On days when temperatures climb above zero, work continues on an unusual project in Fridley: laying 650 feet of polyethylene cylinders to replace two big iron pipes carrying sewage across the Mississippi River from Brooklyn Park.

One of the 42-year-old iron pipes sprang a leak in 2012; it was detected after Fridley residents on Hartman Circle noticed a distinctive odor wafting up from the riverbank that June. A 20-foot-long, cylindrical patch has provided an interim solution, but the new pipes will be a long-term fix.

In October 2013, big backhoe excavators began digging a 10-foot-deep trench in the river­bed that will carry the new cylinders 650 feet across the river. Welders have begun connecting 50-foot-long sections of the pipes, which rest on the riverbank, said Dan Banken, project manager for contractor Lametti & Sons of Hugo.

He said the project is being done in winter (above-zero weather permitting) because the river flow is lowest and boaters aren’t a concern.

The $6 million project is scheduled to be done by June, said Bryce Pickart, a technical service manager for the Metropolitan Council, which owns the pipes. He said the Fridley interceptors collect sanitary sewage from the northwest metro area extending to Maple Grove, parts of Plymouth and northward to Champlin and Anoka.

The new pipes will connect to pipes that run about 16 miles through Minneapolis to the metro wastewater treatment plant on Pig’s Eye Lake in St. Paul.

John Simmelink, who lives on Hartman Circle a few doors south of the worksite, said some of his neighbors were upset about losing more than 100 trees on the site where a temporary dirt ramp and 25-foot-deep trench were built from East River Road to the river. At the same time, he said it was impressive to watch the 80-foot crane and backhoes that built a rock causeway halfway into the shallow river, as well as trenches on land and in the riverbed.

“The steam shovels go right down into the river,” Simmelink said. “I was impressed how quickly they built that causeway.”

By spring, the riverbed trench to Brooklyn Park will be ready for the 650-foot pipes. Banken said the pipes will be pulled and pushed into the trench by three backhoes: one each on the bank, the causeway and a barge.

During the installation process, the twin polyethylene pipes (one is a backup) will be capped on both ends, causing them to float. Twelve-foot-wide concrete collars will be bolted around them every 20 feet, Banken said. Heavy chains will be attached to each collar. The backhoe shovel buckets will hook the chains to move the pipes across the river. The pipes will float despite carrying about 30 collars, each weighing almost 5 tons, he said. Once in place, one cap on each pipe will be removed, allowing river water to enter as the pipes are slowly lowered into the trench.

When the pipes are in place, causeway limestone will be removed, mixed with river sediment and dumped in the trench to cover them. The riverbanks will be restored and new trees planted, Pickart said.

The old pipes, which underwent an $800,000 rehabilitation last year, will remain in the river as backups and for future growth in sewage system capacity, Pickart said. The leaky pipe rehab included a fiberglass, inch-thick patch that cost roughly $100,000 and was custom made inside the iron pipe to cover a 3-inch eroded hole in an elbow on the Fridley bank, Banken said.

For Lametti’s dozen or so workers, the toughest part has been working in the frigid weather. “It’s been a challenge,” Banken said before the latest cold snap. “We can’t work below zero. There’s too much ice.”