The toilet in an aircraft works quite differently from a normal toilet. Each toilet bowl has two valves. A water valve and the flush valve. The water valve provides water from the potable water tank for flushing the toilet waste and the flush valve drains the waste. When the flush button in the toilet is pressed, the flush control unit opens both the water and flush valves, allowing water to flush out of the toilet bowl into a waste tank.

At ground level, the flushing pressure is provided by a vacuum generator. This generator is electrically powered and is connected to the waste tank via a pipeline. Whenever the flush button is pressed, the vacuum generator comes online, creating a vacuum in the waste tank, which allows the waste content from the toilets to fall into the tank. When in the air and above a certain altitude, the pressure difference between the cabin and the atmosphere is sufficient to pull the waste from the toilet bowl into the waste tank. The vacuum generator is therefore no longer necessary.

In some older and smaller aircraft, the toilet is of recirculation type. In this toilet system type, a waste tank is not exist. Instead a holding tank with a flushing solution is used. This flushing fluid contains disinfectants that give the water a blue color. In the holding tank, there is a pump and a filter. The service of a recirculation toilet is similar to a vacuum toilet. The only difference is that once the holding tank is emptied, it needs to be refilled or recharged with new flushing fluid as it requires this liquid for the flushing job.

Sometimes, these types of toilets may leak. This occurs mainly owing to improper toilet maintenance or component failures. In flight, any wastewater from the toilet system can freeze, which can leave a line of frozen flushing fluid on the aircraft fuselage. The frozen solution can even  fall off the aircraft. As if the toilet is serviced with blue disinfectant, the ice formed can be blue. This is sometimes called blue ice.