Distribution System Water Quality Study, Norwich, CT

Tata & Howard, Inc. was retained by the Norwich Public Utilities (NPU) in Norwich, Connecticut to perform a distribution system water quality study to evaluate the water age, water quality, and hydraulic gradient in the Taftville/Occum Service Area. The Taftville/Occum Service Area has high water age, due primarily to the large volume of the Occum Tank.  In addition, high levels of total trihalomethanes (TTHM) are observed throughout the Taftville/Occum Service Area.

This study examined the TTHM concentrations and high water age in the Taftville/Occum Service Area and presents various options to help mitigate these issues.

During the course of this study the following tasks were completed:

• Updated the existing WaterGEMS hydraulic model using recent system data
• Collected water distribution system data on current operating conditions,
  historical and current disinfection by-product samples, water age, pressures, flows, and tank levels and supply and demand data for the entire NPU system
• Conducted a gradient evaluation of the affected service area and investigated potential alternatives to improve the current conditions, including making short and long term capital improvements to provide improved tank level fluctuation and improved customer pressure ranges
• Performed water age modeling of the existing service area and for potential
  alternatives that improve the current conditions and graphed the results
• Provided information and costs for alternatives to the tank passive mixing system
• Provided recommendations for improving the service area gradient(s), water age, and water quality
• To evaluate the Taftville/Occum Service Area, the entire NPU distribution system needed to be examined to determine the quality of the water as it enters the service area.

The key finding from this study related to the TTHM levels in the Taftville/Occum Service Area was that the formation potential in the effluent water from both of NPU’s surface water treatment plants was high enough for the TTHMs to exceed 80 µg/L at the entry point to the service area.

Based on this study, NPU has decided to add dissolved air flotation (DAF) to its Stony Brook water treatment plant. Tata & Howard designed the DAF system and other related improvements.