For years, the sewage flushed away by Portland households has decomposed into methane gas captured by the city's wastewater plant and turned into energy and electricity.

Related: The Power Of Poop Vs. The Might Of Methane

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Now, the city’s wastewater agency, the Bureau of Environmental Services, has a new plan for the gas that’s produced by your solid waste.

It plans to convert the methane into renewable natural gas and sell it as fuel for trucks and buses, with the help of Northwest Natural Gas. The bureau says it can produce enough natural gas in a year to power the equivalent of 154 garbage trucks.

Portland City Council approved the proposal as an emergency measure in a unanimous vote Wednesday.

'Poop Into Power'

“When I told my 13 year old about it, he said I should explain it this way: We’re going to be turning poop into power,” said Nick Fish, the commissioner who oversees environmental services.

The Bureau of Environmental Services is touting the project as a major “triple bottom line” win that produces environmental, social and economic benefits.

The agency says the natural gas produced by Portland’s waste will replace diesel fuel, cutting emissions by 20,000 metric tons of carbon dioxide — more than any other city project to date. It would also reduce particulate pollution.

Local climate change experts said transportation fuel is a smart way to reuse waste methane gas, but added the city’s carbon savings claims could be inflated.

At present, the Columbia Boulevard Wastewater Treatment Plant produces an average 600 million cubic feet of biogas, primarily methane.

The Columbia Boulevard Wastewater Treatment Plant serves residential and commercial customers in Portland. Courtesy of the City of Portland

Bacteria that breaks down solid waste in giant tanks produces methane, a particularly harmful greenhouse gas.

The treatment plant currently recovers about 77 percent of the methane. It’s used to heat boilers and generate electricity on site, and some of the gas is sold to Malarkey Roofing Company to power its manufacturing processes.

The remaining 23 percent is flared through waste gas burners, releasing carbon dioxide into the atmosphere.

The new proposal will capture and reuse 100 percent of the methane produced at the treatment plant, according to BES.

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A $9 Million Price Tag

Repurposing the gas will cost roughly $9 million. The agency will start by building a processing facility to convert the methane into renewable natural gas and an on-site RNG fueling station by the end of this year.

Next year, BES will build a pipe connecting its facility to Northwest Natural’s natural gas distribution system.

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There aren’t currently local buyers for natural gas as a transportation fuel, so in the short term the bureau plans to sell the gas to a company — Clean Energy Renewables — which will resell it on the national market.

The company has agreed to use a system known as traceable credits to guarantee the natural gas is used as a vehicle fuel.

“Those credits allow us to get a premium price for the product, and they allow us to ensure that it displaces diesel fuel and goes into the transportation sector,” said Michael Jordan, the director of the Bureau of Environmental Services.

The bureau is in the process of converting some of its vehicles to run on renewable natural gas. In the long run, the agency hopes to sell its RNG locally as fleets convert from diesel to RNG.

BES estimates the project will pay for itself within three to eight years, and after that will generate income for the agency and its ratepayers.

Praise and Questions

Vivek Shandas, a professor of Urban Studies at PSU who focuses on sustainability, had both praise and questions about the project. “Methane is a particularly potent greenhouse gas, so as much of it as we can capture is probably a good thing, ” said Shandas.

He said the renewable natural gas project involves proven technology shown to have a good return on investment.

Shandas also said engineers have come up with a variety of ways to covert or retrofit diesel engines so they can run on natural gas. A pilot project at the Port of Seattle recently converted four diesel container shipping trucks to run on compressed natural gas.

Related: Washington State Plowing Ahead With Greenhouse Gas Limits

But Shandas was skeptical of the claim that it’s the city’s single greatest effort to reduce greenhouse gas emissions. “Arguably, we’ve done more with the urban growth boundary, with a number of density policies," he said, "than we have with any single win, any single project like this."

Shandas also pointed to the Bureau of Environmental Services’ existing projects developing green infrastructure like rain gardens and bioswales for both reducing emissions and mitigating the effects of climate change.

“This is a wastewater treatment proposal. Yes, it has a climate component. To bill this as a climate proposal, I think is taking this pretty far,” he said.

Andrew Rice, a professor of physics at PSU who studies emissions, said it was hard to assess BES's emissions reductions claims without more detailed information. But he said it appeared at first glance to be a generous estimate.

“If you assume the project's output will be 100,000-340,000 dekatherms per year, as they state, the resulting range of carbon reductions is 5000-19,000 metric tons CO2,” Rice said.