Let's say the average person poops 0.5 L per day, including toilet paper and sawdust for covering it. A composting toilet tank that's 1 m^3 would be ample year-round capacity for 5 people; a tank that's 2x2x6 meters would be enough for over 130 people. It could comfortably be contained in a 1.5 hectare area (3.75 acres) that gives a standard urban density for those 130 people. It does not draw any freshwater reserves, it does not cause any potential runoff issues, it does not rely on any technology more complicated than saws for sawdust, and it literally cannot break or get clogged.

The only cost is that you might have to walk 50 meters to go to the bathroom instead of 5, and once every 6 months someone shovels out half of it (maybe 3 tons in a day's work) to mulch the trees or the flower garden.

The solutions to "who would do this under communism" usually boil down to "we wouldn't need to do that wasteful bourgeois individualist stuff in the first place". Like having a water tower and a treatment plant so that people can flush their poop through pipes in their house. Or having food cooked and served to order in individual portions. Or using a vehicle made of 2 tons of steel to travel past all the infrastructure required for everyone else's steel behemoth. Or buying a new outfit which gets worn 10 times and then thrown in the trash.

Our baseline needs are really not that hard to meet, and capitalism persists by feeding us the illusion that they are hard, and that capitalism alone can meet them.