Catskill-Delaware Ultraviolet Disinfection Facility (2,020 mgd)

Client: New York City Department of Environmental Protection
Location: Westchester County, NY

Although New York's Catskill and Delaware water supplies are known for their excellent water quality, and has been granted avoidance of the need for filtration, concerns over heightened development in the watershed, coupled with increased vigilance over Cryptosporidium- and Giardia-related illness following outbreaks in other parts of the US and EPA’s Long Term 2 Enhanced Surface Water Treatment Rule, prompted the New York City Department of Environmental Protection (NYCDEP) to initiate the design and construction of a 2,020-mgd Ultraviolet (UV) Disinfection Facility in Westchester County. Hazen and Sawyer, in joint venture, undertook pilot testing, planning, environmental assessment, design, permitting and construction services for the innovative Catskill-Delaware Ultraviolet Disinfection Facility.

Project Outcomes and Benefits

  • We took an in-depth look at UV disinfection, and determined that coupled with a strong source water protection program, this alternative to filtration will provide increased public health protection for the NYC public for a cost substantially lower than that for a filtration plant.
  • An innovative approach for evaluating large-scale UV disinfection equipment – a total of 56 units each capable of treating up to 60-mgd - was used to select the best equipment of this project and demonstrate UV could be designed to treat over two billion gallons per day.
  • The facility will pose fewer impacts to the environment than a filtration plant, due to a shorter construction period, considerably smaller footprint, and significantly lower power requirements.
  • Safely treats and controls the flow of more than two billion gallons of water per day to nine million customers, by far the largest facility of its kind in the world.
  • Selected as a 2013 ACEC National Grand Award winner.

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.

To inform design, we built a validation facility to test the UV disinfection unit’s ability to meet the treatment, headloss, and power consumption goals, while also enabling competitive procurement of UV units.

Testing established the design criteria and validated the disinfection capability of the 60-mgd UV unit to be used in the facility.

The disinfection study and the environmental impact study for the Catskill-Delaware UV Disinfection Facility each received top honors from NY-ACE in the Engineering Excellence Competition.

The Catskill-Delaware facility is the largest drinking water ultraviolet disinfection installation in the world.

The UV disinfection facility will have a smaller footprint and lower energy needs than a filtration plant of the same capacity, saving New York City more than $1.5 billion in construction costs and $25 million/year in operation costs.

The design incorporates inlet structures, screening, dedicated flow meters, UV reactors, and flow control valves.

Also included in the designs were hydrogenerators to recover energy from the available head in the system.

The design required planning for integration of the new facilities into the existing hydraulic grade of an operating system without interruption of the water supply to the City of New York.

While NYCDEP initially was granted a Filtration Avoidance Determination (FAD) in 1993, relieving the City from having to filter the Catskill and Delaware supplies, the FAD did require that the City plan for filtration in the future if the water supply were to change. However, the FAD does not exempt the City from needing to provide a form of disinfection that targets Cryptosporidium as required by the EPA’s Long Term 2 Enhanced Surface Water Treatment Rule. By evaluating the effectiveness of UV disinfection against inactivating Cryptosporidium and Giardia, Hazen and Sawyer helped ensure that the City would not have to construct a filtration plant in order to maintain a safe and reliable water supply. The design and construction of the UV facility was then integrated into the FAD, which currently extends through 2017.

Hazen and Sawyer’s conceptual design for the facility evaluated both low-pressure high-output (LPHO), and medium-pressure UV units. LPHO was ultimately selected and we developed an approach that achieved testing to validate the unit’s effectiveness to meet the treatment, headloss, and power consumption goals, while at the same time providing for competitive procurement of UV units. The design was then customized around the selected equipment and required planning for integration of the new facilities, as well as potential future filtration facilities, into the existing hydraulic grade of a completely gravity fed system, all without interruption of the water supply to NYC.

As part of the joint venture team, the firm completed final design for the facility, which began operations in 2012. This facility, an ACEC National Grand Award winner, has the landmark designation as the world’s largest for treating municipal drinking water.

For more information on this project, or to discuss a similar project in your area, contact

Richard Peters, P.E. at