Potable water is not expensive everywhere. Also, laying a second, redundant network of water pipes so that we can flush with "gray water" would be ridiculously

Potable water is not expensive everywhere. Also, laying a second, redundant network of water pipes so that we can flush with "gray water" would be ridiculously costly. In buildings that can recycle their own water, gray water systems work, but it's a whole new problem to retrofit buildings with one.