Ultraviolet Light Disenfection
Ultraviolet Disinfection For Public Water Supplies
By Michael Montysko
Reprinted from http://www.nyruralwater.org/aquafacts/summer2002/14.shtml
The NYSDOH (New York State Department of Health) Bureau of Public Water Supply Protection has recently revised its policy of accepting Ultraviolet light (UV) as primary disinfection at public water systems in New York State (A copy of the new policy called "Environmental Health Manual Item PWS-186" can be obtained from your local health department.). UV disinfection can now be considered for properly filtered surface water sources in addition to groundwater sources. In addition, UV can now be employed at community water systems with external distribution systems as long as a minimum chlorine dosage is provided for maintaining a distribution residual. In the past, the DOH and local health units have permitted the use of UV for the primary disinfection of only properly protected ground water sources. The recommended dosage was originally 16,000 microwatt-seconds per square centimeter (16 millijoules per square centimeter).
The use of UV was permitted only on good groundwater sources since it was thought that surface water pathogens were virtually unaffected by UV. The reason for this misconception was that when in-vitro analysis were conducted on UV irradiated water samples containing a known number of cryptosporidium or giardia lamblia cysts, the number of cysts in the irradiated water versus the pre-irradiated water did not change appreciably. Later, mouse ineffectivity studies showed that although the cysts were not killed, they became sterilized by the UV light. The sterilization of these pathogens occurs when the UV light photons collide with the organism and cause damage to its DNA destroying the organism's ability to reproduce and therefore rendering it unable to cause illness. When applied at a dosage of 40,000 microwatt-seconds per square centimeter it has been shown that a 3-log inactivation of cryptosporidium and giardia lamblia is achieved. Most viruses of concern are also inactivated at these doses.
The new policy requires that all new UV installations provide a minimum of 40,000 microwatt-seconds per square centimeter. The policy also requires the installation of light intensity meters, automatic shut off valves and alarms. Limitations are also placed on inorganic parameters of the raw water, which could potentially foul the device. Many manufacturer's units are rated at a dose of 32,000 to 38,000 microwatt-seconds per square centimeter for a specific flow rate. It may be possible to flow restrict a lower rated unit to increase the dosage since in theory the dosage is directly proportional to the contact time within the unit. An example is a unit rated at 10 gpm with a dose of 32,000 microwatt-seconds per square centimeter that would require flow restriction of 8 gpm to achieve the required 40,000 microwatt-seconds per square centimeter. The manufacturer of the UV unit should provide bioassy validation results for a specific model for a given flow rate. As mentioned previously, systems with external distribution systems are required to maintain a residual through the use of chlorine or other acceptable disinfectant. The residual disinfectant may also provide the secondary benefit of additional virus inactivation.
The use of UV may benefit many systems in the future by reducing disinfection by-products since significantly less chlorine and chlorine contact time may be needed to achieve proper treatment. Small systems with true ground water sources especially non-community systems without distribution systems may find UV to be more operator friendly than chlorination systems. As multi-barrier approach to treatment, UV on surface water sources may provide an additional factor of safety to the normal filtration removal and disinfection inactivation processes.
New York Rural Water Association
P.O. Box 487 Claverack, NY 12513
Phone: (518) 828-3155
Fax: (518) 828-0582www.watertechonline.com
Plant to attack Cryptosporidium Ultraviolet helps city exceed water disinfection goals
When the City of Henderson’s Department of Utility Services wanted to upgrade its water disinfection system, it turned to ultraviolet (UV) disinfection. Henderson, now the second largest city in Nevada, decided to upgrade and modernize its water treatment system to enhance water quality and meet increasing demand.
Following a public bid process conducted by the city, in conjunction with the Nevada State Health Division and engineering consultant CH2M Hill, Aquionics and Hanovia were chosen to supply a new state-of-the-art UV water disinfection system. Construction began in 2001 on one of the first large-scale UV potable water installations in the United States. Following several trials and a rigorous validation process, the system is now complete and in the final test phase for approval by the Nevada State Health Division.
Treating 15 million gallons per day (mgd), the UV disinfection plant consists of four medium pressure UV chambers, each with an independently validated capacity of 5 mgd. The system is completely automated with UV dose control, fail-safe features and reliable automatic cleaning for ease of maintenance. Provision has also been made for a fifth UV chamber should demand rise in the future. The water, which originates from Lake Mead, is coagulated with ferric chloride then flocculated and filtered before passing through the UV chambers prior to distribution.
The city's Technical Services Manager Mike Morine said, “We implemented this project to provide the best possible disinfection for our community. UV is a cost-effective way of achieving this, and it is expected to help us meet and exceed anticipated USEPA water quality regulations. Aquionics and Hanovia have worked diligently with Henderson, CH2M Hill, and the contractor to provide a system that exceeds our requirements and expectations."
The Henderson Department of Utility Services recently received a Gold Award for Competitive Achievement by the Association of Metropolitan Water Agencies (AMWA). "AMWA's Gold Award winners are among the best-operated water systems in the nation," said Diane VanDe Hei, Executive Director of AMWA. "Their performance reflects very positively on the industry, and the consumers and public officials they serve have good reason to be proud of their accomplishments."
HENDERSON, NV - Henderson will be getting the first state- and US Environmental Protection Agency (EPA)-approved ultraviolet (UV) drinking-water treatment plant with Cryptosporidium-inactivation capability in the United States.
The city has awarded a contract for the plant to the British company Hanovia, through its American sister company, Erlanger, KY-based Aquionics Inc.
The UV plant, scheduled for completion this year, will provide at least 99-percent inactivation of Cryptosporidium oocysts, the infective stage of the organism. The system allows for 33-percent redundancy and will treat up to 70,000 cubic meters of water per day from nearby Lake Mead.
"We are excited about utilizing this technology," said Mike Morine, project engineer for the city of Henderson. "We take our commitment to the health and safety of our residents very seriously and we are implementing this project to provide the best possible disinfection for our community."
Henderson selected Hanovia and Aquionics after an international search in conjunction with the Nevada State Health Division and the engineering firm CH2M Hill.