One of the most difficult engineering challenges faced by NASA was the design of the toilet used on the Space Shuttle and ISS. The liquid urine wastes were collected, filtered and recycled as potable water. The solid wastes were collected by vacuum and deposited against the sides of a cylindrical receptacle liner by a spinning impeller. This prevented the solid waste from floating back into the crew cabin.
One thing that every astronaut notes when first entering a space station (like Mir or ISS) is the odor of sewage.