Alternatives to flushing our waste with drinking water is worth some attention, but I far prefer a composting option to using so much energy to neutralize our poop and pee.

"Humanure" is easier and safer when we're not so numerous.

Composting converts waste into resource (in economic terms, a bad into a good). Human urine is very useful for fertilizer. Figuring out how to leverage that would be one of many pieces to be "sustainable".