When flushing the toilet in a passenger airplane, I'm amazed by the huge noise it makes. Like a powerful vacuum cleaner. How Stuff Works article entitled How Toilets Work , then you know my philosophy on this topic. The typical home toilet as described in the article uses a bowl filled with water. When you flush the toilet, it starts a siphon that drains the bowl.
Gravity then carries the water into the septic tank or the sewer system.
See How Toilets Work for details. Since there is no bowl of water, you cannot use a siphon or gravity to empty the bowl. Airplane toilets use an active vacuum instead of a passive siphon, and they are therefore called vacuum toilets . When you flush, it opens a valve in the sewer line and the vacuum in the line sucks the contents out of the bowl and into a tank. Since a vacuum system does not use gravity to move the water, there is nothing to stop the sewer pipe from going straight up.