Norwich — The Board of Public Utilities Commissioners on Tuesday approved “the framework” of an agreement to govern how five surrounding towns will use or reserve capacity in the Norwich sewage treatment system for the next 20 years.

The intermunicipal agreement with the towns of Bozrah, Franklin, Lisbon, Sprague and Preston must be approved by the towns, with Bozrah officials expected to vote on the deal Wednesday.

The agreement, in the works for the past six years, calls for Norwich Public Utilities to provide approximately 20,000 gallons per day of sewage capacity to each town, which would pay the same sewage usage rates to NPU as Norwich sewage customers pay, including the $2,500 capital connection fee per equivalent dwelling unit — with projected usage per unit of about 200 gallons per day.

NPU would bill the towns directly for usage recorded at the town border, and the host towns’ water pollution control authorities would collect usage fees from customers within the towns.

NPU has estimated the intermunicipal agreements to generate up to $25 million in revenue to the city utility over the next 20 years to help pay for major planned sewage treatment system upgrades, NPU officials said Tuesday.

NPU General Manager Chris LaRose said Norwich has about 2 million gallons of sewer capacity to offer at this point, but if a single large customer comes to the region, that could consume much of that capacity. But the projected trend is for reduced sewer usage over time, LaRose said, with new low-flow toilets becoming more common and NPU repairing old, leaking sewer pipes.

NPU currently processes about 5 million gallons of sewage per day, LaRose said.

Towns in the agreement could pay NPU in advance to reserve capacity if they wish, but LaRose expects most towns will pay for the capacity as needed.

The towns already have been building infrastructure to prepare to tie into the Norwich system. With the new Optimus senior housing complex on the verge of opening on Route 82 in Bozrah, lines already have been installed in the town.

Construction of sewer mains is underway in Franklin and is expected to be completed in spring. NPU has a main running down Route 12 to the Preston trash incinerator, and in the future expects to serve the development at the former Norwich Hospital.

Sprague has its own small sewage treatment plant in Baltic, but NPU provides service to about 60 customers in the Versailles area, totaling about 3,000 gallons per day, NPU officials said. NPU sewers now serve only a small section of Lisbon near the Lisbon trash incinerator.

c.bessette@theday.com