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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I believe there are toilets in Japan that do just this. You wash your hands and the water from that is used to flush the toilet, which is directly under the sink.
A lot in here are giving explanations of why you can’t use grey water for X. OP asked solely about flushing toilets. And when it comes to toilets, yes, you can absolutely use grey water to do so. The problem is more in the “how” though. In the previous example the sink is directly over the toilet. So that’s super easy. But let’s take a shower, for instance. How would you get the water into the toilet tank, which is higher than the shower drain? Now you need a pump, which uses electricity so any gain in water savings is negated by energy costs. It’s more a practicality than anything. In my house in order to get the dishwasher water to the toilet it’d need to run 20 foot horizontally and then up 3 feet. It just doesn’t make sense, given how unrelated the number of dishwasher runs to the toilet use are as well. Because now you need a reservoir tank elsewhere. Run the dishwasher 5 times before you need to flush? So the only practical use of grey water for toilets is what is already being done: use the hand washing sink above: It’s a 1:1 use (unless you’re one of my coworkers apparently) and gravity is in your favor. Logitistically nothing else makes sense. And for any other uses than toilet flushing, yeah. You obviously don’t want to use grey water, but OP seems to already know that.
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