<i>What do rocket scientists have to do with designing water purification systems?</i><p>On the assumption that you may not be a native speaker of English, I&#x27;ll mention that here in the United States, at least, the term &quot;rocket scientist&quot; (which some people who work on the International Space Station project literally are) is a synonym for &quot;very smart person.&quot; In other words, my statement is that even though the International Space Station presumably has enough prestige and budget as a project to hire very capable engineers, those engineers have not found it easy to design a toilet-to-tap water treatment system (as the linked article in my first comment reports). Yes, designing a system to work in free fall while confined to the space station is itself a difficult engineering problem, but even here on earth in normal-g conditions, it looks like there is a lot of engineering work still to do on the problem of potable water recycling.