Have you ever wondered what happens when you flush the toilet in an aeroplane? Well I will explain for you.
The typical home toilet uses a bowl filled with water. When you flush, the toilet starts a siphon that drains bowl, gravity then carries the water into a septic tank or the sewer system.
But in an aeroplane, the airplane uses an active vacuum. When you flush, it opens a value in the sewer line and the vacuum in the line sucks the content of the bowl into a tank.
This is why you have to close the toilet in the airplane before you flush, the vacuum does all the work taking in little water 2 liters of water compared to 6-19liters normal toilet uses. Aside using very little water, vacuum toilet can flush in any direction and can be put anywhere in a building.
So when you use the toilet in the airplane for the first time read the instructions written inside the toilet.