What happens when you flush a toilet on a plane?

Using toilets on airplanes is sometimes inevitable. When you do, a million questions race through your mind: Why does it say not to flush while seated? Why is the toilet water blue? Why are the flushes so loud? Does the poop fall from the sky?

Looking at the technology involved, toilets can be really fascinating devices.
The typical home toilet uses a bowl filled with water. When you flush, the toilet starts a siphon that drains the bowl; gravity then carries the water into a septic tank or the sewer system.

On a moving vehicle, however, you can’t use this system because water will splash every time it shakes and you can’t empty the toilet using a siphon or gravity.

Instead, airplane toilets use an active vacuum. When you flush, it opens a valve in the sewer line and the vacuum in the line sucks the contents of the bowl into a tank. This is why you have to close the toilet before you flush. The vacuum does all the work, taking in little water – 2 litres of water, compared to 6-19 litres normal toilets use – or the blue sanitising liquid used in planes to clean the bowl for the next person.