In #479 Todd mentioned about how much happier he was to be drinking tap water than the re-cycled urine the ISS occupants were looking forward to. Now I was taught a long time ago about the water cycle of ocean, to rain, to river to ocean. During this process animals drink it, or eat it in their food, then dispose of it in urine. It got me thinking on a strange tangent about what the chances that Todd was actually drinking re-cycled without knowing.
So armed with Google and some very liberal over-simplification I have made a quick back of the envelope calculation. There is no point in making any claim of accuracy in the amount of urine produced per day over all of time. Taking today’s population of humans, cows, pigs and sheep we get roughly 87 Billion liters per day which will be substantially less than the actual total. It needs to be because I am going to assume that this same volume is produced every day stretching back to when large animals are first recorded as being present 230 million years ago.
There’s a lot of water in the world, approximately 1.4 trillion cubic kilometers. At 87 Billion liters a day it would take 16 Billion days to convert it all to urine. Since the recorded beginning of large animals though, there has been 84 Billion days. That would mean an average of 5 re-cycles for any given amount of water.
In reality there are lots of complications with these sums even outside the extremely inaccurate (but lowball) daily volume. A lot of the water we drink leaves in sweat and our breath, and a lot of the water in urine comes from breaking down sugars fats. The water gets into these through plants and then the animals in the food chain. I think the numbers are good enough to make a solid claim that at least 10% of any volume of water has previously passed through a urinary tract. The ISS is just increasing the percentage.