Airliner toilets use either a “closed waste system,” which works much like a common house toilet and flushes the wastewater into an onboard sewage tank, or the more modern “vacuum waste system,” which sucks wastewater into the tank.
While up in the air, the latter is powered by the difference between the air pressure outside the airplane and inside the cabin, and produces a roaring vacuum whenever a passenger activates the flush. (The noise may make it seem like the toilet is flushing your waste out into the atmosphere, but it’s not.)
Under normal circumstances, the ground crew disposes of the sewage after the plane lands. Even if the pilot and flight attendants wanted to empty a tank midflight, they couldn’t, as the valve is located on the outside of the plane, and can only be opened by the ground crew.
Waste can seep out of the tanks in a malfunctioning aircraft. This happens when a sewage tank or drain tube develops a leak, usually resulting in what air travel experts refer to as “blue ice”—which is what you get when the blue waste treatment liquid from a plane toilet is exposed to freezing temperatures at high altitude.
Blue ice typically gathers and stays on the outside of the aircraft, but sometimes it breaks off before landing. When blue ice does come loose, is often melts and evaporates before reaching the ground.
Leakage of waste is also a serious safety concern in the air. Blue ice has been known to damage aircraft, in one case even knocking an engine off the wing. The Federal Aviation Administration says that some falling blue waste might be nothing more than tinted excrement from a migratory bird that happened to have eaten a blue-colored fruit.
Some other vehicles do intentionally dump waste while in transit. Cruise ships, which can be thought of as small cities on the sea, routinely discharge thousands of gallons of human sewage a day.
Until the 1990s, U.S. trains commonly flushed their waste onto the tracks, simply asking passengers not to use the bathroom while the train was in the station, and this is still common in some parts of the world.
Now they store waste in an onboard holding tank. Aboard the International Space Station, solid waste from astronauts is tightly bagged and fired back toward Earth in an unmanned vehicle.