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.

Such maintenance is common at the facility, but offers a window into the web of operations — from tiny microorganisms to massive incinerators — that purify the wastewater of a third of Minnesota's population. The process churns 24 hours a day on the sprawling campus tucked between the Mississippi River and Pigs Eye Lake, at the tail end of an industrial road.

It's the kind of place with buildings labeled "sludge processing," where staff members say things like "there's really a high BTU value in scum." But it also has a good reputation, winning perfect-compliance awards for three consecutive years from a national industry group that tracks plants adherence to state discharge permits.

"For a typical plant, there's hundreds of opportunities every year for something to not be quite what it's supposed to be," said Glen ­Daigger, a professor at the University of Michigan who has worked at the plant as a consultant. "For a plant to go years and years and be in full compliance, it's a pretty difficult thing."

That's important, since the final product ends up in the Mississippi River about 24 hours after it arrives.

"The [treated] water is cleaner than the river it's going into," said Mike Mereness, assistant general manager of operations at Metropolitan Council Environmental Services, which oversees all the plants.

The plant's 250 million gallon daily capacity makes it the 10th-largest in the country — the largest is in Chicago — and it normally handles about 170 million gallons a day. It is by far the largest of eight plants owned by the Met Council, serving more than 60 Minnesota cities.

'We've had bowling balls'

Arriving water has already traveled from smaller city sewers to larger pipes owned by the Met Council. Its first stop at the plant is a series of screening bars half an inch apart that catch larger debris and deposit it onto conveyor belts.

That grayish pile of material, largely unidentifiable except for plastic pieces, rags, condoms and feminine hygiene products, is sent to a landfill. "We get rubber duckies. We've had bowling balls. Two-by-fours, etc.," Mereness said.

Flushed medications are a bigger concern, Mereness said, since many can't be fully removed in the treatment ­process.

He added: "We've had phone calls where somebody said, 'I've flushed my diamond ring down the toilet. Can you keep an eye out for it?' And we'll say yeah, but the reality is we don't find those diamond rings very often."

As water settles in special tanks, gritty "sludge" and floating "scum" are removed. Then oxygen is added to support microorganisms that feast on nutrients and organic matter, such as feces, dissolved in the cloudy water. Another round of settling removes the microorganisms. A mixture of the sludge and microorganisms are then incinerated, helping power or heat the facility depending on the time of year.

The biological process is a ramped-up version of what occurs in nature because of ideal conditions in the tanks. But the population of bacteria, protozoa and rotifer microorganisms must be limited. Otherwise, Mereness said, "You get too many people at the table and not enough food."

Renewing the plant

A public hearing on the package of improvements, which must be approved by state regulators, is planned for January. The changes over seven or eight years include replacing old equipment in the aeration tanks, updating the electrical and drainage systems, and adding a second pipe accepting water from the South St. Paul area. The upgrades will also allow the plant to burn fatty material that is now sent to a landfill.

Renewing the plant is a constant job. Improvements added up to about $135 million between 2004 and 2014, and about $200 million in the decade before that, according to the Met Council.

Most of the funding comes from payments made by cities, which pass along those costs to residents. The average annual rate for households served by the Met Council was $242 in 2013, though it was about $290 in Minneapolis and St. Paul — the largest cities served by the Metro plant.

Those are below the national average rate of $435 in 2013, as tracked by the National Association of Clean Water Agencies (NACWA). The Metro plant has received three consecutive NACWA "gold" awards for full compliance with its state discharge permit, ranking it among the top third of plants represented by the association, said Mark Hoeke, a consultant for NACWA.

Scott Strand, executive director of the Minnesota Center for Environmental Advocacy, whose organization is suing the state for not imposing more stringent phosphorus limits on sewage treatment plants, speaks positively about the Metro facility.

"I think they're timely with making upgrades when they're necessary," Strand said. "They do a good job of monitoring."

Eric Roper • 612-673-1732

Twitter: @StribRoper