Project Summary

The Town of Hillsborough obtains its water supply from a reservoir – Loon Pond. Prior to the project, the water received slow sand filtration, chlorination and pH-adjustment with potassium hydroxide prior to discharge to the system. 

In general, the reservoir produced excellent water quality except for the development of chlorinated disinfection byproducts (DBPs) – total trihalomethanes (TTHM) and haloacetic acids (HAA5) – which had regularly exceeded state standards. The Town subsequently entered an Administrative Consent Order with NHDES to reduce the DBPs and selected Hoyle Tanner to engineer a solution.  

Working with the Town, available treatment alternatives and technologies were narrowed to two approaches for testing: post-filtration with granular activated carbon (GAC); or UV disinfection followed by chloramination. These two approaches were selected, in part, to maintain two highly valued existing system advantages: 
• A completely gravity-fed system with virtually no pumping; and 
• No daily or regular spent backwash or comparable waste product. 

These advantages currently resulted in high energy-efficiency, very little waste product for disposal and reduced daily operator attention. 

Hoyle Tanner retained the University of New Hampshire to assist with the bench-scale testing and then performed GAC pilot-testing at the water treatment facility. The testing indicated that GAC would not be cost-effective, so UV disinfection followed by chloramination was selected. 

Hoyle Tanner engineered a direct-purchase procurement of the UV system equipment based on lifecycle costing and other considerations and developed the installation construction bid package. A unique, compact UV system was selected to fit within the space constraints of the filter plant pipe gallery.  The system was successfully completed and has operated as expected.