Re: How could I clean the OCEAN water to be drinkable?

Dear David:

There are several means by which you can purify ocean water. Removing salt from salt water is called desalination. Of course, the first thing to do would be to filter out stuff like dirt, this can be done with a filter, e.g. a charcoal-sand filter.

The first, and most obvious, means of getting clean water would be to distill it. Salt and water have very different boiling points. When you boil a salt solution (i.e. the ocean water), the water will evaporate off as steam and a concentrated brine (salt water) solution will be will be left behind. If you boil off (or dry in the sun) all the water, you will have the salt remaining behind as a powdery solid. If you can get access to a fire, a kettle, and an aluminum pan, you can try this: boil the sea water in the kettle over the fire (wood, gas, or electric), then hold the cool aluminum pan over the spout of the kettle. The steam and water vapour coming out of the spout will hit the pan, which is much cooler, and so condense on the pan. A cup or bucket can be used to hold the water that drips off the pan. If you can afford it, then you could get a specialized distillation apparatus like they use in the laboratory in school. You might have seen or used one before in science class. Distillation is also used on a large industrial scale to produce drinking water in some countries.

Another popular means of getting clean water is by freezing the sea water. You have seen pictures of the Arctic and Antarctic, where the large icebergs are floating on the salty ocean. As you know, these icebergs are often quite pure water, and some sailors have actually melted them down to drink before. Some countries have even considered towing icebergs from the polar regions to 'home' so that they can be melted down to use as drinking water! Similarly, we can imitate nature and simply freeze up the sea water. The ice which we obtain is hosed down with clean water to get rid of any excess salt water clinging to it, and then melted to drink. However, in an emergency, it's hard to get access to a freezer, so it's not so practical.

The third method I'll talk about is reverse osmosis. You may have heard of semipermeable membranes, which are special membranes which allow molecules like water to pass through, but not other (presumably larger) molecules and ions (e.g. sodium and chloride ions in salt water). In reverse osmosis, the salt water is forced through the membrane, and the salt is filtered out by the very fine membrane. This page gives more information on this very interesting process. It is actually used on an industrial scale to purify water, in dry countries close to the sea like Saudi Arabia, among others.

Therefore, in an emergency, it is possible to construct a small reverse osmosis device, operated by a portable generator. This would be quite useful in an emergency, though it may be too bulky to be carried about easily. Alternatively, one could distill the ocean water to boil off the water and condense it. Another means of doing this is to let the water evaporate normally at room temperature and have a piece of clean sheeting above to collect and condense the water, but that is much slower. You could do a simple 'net search on 'desalination' to help you find more info.

Hope this helps! Keep up your interest in science!

Thiam Hock "Visking" Tan