New York City Embraces UV Disinfection of its Water Supplies

Last Modified: Nov 18, 2009


  • Paul D. Smith, PE, NYC Department of Environmental Protection
  • Deborah Keesler, PE, NYC Department of Environmental Protection
  • Matthew Valade, PE; Ed Barboe, PE; and Steve Farabaugh, Hazen and Sawyer
  • James Schaefer, PE, Metcalf & Eddy, Inc.
  • Gary Kroll, PE, CDM

In a UV disinfection reactor, UV light inactivates microorganisms by causing photochemical damage to nucleic acids. Study results determined that it is feasible to use UV light for the disinfection of the Cat-Del water supplies to meet current and anticipated regulations.

New York City is fortunate to possess what is arguably the greatest metropolitan water supply system in the world. The New City Department of Environmental Protection (DEP) maintains two water supply systems that together meet an average daily water demand of about 1,200 mgd. The Catskill-Delaware System (Cat-Del) normally provides approximately 90% and the Croton System normally provides the remaining 10% of the City’s water supply.

Because the Cat-Del system contains a high quality raw water supply with an extensive watershed protection program, the DEP has been granted a Filtration Avoidance Determination (FAD) by the United States Environmental Protection Agency (EPA). The supply of the Croton system is located closer to the city and the watershed has been subjected to suburban-type development over the years. This has affected the quality of the water source and filtration is now required by consent decree. In order to continue to provide high quality drinking water to the city, NYCDEP is providing UV treatment for both these supplies. The designs have been complete and construction is underway for the 2020-mgd Cat-Del UV Disinfection Facility and for the 290-mgd Croton Water Treatment Plant.

The Cat-Del UV Treatment Facility will provide UV disinfection for the Catskill-Delaware Water Supply System in compliance with the EPA FAD. UV disinfection in combination with chlorine disinfection, which is currently in place, will provide a multiple disinfection barrier. The Cat-Del UV Treatment Facility has been designed to achieve a 3-log inactivation of Cryptosporidium with UV disinfection alone, a 3-log inactivation of Giardia through a combination of UV and chlorine disinfection and a 4-log inactivation of viruses with chlorine disinfection.

The Croton Water Treatment Plant will provide filtration and disinfection for the Croton water supply. The disinfection component of the plant will consist of a UV disinfection system followed by chlorine disinfection. It has been designed to achieve a 12-log inactivation of Cryptosporidium and Giardia with UV disinfection alone, a 3-log inactivation of Giardia through a combination of UV and chlorine disinfection and a 2-log inactivation of viruses with chlorine disinfection.

This paper presents the role of UV disinfection in New York City’s treatment water program. It provides an overview of NYC’s water supply system, the evaluations and decisions that resulted in the implementation of UV, similarities and differences in UV between the two facilities, and a status on the current progress of both projects. Embracing UV for New York City is a multi-year, multi-billion dollar program that is of interest to waterworks planners, design engineers and operations staff.

For a copy of the full paper, please contact the author at

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