be injected into a natural gas pipeline. Unfortunately, the process is costly and far from perfect.

The treatment of wastewater accounts for about 4% of the United States' electricity demand, contributes 21 million metric tons of greenhouse gases into the atmosphere annually and is a process that's known to be inefficient and leaves leftover sludge to pile up in landfills.

In order to improve the "carbon conversion efficiency," or the percentage of raw carbon that ends up as something useful, the Department of Energy sent out a call for researchers to tackle the problem.

Birgitte Ahring, a professor at WSU's Bioproducts, Sciences and Engineering lab and co-author of a study published in the Chemical Engineering Journal, has spent nearly two years answering the DOE's call. In partnership with the Pacific Northwest National Laboratory and a Richland-based clean technology company Clean-Vantage LLC, their work was recently published in the Chemical Engineering Journal.

"Biogas is 60% methane and around 40% carbon dioxide," Ahring said. "And this carbon dioxide is something you have to remove if you want to put the gas into the gas grid."

For the pilot study, the research team introduced a pretreatment step to treat sewer sludge with oxygen-rich steam at high temperatures and immense pressure. That bit of oxygen in a high-pressure environment works to break down large molecules before the sludge can be introduced to the anaerobic digestion process.

Researchers with the Pacific Northwest National Laboratory said that this pretreatment reduced the cost to treat leftover sewage from $494 to $253 per ton.

"Because you have so much less sludge, now you have much less cost for getting rid of the rest of the sludge (that can't be used)," Ahring said. "In many ways, the economics of this process we have here is actually creating value."

The next step the team took was to introduce their freshly treated biogas to a kind of methane-emitting microbe, called an archaea, that some scientists believe is the earliest form of life on Earth.

Unlike bacteria that often needs oxygen to survive, archaea came from a period of time, roughly 3.8 billion years ago, when our whole planet was anaerobic, meaning life didn't need oxygen to live and grow.

"We were actually very lucky when we found that microbe that is methanogenic," Ahring said. "Many times when you grow a microbe, it needs a little of this, a little of that. This microbe, it can grow in wastewater. You can use water, a vitamin pill and a little nitrogen, then it's happy."

This strain of archaea, once it's isolated and mixed with a little hydrogen, converts the remaining carbon dioxide into renewable natural gas. Researchers determined their final product consisted of 99% pure methane.

Ahring said the university has already patented their "dream microbe" and are partnering with Clean-Vantage to scale-up the project.

The challenges that remain, Ahring said, revolves around the inflated cost of hydrogen and convincing waste water treatment facilities to add extra steps to their existing process. But if those facilities can see the savings on disposal costs while increasing productivity, she's optimistic that those companies would at least explore implementing the process she helped develop. She's already gotten some calls from waste water treatment facilities interested in her work. The next step is to upgrade the pilot study to a more commercial scale, which could occur in the next year or two.

"I'm always very focused on making things that go to the real world," Ahring said. "All of us produce a lot of wastewater, and it ends as sludge. This makes a good solution in the long run for actually getting something out of that sludge. For instance, making the electricity to run the wastewater treatment facility. That would be a good thing, wouldn't it? The main thing here is that there's a better solution."

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