A solar still is perhaps the optimal method of obtaining fresh water when you are caught in the great outdoors without a means of purifying a local water supply.

As the name suggests, a solar still is a way of distilling fresh water from a contaminated supply. It requires a little equipment, but nothing that you wouldn't bring on your average camping trip - and nothing that you wouldn't have if, say, you were stranded on an island with a FedEx jumbo-jet's worth of stuff washing up on the shore:

  • A hole, or the means to dig yourself one.
  • A good solid pot
  • A (contaminated/undrinkable) source of water
  • Some rocks
  • A piece of sturdy, watertight plastic - three feet square or more should do.

Begin by digging a hole roughly two feet in diameter, about one foot deep. In dry conditions, you will want to dig down far enough to find wet soil - specifically, soil that will not absorb water rapidly. Next, situate your pot at the bottom of the hole, in the center. If your pot is very shallow, you will want to boost it up slightly so that you can fill the hole (e.g. the part of the hole that isn't taken up by the pot) with a good amount of your contaminated water.

Next, fill the hole with your unpotable water. *Be sure that you do not get any contaminated water in the pot - that pot is going to eventually collect the fresh water you need*. Wait a few moments to see if the water will drain. If it appears that your hole will retain the water for a while, you're good to go.

This last part is the trickiest. Cover the entire apparatus with your plastic and pull it taught over the hole. Anchor the corners and edges down as best you can so that the surface of the sheet is pretty tight. Now *carefully* place a rock in the center of the sheet; its best to use a rock that is slightly pointed and to have it pointing down. This should push the tarp into an inverted dome shape with the apex right over the pot - you may have to adjust some things to get it just right, but its critical that this be the case. You may also have to add weight to push the tarp down far enough.

Now, pray for sun. If you're lucky enough to pull this off in the wee small hours of the morning, the day's sun will heat and evaporate the water, which will then deposit on the underside of the tarp. As it accumulates, the water will form drops which (hopefully) will run down to the apex and then fall into the pot, as all the bad stuff you don't want to be drinking is left down in the hole.

For this reason it is best to use clear plastic, is it will allow the sun's rays to heat the water sufficiently; opaque plastic is ok, but it will tend to get hotter than clear plastic and thus be less likely to collect condensation, reducing the efficiency of the still. Also, you must be aware that the size and shape of your hole is going to determine how successful you are. A shallow hole will mean a reduced water capacity as well poor downward slope on the tarp. A deep hole will have a high water capacity and will allow you to create a steep slope on the tarp, but it might also be too shielded from the sun to get really cooking. Finally, this method will only purify water contaminated with salt, bacteria and other non-volatile substances. If you try to use this to purify water contaminated with, say, methanol, you will actually end up with a pot more contaminated than before - as methanol has a lower boiling point (e.g. generally evaporates at a lower temperature) than water.

If you can't find any water at all, try urine.

A solar still is a device for turning unpotable water into potable, through the power of the sun.

You can improvise one on land quite simply*, but they are also available to buy, particularly for use at sea, where the improvised method will not work.

A typical still consists of an inflatable ring (in black to absorb the sun's energy), and a cone-shaped cover of clear plastic. The whole unit floats in the sea, tethered to your life raft.

         / \
        / c \
       /     \
      /       \
     /         \
    /           \
   /             \
___|   |_____|   |___________Sea Level
   | d |  b  | d |
   ++ ++-+ +-+---+
    |e|  | |
    | |  |a|
    | |
  +-+ +-+
  |  f  |

Sea water can enter through the hole a and makes a pool b in the centre of the still. Sunlight evaporates the water, which condenses on the plastic cover c.

The condensed water - which is now free from salt, runs down the sloping sides of the cover to the outer ring d, exits through hole e, and collects in the container f. When you need a drink, simply fish the container out of the sea, and hopefully it will contain at least a little drinking water.

A possible problem with the still is that if the sea is too rough, then the saltwater may splash into, and contaminate, the drinking water. However, it has saved many lives, and should be a staple in the kit of the adventurous sea-farer.

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