How does an astronaut toilet work?

A space toilet is, at its core, an air pump. A fan creates suction that draws urine and solids into the system while also preventing odors from escaping into the module; the extracted air is filtered, among other things with activated carbon, and returned to the module. Urine passes through a funnel with a hose into a separator that distinguishes it from the air stream. Solids drop into a bag held inside a container that is sealed after use.

Separated urine is a raw material. It passes into the station's water recycling system, is distilled and filtered, and together with condensation water from the cabin air it returns to the drinking water loop; that's why the crew only receives a fraction of its water needs from Earth. Solids, on the other hand, are not reused: the filled containers travel on a cargo spacecraft that burns up on reentry into the atmosphere. On a trip to the Moon or Mars, this method of disposal would also need to be rethought.

The newest system on board, the Universal Waste Management System, was designed above all to be more compact and lighter than its predecessor, because the same technology must also fit into a cramped spacecraft capsule. Some parts are manufactured through 3D printing in titanium and alloys that withstand aggressive urine and pretreatment chemicals; the seat is shaped so that thigh stirrups ensure proper positioning. For the crew, in the end one thing matters most: a system that works reliably and can be repaired in orbit, because spare parts don't arrive until the next cargo spacecraft, at the earliest.