Norwich — One of the biggest construction projects slated to start next summer should be a game-changer for economic growth, bring cleaner water to Norwich Harbor and hundreds of workers downtown for five to seven years.

And the offensive odor of sewage that sometimes permeates the city’s waterfront on humid days should be a thing of the past.

The long-awaited sewage treatment upgrade — now tagged at $167 million — is in the final design stages, with the first bids advertised in winter and construction to begin next summer, Norwich Public Utilities officials said.

Already plant operators are moving out of a three-story brick administration building into temporary trailers. The building will be demolished to make way for a new single-story administration and testing laboratory building.

The project has been in the works for decades, as NPU negotiated with state environmental officials on how best to comply with state orders to correct the city's persistent water pollution issues.

The state Department of Energy and Environmental Protection in June approved NPU's “Basis of Design Report” for the project. A complex grant and loan package is in place for NPU to move forward.

“It took an awful lot of time and effort, but after a lot of delay for a variety of reasons, it’s moving along very, very well,” NPU spokesman Chris Riley said. “It’s an enormous challenge, but it’s also enormously exciting as well.”

Upgrades to the sewage treatment plant are estimated at $80 million. Building a new wet weather treatment facility, to capture and treat rainwater overflows, is pegged at $53 million. And a new high-capacity pump station at Rose Alley is estimated to cost $34 million.

Once final designs are completed, NPU will apply to DEEP for approval and go out to bid to finalize the costs. Work to eliminate sewer-rainwater overflow qualifies for 50% grant funding, 50% low-interest loan; work to remove nitrogen from the Thames River qualifies for 30% grant and other work, 20% grants and 80% loan.

The loans will be paid through the three years of 6% sewer rate increases the utilities commission enacted. But NPU also hopes that Congress approves a federal infrastructure package that could make at least part of the upgrade project "shovel ready" eligible.

NPU General Manager Chris LaRose said the project is an investment in economic development for Norwich and surrounding towns. NPU has agreements with Bozrah and Franklin to use the system, and is talking to Sprague, Preston and Lisbon.

“When you look at what a sewer brings to the city of Norwich, this is economic development,” he said. “You can’t grow anything in town without an excellent sewer system. Nobody wants to be in the sewer business, but it is a necessary requirement for business.”

Larry Sullivan, NPU wastewater integrity manager, said the funding isn’t the only complexity.

The plan will replace the antiquated digester system under the big gray dome with a liquid waste system. New screens will filter material to a quarter-inch size. No more will raw sewage sit in open-air pools.

But there’s a catch.

All the new construction will take place on the cramped, manmade Hollyhock Island that splits the Yantic River where the current plant stands. The old plant must remain in operation without pause throughout construction.

“These all have to stay in operation, while the new plant gets built around it," Sullivan said. "They’re still working on the construction sequence. ... It’s going to be an interesting five years.”

And those five years will bring dozens of workers to Hollyhock Island each day. Contractors and subcontractors, suppliers and workers needing lunches.

NPU officials said people only have to recall Tropical Storm Elsa to realize the need for the upgrade. The city received 4 inches of rain in eight hours, dumping 8.3 million gallons of untreated sewage and rainwater into Norwich Harbor. The system saw 20 million gallons of flow for six straight hours, Sullivan said, when a normal dry summer day might get 3 million, and a normal wet weather day 5 million to 7 million gallons.

The Rose Alley pump station now can handle 9 million to 10 million gallons. The new station will be capable of sending 60 million gallons of flow to the treatment plant, and the new wet weather system will treat it.

“That first inch of rainfall from the streets is going to have nitrogen from lawns and oil,” Sullivan said. “Why not treat that stuff? Now, Rose Alley is not capable of transporting it, and our plant is not capable of handling it.”

c.bessette@theday.com