This is what a typical, modern commercial airplane toilet looks like underneath all the plastic coverings:
When the flush button is pressed, the flush control unit will check the altitude of the airplane using a pressure gauge. Then, the water valve is opened to let a little bit of water out to rinse the toilet bowl. Finally, the flush valve opens to evacuate the waste matter.
The pipes and waste tanks are subjected to suction pressure from the atmosphere because there is a pipe that connects the waste tanks to the outside of the plane. But don’t worry, only air can pass through this particular pipe.
At approximately above 15,000 feet, there will be sufficient differential pressure between the cabin pressure and the atmospheric pressure, which helps create the suction force needed to flush the toilet.
Air will always flow from an area of higher pressure to an area of lower pressure. When the flush valve is opened, the pressuried air in the lavatory will be exposed to the atmosphere. As such, it will rush out of the airplane explosively, through the toilet bowl’s discharge hole, pipes, the waste tank and the air outlet. While doing so, the air will push the waste matter that is blocking its way into the waste tank.
However, when the plane is below the necessary altitude, or, while the plane is on the ground, a vacuum generator will provide the suction pressure instead.
So how does the toilet know when the vacuum generator is needed?
As mentioned earlier, the flush control unit — capable of actuating the generator — will check the altitude of the plane when the flush button is pushed.
The flight attendants are able to monitor the level of waste in the tanks using the forward attendant panel.
When the container needs to be emptied, the cabin crew will inform the flight crew, and they will radio the airport to request for waste water service. A specialised airport sewage truck will be sent to the airplane.
A worker will attach a hose to the drain hose connector situated at the rear of the aircraft. Then, a switch is activated and waste is drained from the airplane’s tanks and transferred into the truck’s tank.
The truck is then driven to the airport’s waste disposal facility. A valve will open beneath the truck’s tank for its contents to be discharged into the underground sewer system, through a hole in the ground.
If you’re curious to know what the contents look like, here is it:
Thanks for reading.
