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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In practical terms, greywater still has enough oils and soap residue that the toilet tank and bowl would get grimy pretty quickly.
In some jurisdictions that permit separate greywater plumbing systems, it can be sent out of the house to a settling or treatment tank and then used for things like plant irrigation.
Also, if it’s going to be distributed in pipes indoors, the pipes need to be clearly marked “not potable” so someone in later years doesn’t assume it’s clean water and hook it up to a sink. The same is true for rainwater: if it’s collected it’s difficult to get permission to use it for things like toilet flushing. (I negotiated with the State of Illinois for two years to come up with a system design that would be allowed by the health department.)
In Japan, I encountered two interesting things related to greywater reuse. One was toilets that had a hand-washing sink just above the toilet bowl, so the water from that would be used to flush. The other is that once the whole family has used the soaking bathtub (ofurō) they would use a siphon from the washing machine and transfer the hot water from the tub to do laundry with.
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