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I have recently watched "How Many People Can Live on Planet Earth?" documentary with David Attenborough. In it, in the main factors that will limit total human population Earth can support the limited amount of fresh water was named. There also was a part where it described how on International Space Station fresh water is distilled from astronaut urine.

My current understanding is that all the water (fresh or not) in nature (i.e. on planet Earth) is infinitely going through water cycle, and thus non-fresh water is sort of distilled into fresh, by this natural process.

My questions are:

  1. Where am I wrong in my assumption about natural water distillation (i.e. is rain water not as clean as it should be in theory)?
  2. Why don't we collect rain water (at least for plant watering, if it is not suitable for drinking)?
  3. Theoretically: after an animal consumed some amount of water, say a glass, how long will it take, given that animal just urinated on the ground, for that water to go through water cycle and become ready for drinking again?
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  • $\begingroup$ Just because something is cycled does not make it infinite. $\endgroup$ – John Dec 13 '18 at 22:24
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Rain water is still one of the most cleanest water sources available. There may be some ancient underground aquifers that have clean uncontaminated water.

One issue with rain water is that atmospheric moisture - part of the water cycle - needs to condense on aerosols in the atmosphere before it can develop into rain. With the amount of pollution humans are dumping into the atmosphere some of that can form the aerosols that eventually develops into rain thus making rain water dirtier than it used to be - such as acid rain being formed when sulfur dioxides are dumped into the atmosphere when high sulfur coal is burned in power stations. Other aerosols include salt from the oceans.

The other issue with rain water is that the soil in water catchments can be contaminated by pollution. Rain falling on the catchments can absorb some of the contaminants thus making the rain water in the resultant catchment dirtier that it might have been in the past.

Regarding the rate of urine production of animals, it depends on the type of animal. A mouse will process water at a different rate to an elephant or domestic live stock. It is also influenced by circadian rhythms for each type of animal.

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  • $\begingroup$ Thanks for such elaborate answer. It seems I didn't make my point about animals (and their urine) clear though, my actual question was "How long will it take for urine poured on the ground to come back as "distilled" rain?" $\endgroup$ – Filipp W. Dec 15 '18 at 11:00
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The problem about rain water catchment is, that it prevents ground water from being formed, thus depleting these ressources and making them unavailable during droughts. Plus, ground water tends to be cleaner than rain water as far as I know - provided there's no fracking or subterranean atomic waste storage around.

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