A space toilet, or zero gravity toilet, is a toilet that can be used in a weightless environment. In the absence of weight, the collection and retention of liquid and solid waste is directed by use of air flow. Since the air used to direct the waste is returned to the cabin, it is filtered beforehand to control odor and cleanse bacteria. In older systems, waste water is vented into space, and any solids are compressed and stored for removal upon landing. More-modern systems expose solid waste to vacuum pressures to kill bacteria, which prevents odor problems and kills pathogens.
To ensure that the waste also doesn’t float around, the toilet uses flowing air instead of water to flush the toilet. The air pulls the waste away from the astronaut’s body and flushes it away. After the air is filtered to remove bacteria and odors, it’s returned to the living cabin. Solid wastes are dried to remove all moisture, compressed and kept in an on-board storage container. They’re removed and disposed of once the spacecraft has landed. The liquid waste is sent into space.