Norwich — After years of negotiations with the state, revised plans, rising costs, Norwich Public Utilities hopes it has developed a plan to upgrade the city’s nearly 100-year-old sewage treatment system that continues to pollute Norwich Harbor and the Thames River when heavy rains overwhelm the system.

NPU will propose a $167 million compromise plan to the state Department of Energy and Environmental Protection in January in response to the agency’s rejection of NPU’s previous $150 million proposed upgrade plan and to counter DEEP’s request for a much larger $270 million pollution control plan.

NPU officials presented the new proposal to the Board of Public Utilities Commissioners on Thursday and will meet with DEEP officials in January in an effort to win state approval to move forward finally with the upgrade project. The state issued an order to the city in 2016 to correct the frequent sewage and stormwater overflows into the Thames River, which happen even with storms with as low as a quarter- to a half-inch of rain.

NPU General Manager Chris LaRose said the city utility will stress to the state that Norwich could not afford the state's proposed $270 million upgrade, which would call for installing controls on sewage overflows that would capture untreated runoff from all storms up to the worst average expected event in a typical two-year period.

NPU’s compromise plan would require average sewer rate increases of 5 percent per year for the first five years starting in 2021 for the project to be funded with a combination of state grants and low-interest loans through the Clean Water Fund. Construction would be done over a five-year period, and afterward, NPU is seeking state approval for a “capital recovery break” to ease the costs to ratepayers before continuing with longer-term system upgrades, LaRose said.

The proposed compromise plan has three main components: an $80 million major upgrade to the sewage treatment plant on Hollyhock Island on Falls Avenue, a $53 million new wet weather combined sewage overflow treatment plant to be built on Falls Avenue and a new Rose Alley pumping station underground at the city-owned viaduct parking lot.

Larry Sullivan, NPU wastewater integrity manager, said the new wet weather treatment plant would come on during storms when combined sewage and stormwater flows typically would overwhelm the existing treatment plant. The flow would be diverted into the new treatment facility, where solids and debris would be screened out and liquids treated with chlorine before the water is released into Norwich Harbor. When the storm ends and flows drop, the diverted system would shut down gradually.

Sullivan said in designing the new proposal, Norwich has learned from the experiences of the four other Connecticut cities already working to upgrade aging and polluting systems. New Haven learned that just cutting off stormwater collection from the sewer system meant the water ended up in private lawns and basements of homes and businesses. The better plan would be to divert the overflow into a wet weather treatment system.

Currently, the Rose Alley pumping station behind Main Street, the system’s largest, can handle 11 million gallons per day. The proposed new pumping station would handle 60 million gallons per day with a screening system to collect all manner of debris that ends up in the sewage system — 2-liter plastic bottles, a full-size mop and handle and a bath towel, to name a few recent items, Sullivan said.

NPU officials estimated the proposed upgrades would reduce the number of overflow events by 93 percent and the overflow volume by 80 percent.

If DEEP approves the compromise plan, NPU officials estimated final design work would be done in 2020 and 2021, and construction could start in 2021.

No interruptions in the daily operations are anticipated, Sullivan said, which is why it would take some five years to build. He likened the timeframe to state repairs and upgrades to a major highway. If you could close Interstate 95, for example, you could repair and repave it within days. But by keeping it open, the work could take years.

c.bessette@theday.com