Distilling water

[Boneman]

Well, I think this is a technical question, and for the life of me, I thought I'd asked it before, but cannot find it anywhere...

So, hero is trudging through the desert and comes across the well/oasis, only to find it's poisoned/ filthy, whatever. It's a well-worn trope, but my question is this: If you boil poisoned/filthy water and collect the steam, is that not distilled water? Okay to drink? Or does the poison come with it? (I only did General Science for O level...)

[The Judge]

If it's just filthy water, then he'd be mostly safe drinking the water itself after it had boiled -- from wikipedia

Boiling can be used as a method of water disinfection but is only advocated as an emergency water treatment method, or as a method of portable water purification in rural or wilderness settings without access to a potable water infrastructure. Bringing water to the boil is effective in killing or inactivating most bacteria, viruses and pathogens. Boiling is the most certain way of killing nearly all microorganisms. According to the Wilderness Medical Society[1], water temperatures above 160°F (70°C) kill all pathogens within 30 minutes and above 185°F (85°C) within a few minutes. So in the time it takes for the water to reach the boiling point (212°F or 100°C) from 160°F (70°C), all pathogens will be killed, even at high altitude. To be extra safe, let the water boil rapidly for one minute, especially at higher altitudes since water boils at a lower temperature.

If it was a poison, there might still be a problem

In distillation, boiling is used in separating mixtures. This is possible because the vapor rising from a boiling fluid generally has a ratio of components different from that in the liquid.

I read that as saying that although the ratio is different, the components are still there. In steam inhalation for a cold, doesn't the vapour still have the eg eucalyptus scent put into the water? Dredging memories here (yes, O-level only) might it have something to do with how quickly the different bits turn to gas? (I read the wikipedia thing on distillation, but still couldn't understand it...)

[I, Brian]

Originally Posted by Boneman

Well, I think this is a technical question, and for the life of me, I thought I'd asked it before, but cannot find it anywhere...

So, hero is trudging through the desert and comes across the well/oasis, only to find it's poisoned/ filthy, whatever. It's a well-worn trope, but my question is this: If you boil poisoned/filthy water and collect the steam, is that not distilled water? Okay to drink? Or does the poison come with it? (I only did General Science for O level...)

I would consider the condensed water vapour as distilled. Any impurities should be left as a residue after the water has been boiled away.

[chrispenycate]

It all depends on the poison. Some of the organic ones could well be destroyed by the boiling (I believe botulinus toxin can be eliminated by a few minutes enthusiastic cooking), some of the stable compounds (like the heavy metals and arsenic) will stay in the water, never evaporating. But a few substances, temperature stable and with boiling points close (within an order of magnitude close, not a few degrees) might make the trip.


As an example.
If the poison were, say, alcohol (yep, that can be classifies as a poison), then the first drops condensing on your cold surface will be quite concentrated poison, as the boiling point of alcohol at sea level pressures is quite a lot lower than that of water. Then would come a load of the water, diluting the mix (but still a higher concentration than in the original solution) Finally, as concentrations rise and the temperature of the sludge follows, other stable volatiles with higher boiling points start evaporating, and they condense faster.

So most of your organic poisons have either broken down or stayed behind; this'll dispose of most of your "dropping rotting corpses down the well" problems, and the vegetable alkaloids and such.. Most of your infectious organisms (the other residue of the meat decomposition) will be dead, thus ineffective, and they won't travel in the vapour, anyway. Your mineral poisons will stay behind.

Which leaves a few volatile organics like methyl alcohol or fusel oils (or gasoline, for that matter; diverting an oil well into a water one would give a problem that couldn't be solved like this, However, since the two are immiscible all you'd have to do is stick a straw through the surface layer and drink the water from underneath), and a number of unpleasant flavours; you're certainly improving your chances a lot.

So all that's left are a few

[TheTomG]

Actually that's an interesting idea, depending on whether your poison is real or imagined - having to do two distillations. The first boils off the poison, which you collect and dispose of. Then with a fresh flask, you collect the water that is now boiling off, leaving behind the other problematic ingredients and giving you both water that is safe to drink, and a measure of poison to do with as you will.

Could be an interesting story element, it sparked my imagination anyway when chrispenycate wrote that!

[Moonbat]

Isn't there some kind of basic chemstry apparatus that allows you to boil the water and then collect (through condensation) the different liquids that boils off at different temperatures? I'm sure I remember something from my Chemistry lessons (Mr Sodhi will be so pleased!) where there was a ipe that had lots of different exit tubes lines up along it and as the liquid boiled each tube caught the next liquid rising in boiling temp. That way you'd get a small amount of alcohol in the first tube, then water in the next, and then the rest in the last tubes. Or am I imagining that piece of apparatus, maybe it was Alchemy class not Chemistry

[chrispenycate]

Originally Posted by TheTomG

Actually that's an interesting idea, depending on whether your poison is real or imagined - having to do two distillations. The first boils off the poison, which you collect and dispose of. Then with a fresh flask, you collect the water that is now boiling off, leaving behind the other problematic ingredients and giving you both water that is safe to drink, and a measure of poison to do with as you will.

Could be an interesting story element, it sparked my imagination anyway when chrispenycate wrote that!

Wrote it extremely badly (looks in disgust at the little trailing phrase that didn't get edited off, but the computer crashed just as I posted this, and I didn't come back to check). But you'd have to multiple distil both your poison, and your drinking water (sounds a bit time consuming if you're dying of thirst); fractional distillation is a percentages game. And a slow one; the faster you boil, the less separation. Better hope they've dropped dead camels down there, or the mercury fulminate from their explosive shell primers.

And Moonbat, yes, though more often used for petroleum products. But it's still 'a higher percentage of', not pure this and that (unless you travel with a tame Maxwell's demon). And, you're travelling across the desert, so unlikely to just happen to have one of these conveniently in your pack, so you have to build it, out of blown glass (plenty of sand, anyway) and discarded copper coffee pots. How many gerbils do you need to poison before you've got it calibrated for the separation you need?

[Dave]

How are you collecting and cooling down the steam generated?

That would be the most difficult parts of the distillation process (I'm assuming a low-tech world, a shortage of water, and a hot climate - as a minimum you would need glass works and ice (which requires large amounts of energy to produce. One reason science in China fell behind that in the West was that they didn't invent glass.)

However, you could always come up with some entirely natural fictitious way. Someone once told me of two parallel streams on a New Zealand mountain. One was ice cold and had fish. The other came from a volcano and was boiling hot. You could fish out of one, and turn your back and cook the fish in the other.

[Boneman]

That's great, thank you all so much. Initially, it's just a sludge-filled well, with small dead animals so the simple distillation will do nicely. But it has given me a good idea about a problem I had in the city (where they could have better equipment to separate) to help out in the siege.

What about boiling urine? I read of someone who tried to filter it through sand, but just ended up with... well, urine that had trickled through sand... A couple of distillations would render it 'safe' to drink? It's desperation time for survival...

[chrispenycate]

Urine, blood, almost any organic liquid; I'm not guaranteeing flavour here, note, just lack of toxins. But the smoke of most combustion contains a fair amount of steam, too; what are they burning to boil the water? (Wonders which of camel dung or priceless tapestries is going to condense down to the least unpleasant tipple?) With a long enough tube through a cool shady grotto you might be able to collect enough to make a difference.

You can get by without ice (although by inefficient and expensive means I can make ice) but desert cities have cool regions for their citernas; evaporation is a problem even when the water is flowing normally. Still, it would be better to do the distillation at night; even in hot climates, desert nights get cold (lack of cloud to trap the heat, and humidity to regulate it).

I'm now considering some way of collecting sweat; absorbent inner garments that you wring out into the distillation tanks protected by waterproof overgarments?

[Vertigo]

Another thing you can do for dirty water (wouldn't be any good for poisons in dilution) is make a filter. Quite easy to do. A plastic bottle with the base cut off or make a funnel from bark (preferably not a poisonous variety!). Put pebbles in the bottom (to stop the rest of the material from coming out). Add layers of charcoal (from campfire etc) and sand (or moss if available). Then pour water through it. You may need to do it a couple of times before it comes out clean. With all those dead animals in the well you would also want to boil it.

You can also make a very simple still with a container to collect the water and a sheet of plastic (eg bin liner). Dig a hole about the same size as the plastic sheet in full sun (desert would be good for this) and close to the oasis where hopefully the ground is damp (or pour the polluted water into the hole). Put your container in the middle. Lay your sheet of plastic over the top held down with rocks at the edges (if no plastic available then waxed cloth would probably work) place a small stone in the centre to make the lowest point of the sheet be immediately above the container. Heat evapourates water, condenses on sheet, drips into container.

[Boneman]

Blimey, it's a wonder anyone dies in the desert... Thanks Chris, I hadn't thought of the blood bit. If one person had a cut, should another just sup the bllod (non vampire style) or would it be better to go through the distilling? Because of the amount of iron/sodium in blood etc.

And thanks also, Vertigo, I don't think there's plastic in my world, but is it correct that if you pile up nicely smoothed stones, get them really hot during the day, there will be condensation on them as they cool in the desert night? No idea where I heard that one...

[Vertigo]

I figure anything impermeable would do instead of the plastic. Eg metal (from armour?) or even leather. I should have said that you collect this water overnight not during the day.

Re the rocks. I'm not sure the getting them hot in the day is important. I can't see what that would achieve, in fact I suspect it would work better if the rocks could be sheltered from the heat of the sun during the day so they are reasonably cool already in the evening. However if there is moisture in the ground under them then the stones would act similarly to the plastic/waxed cloth in that the moisture evaporating from the ground (still warm from the day) would condense on the stones as they would cool faster (being on the surface). Again probably works best if you soak the ground under the stones with the dirty water from the well. So maybe...

[Venusian Broon]

Having watched numerous Ray Mears progs on the telly I believe he stated that it's notoriously difficult to distill water in a survival situation - partly I think because rigging up a system to carry out an efficient distillation is difficult and as chrispenycate points out I think it takes a really long time to do it properly.

So if I was desperate, but not so desperate to just drink it there and then, I'd take Vertigo's idea of a filter then boil. And take a chance with the heavy solutes.

For the traveller in a parched but vegetated land, one cool way Ray Mears used to get water was to tie a transparent plastic bag around the branches of a eucolyptus tree and collect the transpiration water. You can really get a good quantity of water from this method at the end of the day. But it only works with eucolyptus whose other transpired volatiles are not damaging - I think most other tree species release nasty toxins that are pretty harmful to ingest.*

Oh and don't suck the sap of cacti. Meant to be a terrible thing to do, I believe.


*At least I think it's eucolyptus trees - don't try it unless you've researched it kids.

[TheDustyZebra]

Vertigo, that's the one I was thinking of -- dig a hole, sheet of plastic (or whatever) over it, container under that, and pee in the hole. Don't know where I read that one. Boy Scout manual? Probably not.