Does a solar still purify water?

[Mike Barney]

A discussion came up regarding the need to, or not to purify water that comes out of a solar still. A couple of articles I’ve read indicate you can directly drink anything that comes out of it, including that which uses ‘distasteful’ inputs (ie seawater or urine).

Anyone have any idea if that’s correct, for normal or emergency situations?

Thanks,
MikeB

[Sam Haraldson]

This process removes impurities such as salts and heavy metals as well as eliminates microbiological organisms.

[paul johnson]

Sam’s right. It’s a still, and so “distills” the water.

FWIW, what won’t be removed, and you’re likely not to encounter in backcountry water sources are VOC’s (volatile organic chemicals), as these will begin to evaporate before (or with) the water, condense on the hood of the still and run down to the catch basin. Other than activated charcoal which works on some chemicals, there is not much, to my knowledge, that backpackers have available to them that will remove all chemicals from backcountry water.

Another SOLAR option, for pre-filtered fresh water, not contaminated with chemicals, is SODIS (solar disinfection) if you are NOT on the move, or possibly are on the move in a large enough vessel (pack rafting???, i guess it’s kayaking nowadays.) to perform SODIS.

Just hang a clear bag or jug of pre-filtered water in direct sunlight for about 6.0-6.5 hrs and voila’ the UV wavelengths in the sunlight will make point defects in microbial DNA/RNA making self-replication of the little buggers impossible.

Any issues with the type of plastics being used? I’m the wrong one to address this question. Perhaps someone else will help out with this one. Can, for example, a Platy be used? My guess is “yes”, but i would like an authoritative response from someone else familiar with the characteristics of the 7, or so, types of plastics available that are used for containers.

Not sure why the 6.0-6.5 hr time is recommended. Perhaps, and i’m guessing here, is that some plastics and glass, to a degree filter out various wavelengths of UV light.

I know from very simple personally conducted laboratory experiments that such statements that plastics and glass do NOT allow any UV light through are FALSE. Each semester, when i was a senior undergrad and then a grad student i would demonstrate this with students taking their first microbiology course.

Exposing properly innoculated media in both glass and plastic closed petri dishes for varying lengths of time (20min to 2hr) in front of the lab’s closed windows on a sunny day would result in no growth of the E. coli innoculate, depending upon how long they were exposed to the sunlight. [Sorry, i can’t specifically recall the minimum time that produced no growth – it’s been over 30yrs.]

The “controls” (plural – each student’s, plus my own), similarly innoculated, but not exposed to sunlight, would show growth.

Would i personally drink water “purified” by either solar distillation or SODIS? Answer: Yes. Have i tried either method? Answer: No.

[Neil Bender]

Polycarbonate does a pretty good job of filtering short and long wave UV. Window glass transmits and 80% of UV, whereas a thinner sheet of lexan will only transmit about 3% UV. This is why the Aquastar short wave UV water sterilizer comes with a polycarbonate reservoir, to prevent the user’s eyes from being exposed to the lamp’s shortwave UV. Even clear polycarbonat safety glasses do an acceptable job of protecting eyes from UV. CR-39 eyeglass polycarbonate is virtually opaque to UV.

I don’t know what the UV transmittance is for PET bottles, but as long as you use those and not polycarbonate (and I don’t expect an ultralight backpacker to carry a heavy lexan bottle anyway) solar disinfection does slowly work.

I consider this one of those “good to know” techniques rather than something to rely on for routine use.

[paul johnson]

Neil, thanks for the informative reply with some real world numbers. good info.

Wonder about UV transmission of the plastic used in Platys. Probably will work for SODIS is my guess. But, IF that is true, then one wonders about the SteriPen UV-C purifier, since it may be used with non-Lexan containers. For my part, i don’t look at the container (usually a Platy or Nalgene Cantene) when using UV-C purification (paranoid???).

I’ve heard that the water-container interface reflects UV-C back, but then if that were true, SODIS wouldn’t work real well.

Some confusing (to me at least) info out there.

You’re right, SODIS is relegated to the “good to know” category – just in case it’s ever needed.

[Vlad Putin]

When I learned about them in the eighties in Boy Scouting, if I remember right we were taught any water from a solar still was OK to drink. Unless of course, a lizard or frog or bug gets in underneath the plastic and falls into the drainage cup and you find it floating around in the morning <grin>.

I used to know some Air Force Academy guys who had been thru survival training and they told me they were taught to take a leak around the edges of the solar still early in the night, as any moisture of any type nearby is good cause its going to probably evaporate and condense by AM…into your cup at the bottom of the still.

Vlad

[Ryan Jordan]

Don’t rely on SODIS to purify water in a “meaningful” amount of time if the water is contained in any type of plastic container. All plastics are generally very effective at blocking UV light.

Also, SODIS is most effective in direct sunlight on a very clear day, and generally ineffective on cloudy days. Its ineffectiveness is further compounded by altitude (lower is worse) and air pollution.

Does a solar still remove microorganisms?

No.

It actually relocates the clean water away from the old microbes and deposits it into a new container that has a surface coated with new microbes :)

The advantage of a solar still has less to do with disinfection and more to do with concentration of clean water.

You don’t build a solar still to purify water, you build it to obtain water from less concentrated sources (damp soil, wet leaves).

Peeing in the still is simply a side benefit – a joy of using the still to prove that you can drink your own pee!

And therein, friends, lie the reward. Clean water, who cares? You’re drinkin’ your pee!

[paul johnson]

The good Doc is dotting all of the i’s and crossing all the t’s.

w/o asking his permission first, i’d like to clarify some of his statements:

>>”Don’t rely on SODIS to purify water in a “meaningful” amount of time if the water is contained in any type of plastic container. All plastics are generally very effective at blocking UV light.”

as stated earlier, plan on 6.0-6.5 hrs of exposure in direct sunlight

>>”Also, SODIS is most effective in direct sunlight on a very clear day, and generally ineffective on cloudy days. Its ineffectiveness is further compounded by altitude (lower is worse) and air pollution.”

all true enough.

>>”Does a solar still remove microorganisms?

No.

It actually relocates the clean water away from the old microbes and deposits it into a new container that has a surface coated with new microbes :)”

good one. he got me there!! glad you included a smiley as a closing comment to that statement.

hopefully, the bad buggers that are left behind by distillation also don’t exist in the new container. so, use a clean catch container (perhaps disinfected if need be and the means exist – expose the container to the direct sunlight for a couple of hours might be an option?)

>>””The advantage of a solar still has less to do with disinfection and more to do with concentration of clean water.”

i think of both as a benefit, depending upon the water source that is available.

little potable water = concentration

plenty of non-potable water with no other means to make it potable = purification. when does such a situation occur? read on…

>>You don’t build a solar still to purify water, you build it to obtain water from less concentrated sources (damp soil, wet leaves).”

one situation where there is plenty of non-potable water is when at sea. solar stills were (are they still? [no pun intended. ok, ok, a pun was intended.]) included in some Naval life rafts for this very purpose.

>>”Peeing in the still is simply a side benefit – a joy of using the still to prove that you can drink your own pee!

And therein, friends, lie the reward. Clean water, who cares? You’re drinkin’ your pee!”

shades of Herbert’s “Dune”! i’ll take your word for it Doc. But, i gotta’ ask you. Would you drink my distilled pee too? [no need to reply; i think we all know the answer already.]

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