Most Twin Cities residents don’t think twice about what they flush down the toilet, as long as it doesn’t return. But processing human waste is a sophisticated, full-time job for one of the nation’s largest treatment plants, in St. Paul.

The Metropolitan Wastewater Treatment Plant, owned and operated by the Metropolitan Council, is preparing for $137 million in improvements over several years. The upgrades, the second phase of projects identified more than 10 years ago, target everything from air systems to electrical distribution at the 77-year-old plant.