The reason that people want to separate salt from water usually isn't to get salt -- although it can be; it's to get fresh water to drink. Among the available methods to accomplish this separation and recover the water are reverse osmosis, sequential freezing, polymeric filtering and distillation, and of these, distillation is the least complicated. Instead of performing the distillation with flasks and an open flame, it's safer and more energy-efficient to construct a solar desalinator. That way, you can let the sun provide the heat you need to evaporate the water.
Water should evaporate quickly in full sun, so you'll have to monitor the desalinator and fill it frequently. Collect the salt that collects on the bottom of the box by pulling out the black sheet metal. You should do this daily.
Glass makes a better lid than plastic. On plastic, water tends to condense into droplets that fall back into the salty water, whereas on glass it forms a film.
If your box leaks, seal the seams with silicone caulk.
Wear goggles when cutting plastic with a circular saw, and wear gloves when cutting tin or handling glass.
The solvent in solvent-weld glue is volatile and noxious. Work in a well-ventilated area and wear a respirator if you're sensitive to VOCs.
Make the base for a box by cutting a rectangle from a sheet of 1/2-inch clear polycarbonate sheeting, using a circular saw. The size of the sheet determines the size of the box and how much salty water you can treat. To construct an easy-to-manage desalinator, make the base 12 inches wide and 24 inches long.
Prepare a sheet of galvanized sheet metal that is the same size as the plastic rectangle by cutting it from a larger sheet using tin snips. Paint one side with flat black waterproof paint.
Cut the sides, back and front of the box in such a way that when you put on the lid it will slope toward the back of the box. To get an efficient slope angle, which is about 10 to 20 degrees, the front should be about 12 inches high and the back about 8 inches high. The sides are triangular -- the front of each side piece is the same height as the front of the box, but the back is about an inch higher than the back of the box. This creates a gap between the back of the box and the lid for condensation to escape.
Assemble the box with solvent-weld plastic glue. This type of glue forms a waterproof bond by partially dissolving the plastic. Spread it on both surfaces you're joining, press the pieces together and hold them until the glue dries -- typically about 30 seconds.
Construct a lid out of plate glass. Cut the glass with a glass cutter so that it's about an inch wider than the box; it should be long enough to extend about 3 or 4 inches past the back. When the box is in operation, water will condense on the lid and drip off the back edge. Cutting glass isn't difficult, but it can be dangerous. If you don't feel confident, order a piece of glass of the right size from a glass shop.
Drill two 1/2-inch holes in each side of the box about an inch above the base. These overflow holes will keep the level of the salty water low so that it evaporates faster.
Put the black sheet metal in the bottom of the box and fill the box with salty water to the level of the overflow holes. Set the box in the sun. Cut an 18-inch length of 3-inch diameter plastic pipe in half lengthwise with a hand saw and use one half to catch the condensed water. Arrange it behind the box under the lid overhang at an angle so that the water that drips from the lid flows into a jar or bottle.
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