Portland will turn methane produced at a wastewater treatment plant into marketable renewable natural gas — and use it to fuel city vehicles — under a plan proposed by the Bureau of Environmental Services.

The $12 million proposal, in the works for years, is scheduled to come before the City Council on Wednesday.

Janice Thompson, who tracks Portland’s public water and wastewater utilities for the Citizens’ Utility board, said the proposal pencils out for the city, while also reducing greenhouse…