Poop indirectly on crops. Systems like this or the Aztec chinampa system, basically try to keep nutrients in the loop with fish and other aquatic organisms. Obviously, there's a disease risk if you do it wrong, but that's also true for modern water treatment.

Like evasive chimpanzee said we need to poop INDIRECTLY in crops. Hot aerobic composting for example has excellent nutrient retention rates and eliminates nearly all human borne diseases. The main problem would be medication since some types tend to survive.

Also urine contains almost all of the water soluble nutrients that we expel and is sanitised with 6-12 months of anaerobic storage. So that's potentially an easier solution if we can seclude the waste stream. Again the main issue would be medications.

I don't have the answer, if it was easy we would have done it already. The main issue is we don't have a lot of people working on the answer because we're still in the stage of getting everyone in the world access to sanitation. Certainly the way we're doing it is very energy and resources intensive, unsustainable in the living term, and incredibly damaging to the environment. We've broken a fundamental aspect of the nutrient cycle and we're paying dearly for it.

The other problem is, like recycling, there isn't a lot of money in the solution, so it's hard to move forward in a capitalist system until shit really hits the fan.