Why Do You 'Spose the Oceans are Salty?

You've probably heard that one of the items delivered to the International Space Station (ISS) was a machine designed to recycle water... specifically, urine. Now this is a surprisingly complex task, considering that the recovered water needs to meet... ummm... fairly demanding standards of quality. That is, the astronauts will be drinking the machine's output.

On one hand, though we tend not to think about it, we all do this. But we have natural systems that take care of the processes for us: evaporation, condensation, precipitation, uptake of nitrates and other compounds by bacteria, filtration by natural sediments, flocculation by various electrochemical processes, and destruction of potentially pathogenic organisms by phages, to name a few. We also have processing plants that remove most of the undesirable materials in our sewage before we release it into the environment. Finally, we have enough inhabitable volume that we can entertain the illusion of an "open," unlimited system. The earth system is, in fact, for all intents and purposes, closed, and the vast bulk of what we consume- eating, drinking, breathing- has been through countless other organisms over the course of eath's history. But on the ISS, the inhabitable space is very limited; there is no room for illusions, let alone a functioning ecosystem that can cycle materials such that the output of each component matches and meets the needs for input by another component. It's wonderous to consider that we live in a world where this is so- but on the ISS, they need to have a machine that takes care of this.

On the other hand, I guess I wouldn't really have wanted to be involved in the development of such a machine. (Warning: language NSFW; hat tip to ScienceBlog)

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