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Grass Island Wastewater Treatment Plant Upgrades

Cost-effective improvements fortify facility for the future.
  • Grass Island Wastewater Treatment Plant-Gannett Fleming

    Upgrades to the Grass Island Wastewater Treatment Plant ensured its long-term reliability.

  • Grass Island Wastewater Treatment Plant’s ultraviolet disinfection system-Gannett Fleming

    A low-pressure, high-intensity UV disinfection system eliminated the need for chlorine.

  • Grass Island Wastewater Treatment Plant’s return-activated sludge pumping system-Gannett Fleming

    Grass Island WWTP’s return-activated sludge pumping system.

Town of Greenwich, Connecticut, Department of Public Works

Greenwich, Connecticut

Our Role
Design, Construction Management.

34 mgd
Construction Cost
$7.1 million
6 years
  • Ensured long-term wastewater treatment plant operation and reliability
  • Improved water quality without interrupting daily operations
  • Eliminated chlorine from plant’s discharge, protecting water quality in Greenwich Harbor and Long Island Sound.

A creative design and carefully planned construction staging schedule enabled the town of Greenwich, Connecticut, to improve its Grass Island Wastewater Treatment Plant (WWTP) without interrupting daily operations. Utilizing ultraviolent (UV) disinfection, the project improved the water quality of the Long Island Sound by eliminating the toxicity impacts of chlorine.

Gannett Fleming seamlessly replaced all the plant’s pump systems with modern units, ensuring its long-term operation. The Grass Island project provides a blueprint for successful WWTP improvements and an example of cost-saving ingenuity.

What We Did

To satisfy the National Pollution Discharge Elimination System permit issued by the Connecticut Department of Environmental Protection, Gannett Fleming prepared a disinfection feasibility study, examining whether to upgrade the WWTP’s existing chlorination and dechlorination facilities or install a new ultraviolet disinfection system. The most cost-effective solution was determined to be a new ultraviolet disinfection system. Our team oversaw modifications to the chlorine contact tank to accommodate the new system, then installed a low-pressure, high-intensity horizontal UV system. This disinfection system uses an automatic dose-paced control system that ensures disinfection requirements are continuously achieved for the effluent flow conditions, while also conserving power.

Additionally, Gannett Fleming upgraded all 10 pumps at the WWTP, ensuring its long-term operation with improvements to the return-activated sludge (RAS), waste-activated sludge (WAS), and secondary effluent (SE) pump systems. Space constraints within the plant’s electrical room presented a serious project challenge when replacing existing variable-frequency drive (VFD) enclosures for the RAS and SE pumps. To overcome this hurdle, temporary VFD enclosures installed near the existing units were provided for each system. Using existing pump control panels, each pump motor was connected to the temporary drives one at a time. Once existing pumps were operating with temporary VFDs and running efficiently, the existing VFD enclosures were demolished. New pump-specific enclosures were then installed in the same location along with the corresponding new pump with associated piping and valves. Each VFD and corresponding pump was tested prior to installing the next VFD and pump system. These upgrades did not disturb plant operation or impact the community.

Key Features

  • Low-pressure, high-intensity, horizontal UV disinfection system
  • New pumps, motors, and associated equipment optimize plant performance
  • Pump control integration with plant supervisory control and data acquisition system.

Sustainability Features & Outcomes

  • New disinfection system satisfies stringent residual chlorine limits issued by Connecticut Department of Environmental Protection
  • Premium-efficiency motors reduce energy consumption
  • Pump improvements made within existing site footprint
  • Pump seal water systems use recycled plant effluent water, saving approximately 5.5 million gallons of potable city water annually.

Awards & Recognition

  • Platinum Award, 2014, American Council of Engineering Companies of New York, Engineering Excellence Awards
  • Silver Award, 2010, American Council of Engineering Companies of New York, Engineering Excellence Awards.

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