Stonington — Town and state officials gathered at the Mystic wastewater treatment plant Thursday morning to celebrate the completion of the $18 million upgrade of the town’s three aging treatment facilities, a project that was completed on time and just under budget.

In addition, they had a surprise for former Water Pollution Control Authority Director Harold Storrs, announcing that the upgrade would be dedicated to him and a plaque would be placed at the plant in honor of his 17 years of service to the town, much of which involved planning the project. Storrs retired just before construction began.

“We, as a board, thought the honor should be his as a lasting memory of not just what he did for us as a board but for the town,” WPCA Chairman James Sisk said. “He devoted 17 years of his life to the Town of Stonington.”

In November 2010, voters approved $18.3 million in bonding to pay for rehabilitating and replacing the equipment in the Mystic plant and installing a Biomag system to remove nitrogen from sewage to meet state standards.

While most of the money was spent on improvements at the Mystic plant, systems at the borough and Pawcatuck plants also were replaced, resulting in less chlorine being discharged into the Pawcatuck River and Stonington Harbor.

New aeration blowers, which provide oxygen to the organisms that clean the water, were installed at all three plants. Odor control systems also were upgraded at all three facilities.

The installation of the Biomag system at the Mystic plant allowed the needed improvements to take place without having to expand the plant, which could not be done because of the small size of its lot.

Sisk said the three-year project, which followed more than a decade of planning, was challenging because the town had to continue to operate the Mystic plant within its permits limits while making the upgrades.

He said the town was able to compete the work with no interruption of service and within permits limits.

Both Sisk and current WPCA Director Tom Gilligan said the success of the project was the result of the cooperation between US Water, the firm that runs the plant for the town, and CDM Smith, which designed and made the upgrades.

“They did an excellent job making this happen with minimal disruption for residents,” Gilligan said.

The design and construction ended up costing just less than $17 million with $1 million in other costs.

Gilligan said the project and its cutting-edge technology are designed to keep the plants operating for another 20 years.

j.wojtas@theday.com

Twitter: @joewojtas