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Solar Still / Liquid Contaminants? - Page 2

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Posted by daestrom on November 5, 2003, 11:09 pm

Well, you seem to be thinking that boiling occurs at an exact temperature or
some such.  While we often say the boiling point of water is 100C, it is a
bit more complicated than that.

Ethyl alcohol boils at 78.5 C, so one is tempted to think that heating a
water-alcohol mixture to something like 90C will effectively separate the
two.  But a liquid's vapor pressure is not an on/off kind of thing.  Heating
water-alcohol mixture to 90C will boil out the alcohol *and* some of the
water will evaporate too.  This is why when distilling alcohol (say for
personal use, or powering an engine), you can't get more than about 85%
alcohol through distillation alone.  To get higher 'proof' requires another
means of 'drying' such as dessicants.

Similarly, if the water is contaminated with something whose boiling point
is slightly higher than water, there will still be some of that substance's
vapor in the water-vapor coming from the mixture.  It can be enough to
contaminate the water you condense from that vapor.

Boiling water to purify it works great for *some* contaminants.  Salts
(seawater), bacteria and 'dissolved solids' are no problem.  But if the
contaminant's vapor pressure is anywhere near that of water (i.e. 'boiling
point' is near 100C ) than it doesn't work so well.  The closer the two
vapor-pressure curves are to each other, the less effective.  It works great
for seawater because sodium-chloride's 'boiling point' is much, much higher
than water (think about it;-)

Gasoline, fuel oil, dry-cleaning fluid, xylene, any number of liquid
contaminants may not be removed through simple distillation.  Are you being
paranoid?  I don't know;-)  Is your water supply likely to be contaminated
with one or more of these?  I don't know that either.


Posted by Anthony Matonak on November 5, 2003, 4:03 am
nospam@nowhere.edu wrote:

I think it's not uncommon to find that many stills use an active carbon
post-filter to deal with whatever traces of stuff that get through
the distillation process. I'm not sure but I would think there are only
a handful of harmful pollutants that could get through in the first
place and it's likely that these are fairly uncommon. It's also quite
likely that anyone doing this would know (through testing) what is in
the water to start with and would know if there is anything that would
get through.


Posted by repairco on November 6, 2003, 12:16 am
 Reaching way the hell back to some survival training I got in Boy Scouts I
believe that the issue of contaminants entering the water in a survival
still came up.  If memory serves the answer was that the lower boiling point
junk would not re-condense or not much, anyway) because of the higher temps
in the survival still environment. (which is why they told us that the
collection cup should be removed at night and not replaced until the still
was in full sunlight)  Yeah, there would probably be some contamination of
some sort if you start with really crappy ;-) water in the beginning but the
average, run of the mill moisture shouldn't be any worse for you than
untreated well water.  When I was young I lived off a rainwater cistern for
almost 15 years----no problems.  We DID treat the cistern with a couple of
gallons of bleach every spring but that was all we did.

Posted by Dale Farmer on November 6, 2003, 4:41 am

repairco wrote:

    The contaminants found in water in the wild, as it were, are either traces
worth worrying about, or are removable with a common solar powered type
still.  The problem is that there are nowadays a lot of other stuff that has
been added to the drinking water aquifers since the dawn of the industrial age.
When I moved to NJ, I sent off a sample of the tap water in my apartment to
a lab and they came back with a long list of crap that was in the water.  The
biggest ones were various types of solvents.
    I got one of those counter-top water stills in the post-Y2K sales from the
survivalist websites, and rigged up a gravity operated valve and tank that
took the first quart of water out of each batch to a separate tank, which I
would dump out.  Sent a sample of the now distilled water of to the lab,
and they reported is as effectively pure.  ( There was some stuff in it, but
the levels were far lower.  )  That was the water I used for drinking and


Posted by Mitch Dickson on November 6, 2003, 9:59 pm
 HEHEHE!  Yep, if my old grand daddy had had a solar still, he never would
have got caught making moonshine :)  It's the wood smoke that gets ya caught

Iffin ya got a good family recipe for corn mash, a solar still might be
right handy!


"Come by and sit a spell with me at  www.volstate.net/~mitch/    "


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