# Can I purify water with UV light? **URL:** \ **Category:** Factual Questions\ **Created:** [May 27, 2005, 12:07am UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686 "2005-05-27T00:07:15Z")\ **Posts on this page:** 19\ **Page:** 1
**Author:** ![CynicalGabe](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@CynicalGabe](https://boards.straightdope.com/u/CynicalGabe)\ **Post date:** [May 27, 2005, 12:07am UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/1 "2005-05-27T00:07:15Z")
Is it possible to use a UV light to sterilize a bottle of water? If so, what kind of light would be necessary, and how would I go about doing this? ---
**Author:** ![adirondack\_mike](https://avatars.discourse-cdn.com/v4/letter/a/b5a626/32.png) [@adirondack\_mike](https://boards.straightdope.com/u/adirondack_mike)\ **Post date:** [May 27, 2005, 12:17am UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/2 "2005-05-27T00:17:27Z")
UV radiation can be used to **sterilize** not to **purify** it. Milk is (was) pasteurized with UV radiation. To purify water (removing particulates, minerals, chemicals, etc.) can not be done with UV. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [May 27, 2005, 12:20am UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/3 "2005-05-27T00:20:05Z")
IIRC, the penetration of UV into clear water is about 1 cm; any turbidity will reduce that. The usual sanitization setup for water is a filter, then a flow cell that runs a \<1-cm-thick layer of water past a UV emitter in a quartz tube. Again, that’s IIRC – it’s been years since I’ve looked at the specs for one of these things. ---
**Author:** ![CynicalGabe](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@CynicalGabe](https://boards.straightdope.com/u/CynicalGabe)\ **Post date:** [May 27, 2005, 12:31am UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/4 "2005-05-27T00:31:17Z")
> [@adirondack\_mike](#): > > UV radiation can be used to **sterilize** not to **purify** it. Milk is (was) pasteurized with UV radiation. > > To purify water (removing particulates, minerals, chemicals, etc.) can not be done with UV. My bad with the title. OP was correct. 😉 1cm, huh? Next question: how much exposure is needed? Seconds? Hours? And what sort of light? A special lamp? Or will a blacklight do the trick? ---
**Author:** ![CynicalGabe](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@CynicalGabe](https://boards.straightdope.com/u/CynicalGabe)\ **Post date:** [May 27, 2005, 12:33am UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/5 "2005-05-27T00:33:56Z")
> [@Nametag](#): > > IIRC, the penetration of UV into clear water is about 1 cm; any turbidity will reduce that. The usual sanitization setup for water is a filter, then a flow cell that runs a \<1-cm-thick layer of water past a UV emitter in a quartz tube. > > Again, that’s IIRC – it’s been years since I’ve looked at the specs for one of these things. Oh, and a filter is out of the question in this scenario. As is boiling. So I figure I’m left with irradiation of some sort. Not having access to nuclear materials, I figure UV is about it. ---
**Author:** ![danceswithcats](https://avatars.discourse-cdn.com/v4/letter/d/9e8a1a/32.png) [@danceswithcats](https://boards.straightdope.com/u/danceswithcats)\ **Post date:** [May 27, 2005, 12:53am UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/6 "2005-05-27T00:53:52Z")
Blacklight is lower frequency UV than the bandwidth used for purification of water. The water chambers are constructed to pass water next to the quartz tube containing the light source for sufficient time to effect cleansing. They’re installed on the main distribution branch, so they have to be capable of handling the anticipated flow of the residence. **Nametag** is correct-turbidity and iron in water will reduce the effecacy of a UV unit, so if an analysis indicates same, a filter upstream is appropriate. ---
**Author:** ![wevets](https://avatars.discourse-cdn.com/v4/letter/w/e9a140/32.png) [@wevets](https://boards.straightdope.com/u/wevets)\ **Post date:** [May 27, 2005, 1:00am UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/7 "2005-05-27T01:00:42Z")
I seem to recall being told that in developing countries, an appropriate way to sterilize water is to leave it in 2 L containers on the roof with black plastic behind it in the sun for a couple days. Is this accurate? Is it temperature also or is the UV enough - it seems like those 2 L jugs are more than 1 cm thick… ---
**Author:** ![CynicalGabe](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@CynicalGabe](https://boards.straightdope.com/u/CynicalGabe)\ **Post date:** [May 27, 2005, 3:47am UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/8 "2005-05-27T03:47:35Z")
> [@wevets](#): > > Is it temperature also or is the UV enough - it seems like those 2 L jugs are more than 1 cm thick… Sounds like they are using temperature. So where can I find one of the higher-frequency UV lights? ---
**Author:** ![gotpasswords](https://avatars.discourse-cdn.com/v4/letter/g/c57346/32.png) [@gotpasswords](https://boards.straightdope.com/u/gotpasswords)\ **Post date:** [May 27, 2005, 4:18pm UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/9 "2005-05-27T16:18:29Z")
Are you looking to do this to a whole house worth of water, or a drinking water dispenser? If the latter, just grab one that’s already been engineered. One brand I’m aware of is Innowave. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [May 27, 2005, 4:53pm UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/10 "2005-05-27T16:53:08Z")
Comemrcial systms are available. I was looking into this for preventing algea blooms (red tide) in a closed loop chilled water system. \* The UV lamps can be VERY high power density; the water is used to cool them. This kills living “stuff” but does nothing for particulates, heavy metals, acid, etc that might still make water non-potable. \*chemical biocides were deemed more cost effective. ---
**Author:** ![NoPretentiousCodename](https://avatars.discourse-cdn.com/v4/letter/n/82dd89/32.png) [@NoPretentiousCodename](https://boards.straightdope.com/u/NoPretentiousCodename)\ **Post date:** [May 27, 2005, 5:32pm UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/11 "2005-05-27T17:32:17Z")
UV is much less effective on spore-forming bacteria. These include C. botulinum and other soil borne microbes. ---
**Author:** ![Harmonix](https://avatars.discourse-cdn.com/v4/letter/h/3ec8ea/32.png) [@Harmonix](https://boards.straightdope.com/u/Harmonix)\ **Post date:** [May 27, 2005, 8:23pm UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/12 "2005-05-27T20:23:50Z")
Perhaps chlorine tablets would be more cost effective? bleach? > **[Emergency Disinfection of Drinking Water | US EPA](https://www.epa.gov/ground-water-and-drinking-water/emergency-disinfection-drinking-water)** > > How to boil and disinfect water to kill most disease-causing microorganisms during emergency situations where regular water service has been interrupted and local authorities recommend using only bottled water, boiled water, or disinfected water. ---
**Author:** ![CynicalGabe](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@CynicalGabe](https://boards.straightdope.com/u/CynicalGabe)\ **Post date:** [May 27, 2005, 9:15pm UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/13 "2005-05-27T21:15:38Z")
> [@gotpasswords](#): > > Are you looking to do this to a whole house worth of water, or a drinking water dispenser? If the latter, just grab one that’s already been engineered. One brand I’m aware of is Innowave. Just a little bottle ---
**Author:** ![Myglaren](https://avatars.discourse-cdn.com/v4/letter/m/f1d935/32.png) [@Myglaren](https://boards.straightdope.com/u/Myglaren)\ **Post date:** [May 27, 2005, 9:26pm UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/14 "2005-05-27T21:26:45Z")
Distill it? ---
**Author:** ![CynicalGabe](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@CynicalGabe](https://boards.straightdope.com/u/CynicalGabe)\ **Post date:** [May 28, 2005, 12:44am UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/15 "2005-05-28T00:44:15Z")
> [@Myglaren](#): > > Distill it? Wont work for my purposes. I want to sterilize a small container of water while leaving the other properties of the water intact (minerals, etc). Killing bacteria and viruses is the only thing that interests me. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [May 28, 2005, 1:02am UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/16 "2005-05-28T01:02:27Z")
If you just want a few liters, why not sterilze a bottle by autoclave or baking, and shoot water into it through a 100,000MW cutoff ultrafilter? Amicon, Millipore and Cole-Parmer make devices for this or similar purposes. ---
**Author:** ![NoPretentiousCodename](https://avatars.discourse-cdn.com/v4/letter/n/82dd89/32.png) [@NoPretentiousCodename](https://boards.straightdope.com/u/NoPretentiousCodename)\ **Post date:** [May 29, 2005, 3:09am UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/17 "2005-05-29T03:09:06Z")
> [@CynicalGabe](#): > > Wont work for my purposes. I want to sterilize a small container of water while leaving the other properties of the water intact (minerals, etc). > > Killing bacteria and viruses is the only thing that interests me. hmmm… plastic or glass will block the UV in that case. ---
**Author:** ![Rikster](https://avatars.discourse-cdn.com/v4/letter/r/e9a140/32.png) [@Rikster](https://boards.straightdope.com/u/Rikster)\ **Post date:** [May 29, 2005, 5:47am UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/18 "2005-05-29T05:47:42Z")
Try a pond supply store, I have a 9 watt in my pond but they come higher ---
**Author:** ![NoPretentiousCodename](https://avatars.discourse-cdn.com/v4/letter/n/82dd89/32.png) [@NoPretentiousCodename](https://boards.straightdope.com/u/NoPretentiousCodename)\ **Post date:** [May 29, 2005, 6:26am UTC](https://boards.straightdope.com/t/can-i-purify-water-with-uv-light/305686/19 "2005-05-29T06:26:43Z")
Also not sure if UV will kill most viruses. Here is something you may find helpful from [this link to an online microbiology textbook.](http://pathmicro.med.sc.edu/fox/antibiotics1.htm) (Bolding mine) > [@](#): > > Sterilization refers to killing (or removal) of ALL bacteria in a non-selective fashion. For example, autoclaving involves heating liquids (e.g. media) or solids to 121oC under steam pressure. The materials must be heat resistant. Ethylene oxide is sometimes used in hospitals for equipment that cannot be heated. Membrane filters have pores that trap bacteria, but allow drugs and small chemicals to pass through; thus pre-sterilized filters can be used to sterilize delicate solutions. \*\*UV light is used for decreasing bacterial levels on surfaces such as in operating rooms; however it is not totally effective. \*\*Ionizing radiation is more efficient and can be used for sterilizing instruments and food. > > Disinfectants (e.g. phenol-based) can be useful in killing many bacteria on certain instruments, but cannot be used for internal consumption or on skin. Antiseptics (e.g. iodine or 70% alcohol) are used topically (e.g. on skin surfaces) to reduce bacterial load. This fits what I recall from micro class, namely that UV light is sometimes used in hospital settings to keep bacterial counts under control on certain surfaces, but not really as a primary means of disinfecting anything. Usually it is part of a whole system, and can be useful in keeping things under control, but is not a magic bullet by any means.