[at-l] More technical info on uv sterilization

["David F. Addleton" <dfa@wimlaw.com>]

The minimum recommended dose is 16,000 MWs/cm2 (which are 60 billionths of
a kilowatt hour/cm2) but pathogens differ widely in the amount of radia-
tion required to kill or deactivate. Bacteria are the easiest group to
treat and differ the least in amount of radiation required. Viruses are
most resis- tant and variable. Recent research indicates that Polio and
Rotarians virus required 45,000 MWs/cm2 or nearly three times the minimum
recommended dose. A new low-pressure mercury lamp will deliver about 30,000
MWs/cm2 but the level decreases with age so lamps are normally replaced
annually. Cysts and worms are unaffected by ultraviolet light so if present
they must be treated by another procedure. 
An ultraviolet water treatment device is quite simple. The most common
design consists of a small cylindrical tank with a cylindrical mercury arc
lamp located along the center line. Water enters one end, flows through the
annular space between the lamp and the outer cylinder and exits from the
other end within a few seconds. The annular space between the lamp and the
outer wall must contain enough volume to allow the required amount of
radia- tion exposure but small enough to maintain the minimum radiation
intensity at the outer wall surface. The standard is 16,000 MWs/cm2 at 7.5
cm distance which is nearly 3 inches. 
Ultraviolet light is easily absorbed by solids, including particulate
matter in the water or deposits on the lamp surface. As a consequence,
ultra- violet light treatment should only be attempted on clear water. A
prefilter is recommended on most units and periodic inspection and cleaning
as neces- sary. 
Mercury arc lamps require some time to warm up before they operate prop-
erly and various mechanisms allow for this. A safe UV treatment device
should not allow water to flow unless the mercury lamp is operating up to
tempera- ture. There are few industry standards or government regulations
that cover these treatment devices, so the buyer needs to ask questions
about how the device and system work. 
Ultraviolet treatment has the advantage of adding nothing to the water and
not requiring the addition of treatment materials as long as the lamp is
maintained in good operating condition. The major disadvantage is that
there is no residual for treatment beyond the device. If contamination
occurs after treatment, another disinfection method such as chlorination
will have to be used to sanitize the system and treat the water. 
Some pathogens deactivated by UV light may be reactivated when exposed to
oxygen as there is no residual to counteract recontamination. Storing water
in the refrigerator for a cold drink or reconstituted drinks that nor-
mally are refrigerated are the common problems. Water for such uses might
best be boiled or treated with a small amount of disinfectant if UV is the
primary disinfection process. 

This comes from: 
http://hermes.ecn.purdue.edu/cgi/convwqtest?/mf-886.ks.ascii

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