UV Water Purification

[TKM]

Does anyone know how the consumer versions of the UV water purifiers such as
those now sold in Home Depot are set up? Do they just irradiate the water
with UV-C or is there more going on.

Terry McGowan

 

[Victor Roberts]

Does anyone know how the consumer versions of the UV water purifiers such as
those now sold in Home Depot are set up? Do they just irradiate the water
with UV-C or is there more going on.

Terry McGowan

Hi Terry - I've never seen the ones at Home Depot, but I
belong to a sailing club on Saratoga Lake here in NY and we
draw our drinking water from the lake. It goes through a
sediment filter and then a charcoal filter and then a UV
unit that does just what you suggest - the water is
irradiated by germicidal lamps. I also did some work for a
company that makes large "medium pressure" Hg lamps for
water purification. I never saw the complete water treatment
system, but it was my understanding that the water was
simply irradiated by the lamps, which may actually produce
more UV B than UV C.

--
Vic Roberts
http://www.RobertsResearchInc.com
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[Victor Roberts]

| Does anyone know how the consumer versions of the UV water purifiers such as
| those now sold in Home Depot are set up? Do they just irradiate the water
| with UV-C or is there more going on.

I would think the UV would be adequate if strong enough (which would be too
strong to look at I would think). I'd like to find an ice maker/dispenser
that has strong UV lights that shut off when you open the door to scoop out
some ice, and back on again when the door is closed.

UV would quickly destroy the plastic used in most
refrigerators. You would probably have to go back to
porcelain enameled steel.

--
Vic Roberts
http://www.RobertsResearchInc.com
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[George Pontis]

Hi Terry - I've never seen the ones at Home Depot, but I
belong to a sailing club on Saratoga Lake here in NY and we
draw our drinking water from the lake. It goes through a
sediment filter and then a charcoal filter and then a UV
unit that does just what you suggest - the water is
irradiated by germicidal lamps. I also did some work for a
company that makes large "medium pressure" Hg lamps for
water purification. I never saw the complete water treatment
system, but it was my understanding that the water was
simply irradiated by the lamps, which may actually produce
more UV B than UV C.

These lamps are also used in aquaria. The original designs used bulbs similar to
linear fluorescent tubes, but of course no phosphor and presumably a quartz
envelope. A typical model would be the G25T8. Some designs just surrounded the
tube with a water jacket, used a couple of o-rings to seal, and flowed aquarium
water past it. Later designs put the lamp inside a quartz tube which made it
easier to control the sealing interface. Some new models use enclosures with a
deep helical grove to direct the water flow and increase exposure time. The new
generation of UV units often use twins with a 2G11 base, such as the PL-L18W/TUV.

Wavelength is definitely UV-C. Almost all the output is at 254nm. The early
designs had failure points caused by damage to plastic parts that were exposed to
the UV. These things definitely work if applied properly, which usually means that
the exposure time is long enough to kill the bugs of interest. There is a wide
range of dose necessary to kill all the organisms that might be in water. For
example, algae and bacteria are easier damaged than protozoa. The primary
limitations to their use in aquaria are due to them working too well! For
freshwater planted aquaria, they oxidize iron and important trace elements so they
are no longer available to the plants. In saltwater systems, they can kill small
organisms that are intentionally cirulating and feeding reef animals.

The lamps are subject to the same failures that affect regular fluorescent lamps.
They are usually changed based on hours of operation to ensure adequate light
output. Higher reliabability applications use multiple lamps, lest one lamp or its
ballast fail and go undetected for a while.

 

[TKM]

Hi Terry - I've never seen the ones at Home Depot, but I
belong to a sailing club on Saratoga Lake here in NY and we
draw our drinking water from the lake. It goes through a
sediment filter and then a charcoal filter and then a UV
unit that does just what you suggest - the water is
irradiated by germicidal lamps. I also did some work for a
company that makes large "medium pressure" Hg lamps for
water purification. I never saw the complete water treatment
system, but it was my understanding that the water was
simply irradiated by the lamps, which may actually produce
more UV B than UV C.

Hmmm. Gives the germs a tan rather than certain death. PETA has gone way
way too far.

What I was wondering is that, in addition to germ killing, the addition of
materials (perhaps as coatings on surfaces) such as titanium dioxide would
destroy organic molecules that cause odor and taste problems. But I didn't
know what claims were being made for the devices.

Terry McGowan

 

[TKM]

To expand on how these things work.
They damage the DNA.
This is a cumulative dosage per organism, and the figure of merit is
mJ/cm^2 exposure.

There is a variance of some hundred times or more between the easiest
to kill organisms (IIRC, many harmless bacteria) and the hardest
(spores or eggs with nearly opaque coatings protecting the DNA).

If the dosage isn't enough to kill in one pass, many passes with a delay
can work just as well - but not if the breeding rate is higher than the
sterilisation rate of course.

Clean water is very transparent to UV - you get the best bang for the
watt, when the UV can shine along a water column until it's absorbed.
In practice of course, apart from large treatment plants, this is rarely
achieved.

Sounds like there might be extensive dose information available. Is there a
good reference on bug type and dose required?

Terry McGowan

 

[Victor Roberts]

These lamps are also used in aquaria. The original designs used bulbs similar to
linear fluorescent tubes, but of course no phosphor and presumably a quartz
envelope. A typical model would be the G25T8. Some designs just surrounded the
tube with a water jacket, used a couple of o-rings to seal, and flowed aquarium
water past it. Later designs put the lamp inside a quartz tube which made it
easier to control the sealing interface. Some new models use enclosures with a
deep helical grove to direct the water flow and increase exposure time. The new
generation of UV units often use twins with a 2G11 base, such as the PL-L18W/TUV.

Wavelength is definitely UV-C. Almost all the output is at 254nm. The early
designs had failure points caused by damage to plastic parts that were exposed to
the UV. These things definitely work if applied properly, which usually means that
the exposure time is long enough to kill the bugs of interest. There is a wide
range of dose necessary to kill all the organisms that might be in water. For
example, algae and bacteria are easier damaged than protozoa. The primary
limitations to their use in aquaria are due to them working too well! For
freshwater planted aquaria, they oxidize iron and important trace elements so they
are no longer available to the plants. In saltwater systems, they can kill small
organisms that are intentionally cirulating and feeding reef animals.

The lamps are subject to the same failures that affect regular fluorescent lamps.
They are usually changed based on hours of operation to ensure adequate light
output. Higher reliabability applications use multiple lamps, lest one lamp or its
ballast fail and go undetected for a while.

The lamps you are describing are low pressure Hg lamps,
basically fluorescent lamps without the phosphor and in
UV-transmitting envelopes. (As you said.) The dominant
output, by far, from a low pressure Hg discharge is 254nm
radiation.

The lamps I was discussing are called "medium pressure Hg
lamps" in the UV industry. Lighting people would call them
high pressure Hg lamps.

According to one source,

http://www.iupac.org/goldbook/M03816.pdf

the dominant output from these lamps is 300, 303, 313, 334,
366, 405, 436, 546 and 578 nm. There is virtually no output
at 254 nm because the high Hg pressure causes the 254 nm
radiation to be self-absorbed and almost all the radiation
comes from non-resonant transitions. There is no significant
UV C from these lamps.

The main reason why these lamps are use in spite of the lack
of UV C is that high Hg pressure provides much higher power
density than is possible with a low pressure Hg lamp. We see
the same effect when comparing low pressure Hg fluorescent
lamps to high pressure Hg, metal halide and high pressure
sodium lamps..

--
Vic Roberts
http://www.RobertsResearchInc.com
To reply via e-mail:
replace xxx with vdr in the Reply to: address
or use e-mail address listed at the Web site.

This information is provided for educational purposes only.
It may not be used in any publication or posted on any Web
site without written permission.