eli5 Is there a reason the waste water like from showers can’t be filtered through pipes and used for toilet water?
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Non-sewage household water (from, e.g., showers, sinks, dishwashers, etc.) is called graywater and in certain circumstances is captured for particular uses, largely watering lawns or gardens (especially at night). It’s still fraught with potential dangers since graywater can carry pathogens from people washing their hands, etc., so it’s seldom used except in places with serious fresh water limitations. And even then, graywater systems are designed to prevent people coming into contact with graywater.
Circulating such water through toilets introduces risky potential contact points for people and pets. Flushing a toilet spreads a fair amount of water droplets (and other things) through the immediate area, so it’s best to avoid untreated water. Also, there’s lots of engineering and expense needed to create a separate water circulation system – you’d need to address water pressure issues and essentially pump used water into a storage tank connected to toilets with adequate backflow preventers, and ensure that there’s always enough water to flush with (or create an entire secondary supply system). And for all that, you’re capturing maybe 20-30 gallons of water per shower. That’s enough to flush a toilet 20+ times, but that seems an unsustainable ratio – do you flush a toilet 20 times for each shower you take? So you’re going to have to dump water out of the system anyway unless you have enormous tanks or some way of connecting your system to some large venues that use toilets but not showers (like sports stadiums).
In short, it can be done, but the design and engineering difficulties are enormous and the benefit is minimal.
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