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|Ultraviolet (UV) rays are part of the light that comes from the sun. The UV spectrum is higher in
frequency than visible light and lower than x-rays. As a water treatment technique, UV is known to be
an effective disinfectant due to its strong germicidal (inactivating) ability. UV disinfects water
containing bacteria, viruses, and Giardia lamblia and Cryptosporidium cysts.
UV has been used commercially for many years in the pharmaceutical, cosmetic, beverage, and
electronics industries. It was used for drinking water disinfection in the early 1900s but was
abandoned due to high operating costs, unreliable equipment, and the expanding popularity of
disinfection by chlorination. Recently, the safety of chlorination has been questioned and UV has
experienced increased acceptance in both municipal and household systems. There are few large-
scale UV water treatment plants in the United States although there are several such plants in Europe.
Municipal systems use UV in conjunction with chlorine, thus reducing the amount of chlorine
necessary for disinfection. Likewise, disinfection byproducts (DBPs), the chemicals associated with
chlorination, are also reduced. Certain DBPs, such as trihalomethanes, have been linked to increases
in certain cancers. UV treatment’s main advantage is that no chemical input is required. However, UV
treatment lacks residual (remaining) disinfection in the water delivery system, such as that available
with a chemical treatment system like chlorination. Therefore, a secondary disinfection method, such
as chlorine or ozone may be a requirement for a UV system.
Uses of UV disinfection
UV radiation has disinfection properties that inactivate bacteria, viruses, and some other
microorganisms. It effectively treats Giardia lamblia and Cryptosporidium cysts, which may also be
removed from water by filtration. UV is not recommended if the untreated water contains very high
levels of coliform, the indicator organism that is the basis for bacteriological water tests, or if there is
substantial color or turbidity (cloudiness) in the water. UV is effective only if the light intensity reaches
the organism in question; therefore, nothing should be present in the water that shields the organism
from the radiation. Household UV treatment could conceivably be used for chlorinated water from a
public supply if the home has a treatment device, such as an activated carbon filter, that removes
chlorine (and thus allows bacterial growth). In this case, UV provides a final disinfection of the water
Principles of UV disinfection
UV radiation has three wavelength zones: UV-A, UV-B, and UV-C, and it is this last region, the
shortwave UV-C, that has germicidal properties for disinfection. A low-pressure mercury lamp
resembling a fluorescent lamp produces the UV light in the range of 254 nanometers (nm). A nm is
one billionth of a meter (10^-9 meter). Since most microorganisms are affected by radiation around
260 nm, UV radiation is in the appropriate range for germicidal activity. There
are UV lamps that produce radiation in the range of 185 nm that are effective on microorganisms and
will also reduce the total organic carbon (TOC) content of the water.
In a typical UV system, approximately 95 percent of the radiation passes through a special quartz
glass sleeve and into the untreated water that flows in a thin film over the lamp. The glass sleeve
keeps the lamp at an ideal temperature of 104 °F. UV radiation affects microorganisms by altering the
DNA in the cells and impeding reproduction. UV treatment does not remove
organisms from the water, it merely inactivates them. The effectiveness of this process is related to
exposure time and lamp intensity as well as general water quality parameters. The exposure time is
reported as "milliJoules per square centimeter" (mJ/ cm2), and the U.S. Department of Health and
Human Services has established a minimum exposure of 16 mJ/cm2 for UV disinfection systems. Most
manufacturers provide a lamp intensity of 30-50 mJ/cm2. Coliform
bacteria, for example, are destroyed at 7 mJ/ cm2. Since lamp intensity decreases over time with
use, lamp replacement is a key maintenance consideration with UV disinfection. In addition, UV
systems should be equipped with a warning device to alert the owner when lamp intensity falls below
the germicidal range.
Used alone, UV radiation does not improve the taste, odor, or clarity of water. UV light is a very
effective disinfectant, although the disinfection can only occur inside the unit. There is no residual
disinfection in the water to inactivate bacteria percentage of microorganisms destroyed depends on
the intensity of the UV light and the contact time. If material builds up on the glass sleeve, the light
intensity and the effectiveness of treatment are reduced.
Either sediment filtration or activated carbon filtration should take place before water passes through
the unit. Particulate matter, color, and turbidity affect the transmission of light to the microorganisms
and must be removed for successful disinfection.
UV is often the last device in a treatment train (a series of treatment devices), following reverse
osmosis, water softening, or filtration. The UV unit should be located as close as possible to the point-
of-use since any part of the plumbing system could be contaminated with bacteria. It is recommended
that the entire plumbing system be disinfected with chlorine prior to initial use of a UV system.
Types of UV disinfection devices
The typical UV treatment device consists, of a cylindrical chamber housing the UV bulb along its
central axis (Fig. 1). A quartz glass sleeve encases the bulb; water flow is parallel to the bulb, which
requires electrical power.A flowcontrol device prevents the water from passing too quickly past the
bulb, assuring appropriate radiation contact time with the flowing water. It has been reported that
turbulent (agitated) water flow provides more complete exposure of the organism
to UV radiation.
A UV system housing should be made of stainless steel to protect any electronic parts from corrosion.
To assure they will be contaminant-free, all welds in the system should be plasma-fused and purged
with argon gas. The major differences in UV treatment units are in capacity and optional features.
Some are equipped with UV emission detectors that warn the user when the unit needs cleaning or
when the light source is failing. This feature is extremely important to assurance of a safe water
supply. A detector that emits a sound or shuts off the water flow is preferable to a warning light,
especially if the system might be located where a warning light would not be noticed immediately.
NSF International, a nonprofit standard-setting organization, has developed standards for these UV
light systems. NSF-approved systems can be found on the NSF website at www.nsf.org.
Maintenance of a UV system
Since UV radiation must reach the bacteria to inactivate 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. A few minutes for lamp warm-up is needed before the system is used again following
shut-down. 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
with a chemical such as chlorine prior to relying on the UV system for disinfection. Because UV lights
gradually lose effectiveness with use, the lamp should be cleaned on a regular basis and replaced at
least once a year. It is not uncommon 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. As
stated previously, units equipped with properly calibrated UV emission detectors alert the owner when
the light intensity falls below a certain level.
The treated water should be monitored for coliform and heterotrophic bacteria on a monthly basis for
at least the first 6 months of the device’s use. If these organisms are present in the treated water, the
lamp intensity should be checked, and the entire plumbing system should be disinfected with a
chemical such as chlorine.
Capacity of UV disinfection systems
UV is an in-line, point-of-entry system that treats all the water used in the house. The capacities range
from 0.5 gallons per minute (gpm) to several hundred gpm. Certain point-of-use devices (treating
water from a single tap) may include UV as a final disinfection method, as when used with reverse
osmosis, for example. (For more information on reverse osmosis systems, consult Water Treatment
Notes No. 4, Reverse Osmosis Treatment of Drinking Water.) Since bacteria may be shielded by
particles in the water, pretreatment to remove turbidity may be required.
There is also a limit to the number of bacteria that can be treated. An upper limit for UV disinfection is
1,000 total coliforms/100 mL water or 100 fecal coliforms/100 mL.
Prefiltration is required to remove color, turbidity, and particles that shield microorganisms from the
UV source. Water that contains high mineral levels can coat the lamp sleeve and reduce the
treatment effectiveness. Therefore, pretreatment with a water softener or phosphate injection system
may be necessary to prevent build-up of minerals on the lamp. Table 1 lists the maximum levels of
certain contaminants that are allowable for effective UV treatment. It is extremely
important to remember that UV provides no residual disinfection of the water. Microorganisms
that have been shielded from the UV light by components such as particulate matter or color, may not
be completely exposed to the radiation and may be reactivated if they come in contact with oxygen.
Therefore, storing UV-treated water for any period of time could result in recontamination.
Quick Facts about UV Water Treatment
Recommended maximum contaminant levels in water entering a UV treatment device:
Suspended solids 10 mg/L
Iron 0.3 mg/L
Manganese 0.05 mg/L
UV water treatment is an effective way to disinfect home drinking water supplies; it is becoming
increasingly popular as an alternative to chlorine disinfection systems because it adds no chemicals
to the water. There are, however, specific water quality parameters that must be met for the UV
system to produce adequate amounts of bacteriological safe water. In addition, adherence to a
regular maintenance routine is essential.