IIRC there was a collection service, or possibly automated conveyor belts..? The main idea was not to poop into a bowl of water and then use a bunch more water to transport the poop somewhere just to have to clean it out of the water later. Nowadays, of course, we could even use biodegradable cellulose bags.

A composting toilet is probably overall a much better solution. (And a bidet.)

edit: I found this linked from the Dymaxion House article: https://en.wikipedia.org/wiki/Packaging_toilet