Expensive, hard to use, and seemingly incapable of avoiding the pun “out of this world”, NASA’s Waste Collection System is known all over the world, at least in passing. Here’s the real story behind space toilets:
The easiest way to explain the Space Toilet is to not start with the toilet at all, but rather the wet shop-vac, which is much closer to the space toilet than an actual toilet is. You see, without gravity pulling everything down, anything will just tend to pool up, attached to whatever it last was. This problem, in NASA technical jargon as it relates to human waste, is called “lack of separation”, and the space toilet’s function is mostly to facilitate separation. And so, the space toilet is both the fanciest toilet in the world, but also the most advanced shop-vac, since their solution was to narrow the opening for waste and create meaningful suction.
As far as using one is concerned, it’s conceptually as simple as literally vacuuming human waste into the system. Flip a switch, and fans start up and create a vacuum. Number 1 is pretty strait forward, and with a handy flexible tube attachment, it is nearly impossible to mess up aiming for guys (sorry ladies). Number 2 is entirely different, and in reality, usually requires hours and hours of practice for astronauts to get right. Astronauts actually have scheduled space pooping training sessions on the ground, because failure in space is a much worse mess to deal with, and nobody wants to be famous for defecating on multi-million dollar equipment and having bits float about the place (NASA’s term for these are “escapees”). Talk about potential for being a bad roommate.
This is why space toilets are so complicated and expensive, like so many other things in space. This isn’t just a toilet, it’s a whole municipal water treatment system in miniature, that runs with minimal water, and can’t get oxygen or anything else from the atmosphere, and has to contain it’s smells perfectly. Really, zero gravity is probably the simplest problem for the engineers to solve with the device, and the only advantage being in space give you for this is all the free “cold” you could want.
Before the Space Toilet, there was the “Fecal Containment System” and “Urine Containment Device”.
The urine containment device is basically a disposable condom that attaches to a bag for collection, and ultimately then the urine is to be ejected into space.
Since engineers needed the connection to be snug, so to speak, they handed out the different sizes – Small, Medium, and Large and surveyed astronauts about which condom size fit the best. Naturally every one said Large every time, and eventually they actually had to start labeling them Large, Gigantic, and Humongous. Simply so astronauts wouldn’t end up with wet spacesuits out of stubborn pride. At least back then you could watch it spray out into space, which was apparently one of the more unexpectedly pretty things you could see in space.
The all the elegance of a plastic bag, with finger bags (called finger cots in the tech specs) for manipulation. Bonus: Since there is live bacteria that will swell and rupture the bag over a period of many hours, you have the dubious honor of getting to massage some germicidal gel into the bag when you are done. Well, at least you are an astronaut at a time when ticker-tape parades were a thing, so you got that to look forward to. Also, these things probably cost a fair bit of taxpayer money, so it’s sort of decadent for the user in a way. So it’s all in the frame of mind.
As you can see, what amounts to a multi-million dollar shop-vac is a step up, and thus the Space toilet is the ISS preferred choice.