24 Water Purification: Treatments requiring electricity: Ozone: UV Light
Description
This article is from the Water Treatment FAQ, by Patton Turner with numerous contributions by others.
24 Water Purification: Treatments requiring electricity: Ozone: UV Light
Ultraviolet light has been known to kill pathogens for a long time. A
low pressure mercury bulb emits between 30 to 90 % of its energy at a
wave length of 253.7 nm, right in the middle of the UV band. If water
is exposed to enough light, pathogens will be
killed. The problem is that some pathogens are hundreds of times less
sensitive to UV light than others. The least sensitive pathogens to
UV are protozoan cysts. Several studies show that Giardia will not be
destroyed by many commercial UV treatment units. Fortunately these
are the easiest pathogens to filter out with a mechanical filter
The efficacy of UV treatment is very dependent on the turbidity of
the water. The more opaque the water is, the less light that will be
transmitted through it. The treatment units must be run at the
designed flow rate to insure sufficient exposure, as well as
insure turbulent flow rather than plug flow.
Another problem with UV treatment is that the damage done to the
pathogens with UV light can be reversed if the water is exposed to
visible light (specifically 330-500 nm) through a process known as
photoreactivation.
UV treatment, like ozone or mechanical filtering leaves no residual
component in the water to insure its continued disinfection. Any
purchased UV filter should be checked to insure it at least complies
with the 1966 HEW standard of 16 mW.s/cm^2 with a maximum water depth
of 7.5 cm ANSI/NSF require 38 mWs/cm^2 for primary water treatment
systems. This level was chosen to give better than 3 log (99.9%)
inactivation of Bacillus subtillis. This level is of little use
against Gairdia, and of no use against Crypto.
The US EPA explored UV light for small scale water treatment plants
and found it compared unfavorably with chlorine due to 1) Higher
Costs, 2) Lower Reliability, and 3) Lack of a Residual Disinfectant.
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