At the start of the film Waterworld we see a curious looking boat floating on the sea. On board is ‘The Mariner’ ( Kevin Costner) who is peeing into a plastic cup. He pours the urine into a strange contraption, and collects a clear liquid, which he drinks – what’s going on!?

Urine contains many salts. If you are thirsty the salts in urine will dehydrate you further and if you were stranded in the desert the salts would kill you! Ideally then, to drink urine, we need to get rid of the salts. Is it possible to filter out the salts by using say a common tap water filter? Cut a water purifier canister open and you will see that it contains many small black and white granules. The black granules are activated carbon, the white ones are 'molecular sieves' (aluminosilicates). Both these materials have extensive, porous molecular surface structures which readily absorb small molecules and ions. When water is poured through the unit the granules absorb impurities. So could this have been what The Mariner was using? Later in the film there is a clip of a child drawing with a piece of charcoal. Charcoal is, wood that has been burnt in a restricted flow of air. It can be used to make activated carbon so maybe he could have made some?

The Mariner by the way half-human, half-fish. He can stay underwater for a long time and scavenges the ocean floor where there are lost cities and towns - he's a bit like a futuristic scrap man. It's plausible, then (assuming for the moment that we take the naritive of the film seriously!), he might have found a supply of water filters here. Perhaps the contraption in the film uses a pump to pass the urine through a number of filters?

To see if this would work I collected some urine and passed it through a standard water purifier several times. After about 15 passes (!) the colour and smell had gone - it looked ok, so something had been filtered out.

Pure (de-ionised) water does not conduct electricity. I tested the conductivity of fresh urine and that of my clear filtered liquid by using a battery, bulb and meter connected to two carbon rods dipped in the test liquid. The urine conducted electricity well and the bulb lit. Unfortunately the clear, filtered liquid also conducted electricity, though less so. Filtering, it seems, does not remove everything. So drinking filtered urine via a water purifier won’t work very well and anyway the molecular pores in the filters will eventually clog-up and become in-effective. Something else would need to be done to get rid of the salts.

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