Norwich ― The biggest single construction project in Norwich Public Utilities’ history is expected to begin this fall, and like building a new school, the utility will need to ensure the existing obsolete sewage treatment plant stays open while building its $200 million replacement on the same site.

Utility officials and a project design consultant presented the plans to the City Council on Monday.

This came in advance of a Sept. 18 public hearing and council vote on a proposed ordinance authorizing $200 million in revenue bonds for the project to be paid through sewer customer rates.

The current sewer plant was built in 1955 and upgraded in 1973 with equipment that was expected to last 20 years but is still in place, said Craig Wagner, principal engineer for CDM Smith.

NPU General Manager Chris LaRose said the upgrade project must be done now to avoid state fines for water contamination caused by the outdated plant, which frequently sends untreated sewage into the Thames River following major storms and nitrogen contamination.

“We’re basically looking at incorporating 50 years of new technology that the plant doesn’t have today,” Wagner said.

Along with improvements in daily operations, more automation, flood prevention upgrades and expanding the plant capacity from 15 million to 20 million gallons per day, the project also will place a modern cap on the city’s old landfill at the far end of Hollyhock Island.

The landfill never was capped properly. Material excavated to prepare for the plant construction will be spread on top of the landfill, graded and capped. The site then could be used for a solar array to help power the sewage treatment plant, LaRose said.

The entire project will be funded through the state Clean Water Fund, with $72 million in grants and $128 million in a 2% interest loan to be paid by the city’s 10,000 sewer customers over 20 years. The Board of Public Utilities Commissioners is expected to vote Sept. 26 on proposed new utility rates, including a 12.1% sewer rate increase to help pay for the new plant.

LaRose said NPU reached a regional agreement with surrounding towns in the hopes of generating some $20 million in new revenue to help pay for the plant, but new hookups in those towns has been slower than expected. Expanded sewer lines are planned to stretch into Bozrah and Franklin as well as Preston to serve development at the former Norwich Hospital property, LaRose said.

The new plant construction is expected to break ground this fall and take five years to complete. As with a new school construction project, the old plant must continue to operate and remain in compliance with state and federal environmental regulations during construction.

Once the sewer plant project is completed, NPU will pursue other sewer system upgrades recommended by DEEP. One calls for installing a wet weather treatment process adjacent to the existing sewer plant to treat higher volumes of wet weather flows prior to discharging the material into Norwich Harbor. DEEP also wants NPU to rebuild its main wastewater pumping station to reduce combined sewage and stormwater overflows into the harbor, NPU officials said.

“These two other projects will require additional, significant investment by NPU ratepayers,” NPU wrote in a news release providing an overview of the sewer system upgrade project. “We will continue to balance the environmental and regulatory requirements from DEEP with our goal of minimizing the impact on our wastewater customers.”

c.bessette@theday.com