Ultraviolet
Ultraviolet Radiation
(UV) units are usually installed as Point-of-Entry (POE) systems. They
are generally placed at the entrance of the water system into the house to
service all incoming water.
Ultraviolet
(UV) systems expose the water to light from a special lamp. The
light is at a specific wavelength between 100 and 400 nanometres (nm) with light
under 300nm being most effective at killing common bacteria.
UV adds nothing to the
water, produces no tastes or odors, and usually requires
only a few seconds of exposure to be effective.
The minimum dosage for an ultraviolet light water treatment device is
16,000 µWs/cm² at a wavelength of 253.7 (nm) at maximum flow through the UV
device.
UV
is effective on pathogens but not on protozoan cysts such as those of Giardia
Lamblia, responsible for giardiasis. Ultraviolet light
physically alters the DNA
material in cells so that bacteria, viruses, molds, algae and other
microorganisms can no longer reproduce. When a microbe can no longer
multiply it is considered dead.
UV systems, however, cannot process water
which is turbid or has suspended solid particles of inorganic
or organic matter because these can shield
organisms against the light. Because of the possible
presence of protozoan cysts, pre-filtration to clarify the water or micro filtration
after UV processing must
be added to ensure that the system is completely disinfected.
Ultraviolet effective for:
- Bacteria
: Coliform Bacteria,
- Bacteriophage
(E. coli),
- Salmonella
Typhosa (Typhoid Fever)
Considerations:
Because
the process of UV sterilization is generated by a light source the light chamber
through which the water passes must be kept clear of algae, scale build-up and sediment
which can foul the light chamber. UV lamps are separated from the water by
a tube, it is these tubes upon which algae grows, recently Teflon tubes have
been replacing quartz tubes decreasing the algae problem. Consider
maintenance.
UV
lamps have a functional life expectancy of between 12 and 18 months after which
their ability to emit sufficient UV emissions is reduced below the level at
which bacteria will be killed or sterilized. Consider
maintenance.
Special
consideration must be given to "Dwell-time," if the Gallon Per Minute
(GPM), or flow rate is only slightly too high for the power of the UV lamp,
there is little net effect on the microorganisms.
UV
sterilization is most effective when:
-
Total
iron is less than 0.3ppm
-
Manganese
is less than 0.05ppm
-
hydrogen
sulphide is less than 0.05ppm
-
suspended
solids are less than 10ppm
-
Water
hardness below 7 gpg
Have your water
tested!
Holding UV-treated water
within and without the
water system
may cause increase
in the risk of contamination from reintroduction of bacteria. For example,
larger water systems and the storage water in the
refrigerator for a cold drink or reconstituted drinks.
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