"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. Because the vacuum does all the work, it takes very little water (or the blue sanitizing liquid used in airplanes) to clean the bowl for the next person. Most vacuum systems flush with just half a gallon (2 liters) of fluid or less, compared to 1.6 gallons (6 liters) for a water-saving toilet and up to 5 gallons (19 liters) for an older toilet.