For the past week, astronauts aboard the ISS have been alternating between their only other civilized option -- the toilet on the Soyuz return capsule -- and a "back-up bag-like collection system" NASA engineers developed for just such an emergency. Given the challenges surely presented by peeing in zero-gravity (picture splashback droplets floating around the cabin like plankton in a poorly-maintained aquarium), the mechanical toilet surely involves some sort of negative air-pressure device that sucks urine into a receptacle; I'm hard-pressed to imagine how a non-mechanical device can properly function.

On Earth, sanitary waste management relies on two forces: the muscles of the body, which physically propel liquids and solids at velocity through one of two apertures; and gravity, which we trust to ensure that-which-is-no-longer-us goes into the toilet and stays there. Our bodies evolved to rely on gravity as a constant; there's a reason these excretory apertures function best when aimed downward. But in space, while the muscles of the body continue to function, the lack of gravity means that streams of effluvia will not simply drop with a satisfying plop, but instead ricochet about an enclosed area with an equal and opposite velocity to that at which it was expelled (allowing for friction caused by viscosity, of course; but in zero-G, you shouldn't place your hopes for a particle-free oxygen on your waste's penchant for stickiness).