Portland, Oregon has a plan to get polluting diesels off its streets, and it involves sewage.

Since 2008, the city's Columbia Boulevard Wastewater Treatment Plant has been collecting methane – a stinky greenhouse gas that also doubles as a relatively clean-burning hydrocarbon fuel – produced by bacteria as it decomposes solid waste in sewage. It currently captures about 77 percent of the methane generated there, and uses some to generate electricity, and sells the rest. The other 23 percent is essentially wasted, as it's flared off, producing carbon dioxide.

But Portland now wants to use 100 percent of the methane produced at the treatment plant, turning it into renewable natural gas for use in vehicles. City Council approved a plan to build a CNG fueling station at the site, and convert or replace diesel vehicles. The plan could make a significant impact, too, as the Bureau of Environmental Services estimates the plant can produce enough natural gas to fuel 154 garbage trucks, with the diverted methane and the move away from diesel saving 21,000 tons in carbon emissions every year.

As renewable natural gas fetches a higher price than the extracted kind – despite being chemically and practically identical – the city will use the gas they generate for vehicles, and simply purchase the natural gas they need to continue to provide their electricity generation needs. While city vehicles can fill up at the onsite fueling station, Portland also wants to sell its renewable natural gas to outside customers to help pay for the project and CNG vehicles. While the construction and CNG conversions are expected to cost about $12 million, the city expects to generate some $3 million in revenue per year from sales.

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