PORTLAND, Ore. -- The city of Portland just approved a project that will not only improve air quality but also save ratepayers money down the road.

City commissioners unanimously approved a $15 million project to build a system that will convert the methane released during the treatment of waste water, into a compressed natural gas.

The facility will be built into the city's large waste water treatment plant off of North Columbia Boulevard.

It will take 100 percent of the methane produced there, some of which is currently being flared, and convert it to the renewable natural gas.

That gas will then be used to power city owned vehicles currently burning diesel. Those vehicles will be converted to run on the much cleaner burning natural gas. The city say it's the equivalent of replacing 154 large trucks a year that now put out diesel exhaust.

"We're taking waste and turning it into cash and along the way we're replacing dirty diesel with clean energy, so it is a huge advance toward our Climate Action Plan," said Portland City Commissioner Nick Fish.

Any of the natural gas that is not used will be sold, potentially earning the city between $3 million and $10 million a year.

That money would be used to pay off the project and then improve the city's aging sewer system without raising rates.

"The bottom line is that we're turning waste into profit for ratepayers and along the way protecting our environment," said Fish.

The city has also partnered with Northwest Natural to use its existing pipeline that runs right in front of the treatment plant to distribute the natural gas.

The new project should be up and running by the end of 2018.