Drinking Water Treatment – Ultraviolet Radiation

EFFECTIVE AGAINST: Efficient at inactivating vegetative and sporous forms of bacteria, Giardia lamblia, Cryptosporidium cysts, and other pathogenic microorganisms.

INEFFECTIVE AGAINST: Not recommended if the untreated water contains high levels of coliform, substantial color, or suspended solids. (Where total coliform bacteria exceed 1,000 colonies per 100 mL or fecal coliform bacteria exceed 100 colonies per 100 mL.)



Ultraviolet (UV) light has been used to disinfect water supplies for more than 75 years, but only recently have home UV systems become available. Municipalities sometimes use UV instead of chlorination for disinfection to avoid the byproducts that chlorination may produce in the treated water supply. The primary advantage to UV treatment is that it disinfects water without the use of chemicals. Its primary disadvantage is the lack of residual disinfection.

UV devices may either be point-of-use or point-of-entry. UV light kills bacteria, viruses, and some cysts. It does not kill Giardia lamblia cysts or Cryptosporidium parvum oocysts, which must be removed by filtration or distillation. UV is not recommended if the untreated water has a coliform content exceeding 1,000 total coliforms or 100 fecal coliforms per 100 milliliters.

It is important to note that, although UV is an effective disinfectant, disinfection only occurs inside the unit. No disinfection occurs beyond the treatment unit to kill bacteria that survived or were introduced after UV treatment. If residual disinfection is necessary, chlorination may be necessary in addition to or as an alternative to UV.

How ultraviolet radiation works

UV systems expose water to the light from a special lamp. The light is at a specific wavelength, capable of killing common bacteria. The percentage of organisms killed depends on the intensity of the UV light, the contact time that the water has with the light, and the amount of suspended solid particles in the water. The system adds nothing to the water, produces no tastes or odors, and typically requires only a few seconds of exposure to be effective. Treatment of the water occurs as the water passes into the light. The light penetration into water is shallow, usually only 2 to 3 inches.

Suspended solid particles in the water can shield organisms from the light. The untreated water entering the unit must be completely clear and free from any sediment or turbidity to allow all of the bacteria to be contacted by the light. In addition, inorganic constituents such as iron, manganese, and hardness must be below certain specified levels for the UV unit to effectively treat the water. Water with a high hardness (calcium and magnesium) may also coat the sleeve with scale (a whitish deposit of hardness), which may require routine cleaning or addition of a water softener. Therefore, UV devices are often combined with other technologies such as particle filters, carbon filters, ion exchange units, and reverse osmosis systems to remove particles prior to UV disinfection. UV is often the last device in the treatment train (a series of treatment devices), following reverse osmosis, water softening, and filtration. The UV unit can either be a point-of-entry system, treating all the water entering the house, or a point-of-use device, treating water from a single tap as a final disinfection method.

The typical UV treatment device requires electricity and consists of a cylindrical chamber that houses a low-pressure mercury lamp that produces the UV light. A quartz glass sleeve encases the bulb, which prevents the water from contacting the lamp and helps keep the lamp at an ideal operating temperature of 104 F. The lamp produces the UV light. Approximately 95 percent of the radiation passes through this glass sleeve and into the untreated water. The untreated water either flows in a thin film over the sleeve, or it flows through quartz glass tubing that is spiraled around the lamp. The latter design allows for a longer contact time between the UV light and the untreated water. Lamp intensity, contact time, and general water quality determine the effectiveness of the process.

A UV unit should be located as close as possible to the point of use because any part of the plumbing system could be contaminated with bacteria. Before using a UV system for the first time, disinfect the entire plumbing system with chlorine.


There is a limit to the numbers of bacteria that can be treated with UV. An upper limit for UV disinfection is 1,000 total coliforms per 100 milliliters or 100 fecal coliforms per 100 milliliters.

UV systems may have a flow rate capacity of 0.5 gallon per minute to several hundred gallons per minute. Household water requirements dictate the level of treated water needed.


Regardless of the quality of the equipment purchased, it will not perform satisfactorily unless maintained in accordance with the manufacturer’s recommendations for maintenance, cleaning, and part replacement. Keep a logbook to record water test results, equipment maintenance, and repairs.

Because UV radiation must reach the bacteria to kill them, the housing for the light source must be kept clean. Commercial products are available for rinsing the unit to remove any film on the light source. An overnight cleaning with a solution of 0.15 percent sodium hydrosulfite or citric acid effectively removes such films. Some units have wipers to aid the cleaning process.

UV systems are designed for continuous operation and should be shut down only if treatment is not needed for several days. The lamp needs a few minutes to warm up before the system is used again following shutdown. In addition, the plumbing system of the house should be thoroughly flushed following a period of no use. Whenever the system is serviced, the entire plumbing system should be disinfected prior to relying on the UV system for disinfection.

UV lights do not burn out but gradually lose effectiveness with use, so the lamp should be cleaned on a regular basis and replaced at least once a year. It is common for a new lamp to lose 20 percent of its intensity within the first 100 hours of operation, although that level is maintained for the next several thousand hours. Units equipped with properly calibrated UV emission detectors alert the owner when the unit needs cleaning or the light source is failing – an important feature to ensure a safe water supply. A detector that emits a sound or shuts off the water flow is preferable to one with a warning light. Detectors should not supplant annual replacement of the light source or regular cleaning of the lamp housing.

The treated water should be tested for coliform bacteria on a monthly basis for at least the first six months of the device’s use. If bacteria are present in the treated water, the lamp intensity should be checked and the entire plumbing system should be shock chlorinated.

Special considerations

Ensure the system you choose is installed and operated according to the manufacturer’s instructions. After installation, retest both the raw water (prior to treatment) and the treated water at a state certified laboratory to ensure it is working properly and removing the contaminants. You should continue to test the quality of both the untreated and treated water annually. This annual test will also help you determine how well your treatment system is working and whether maintenance or replacement of components may be necessary.

The following affect the disinfection efficiency of a UV system:

   * Contact times and flow rate
   * Depth of water being treated
   * Chemical and biological films that develop on the surface of UV lamps
   * Water quality - the presence of other contaminants
   * Clumping or aggregation of microorganisms
   * Turbidity
   * Color
   * Short circuiting in water flowing through the UV contactor
   * Accumulation of solids on the surface of the UV sleeves

Questions to ask before you buy

Before purchasing a water treatment device, have your water tested at a state certified laboratory to determine the contaminants present. This will help you determine if ultraviolet radiation is an effective treatment method for your situation. See Questions to Ask Before You Buy A Water Treatment System for more information.

Adapted from: Wagenet,L., K. Mancl, and M. Sailus. (1995). Home Water Treatment. Northeast Regional Agricultural Engineering Service, Cooperative Extension. NRAES-48. Ithaca, NY.