No such thing as waste water
Water does not get "used up" when we use it - it simply changes form. Rather than throwing it away. When we learn to use that water many times over before returning it to the environment, we have moved from scarcity to abundance.
Waste water in many cases is simply "water with nutrients". While nutrients in clean water is a bad thing (see eutrophication), this water does have uses:
- in water for irrigation, the nutrients are is a good thing - as long as there are no toxins, or pollutants that the receiving ecosystem can't handle. (Choice of detergent is key when reusing greywater from a laundry.)
- in flush toilets, some nutrients are harmless. This is a matter of the nature of the pollutants, the concentration and how long since productin. (Leaving greywater for a long period, e.g. 24 hours, potentially allows pathogens to breed, so it is then regarded as blackwater, i.e. a health hazard.)
 See also
- No Such Thing As Waste Water, Sarah Kuck, WorldChanging.com, January 19, 2009.