After flushing the toilet during a flight, water waste is flushed by a closed waste system or sucked by a vacuum waste system into an onboard sewage tank. Many people tend to believe that the waste is dumped down while the plane is in flight, which is wrong.
The waste is disposed of by the ground crew only after the plane lands. No one can empty the sewage tank while the plane is in the air because the valve is located outside the plane. Waste leaks only occur if there is a malfunction in the sewage system, and this results in what experts refer to as "blue ice." This is the blue waste treatment liquid that is turned into ice as a result of freezing temperatures at high altitudes.