Flush a toilet in Forest Lake and the waste will spend more than a day traveling more than 30 miles of regional sewers before reaching a treatment plant in St. Paul. But the ride isn’t always smooth.
Corrosion has eaten away at the vast network of hidden pipes that disposes the Twin Cities’ wastewater, which is why the Metropolitan Council is pursuing the largest rehabilitation effort in the system’s history. It is spending about $100 million a year to overhaul deteriorating regional sewers, known as interceptors.
“In terms of an investment in the infrastructure of the Twin Cities, it’s a well-kept secret,” said Met Council Chairman Adam Duininck. “It’s not something that a lot of people are that familiar with, unless you’re one of those communities who are in the middle of one of these big projects.”
The repairs and replacements, already underway, are expected to touch most areas of the Twin Cities over the next 15 years. More than 100 miles of pipe, some of it buried beneath rivers, interstates and railroad tracks, is under construction or scheduled for an upgrade over the next decade.
“Once we realized how widespread the corrosion had gotten, we ramped up our program,” said Bryce Pickart, who oversees wastewater construction projects for the Met Council, referring to work that accelerated in 2013.
The web of regional sewers, which collects waste from smaller city sewers and delivers it to eight wastewater plants, comprises 610 miles of pipe — enough to reach Kentucky if laid end-to-end. The average pipe is about 4 feet across, but they can extend as wide as 14 feet closer to the treatment plant. One pipe alongside Interstate 94 in St. Paul is buried 210 feet underground.
But the system is growing older. Nearly 60 percent of the pipes were built before 1980. And the vast majority are made of concrete or metal, which are susceptible to corrosion.
Some need to replaced, but most are being repaired. In Richfield, for example, workers are restoring a pipe along 66th Street in conjunction with a reconstruction of the street.
“That’s an old, old system. And it needs to be rehabbed,” said Kristin Asher, public works director in Richfield. “The sanitary sewer is one of the most important things for public health.”
Corrosion over time
The system’s gradual deterioration is largely the result of hydrogen sulfide released by wastewater when it becomes turbulent, sloshing around when it reaches a drop in a pipe that relies on gravity to move water, for example. That takes a toll on the concrete and metal pipes. Since conditions vary, some sewers have lasted for 100 years, while others need fixing after 40 or 50.
“It isn’t a matter of when the sewer was built, necessarily, it’s really the circumstances of that system,” Pickart said.
Pipe failures are rare, but do occur. The collapse of an interceptor on the East Side of St. Paul in 2014 created a 25-foot sinkhole. Another collapse in 1995 sent hundreds of gallons of sewage into Lake Minnetonka.
“We have a very reliable system and we don’t have very many breaks and spills,” Pickart said.
The repairs are complicated, starting with the inspections of the pipes, usually by floating camera-equipped devices. But some pipes are so big, with such high flows, that a raft equipped with cameras, sonar and depth-measuring lasers must be lowered inside.
The wastewater must also be rerouted before the repairs can begin, which requires the installation of temporary pipes above ground to handle the flow.
Repairs range from replacing pipes to relining them with corrosion-resistant materials like fiberglass and plastic. Many repairs are made by using water to unfurl a giant liner into the pipe, which then cures into a solid layer, eliminating the need to dig a trench.
Costs mount statewide
Like the rest of the region’s wastewater infrastructure, the interceptor overhaul will be paid for through user fees. Most of the money comes from fees the Met Council charges cities based on the volume of wastewater produced, which cities pass to residents.
The Met Council’s wastewater rates are significantly less than the national average, though the municipal rate will rise 5 percent next year, partly because of the interceptor rehab. Cities will pay just under $2,500 per million gallons, amounting to a tab of about $40 million next year for Minneapolis, the region’s largest producer of wastewater.
The need for upgrades extends across the state, however. A report released by the Minnesota Pollution Control Agency (MPCA) early this year found that the state’s wastewater infrastructure is in need of $4.2 billion of investment. About 40 percent of that is tied to rehabbing existing sewer systems.
More than half the need is outside the metro area, where many smaller communities lack the population to cover the cost economically for residents.
“In general, the smaller communities, if … they have these needs it gets harder and harder to pay for them, for it to be affordable,” said Bill Priebe, who supervises Twin Cities wastewater permitting for the MPCA.
Duininck said the regional wastewater system is largely built out, so most of the council’s attention is focused on maintaining and modernizing it.
“The reason that the Met Council is here — one of the primary reasons — is to have that regional infrastructure that supports growth,” Duininck said. “And the wastewater system is a huge part of that.”