This is what happens when you flush toilet in Aeroplane

Have you ever wondered what happens when you flush an aircraft toilet? What causes the loud noise, and where is the waste stored on the aircraft? It is actually a great pressure-based system, developed in the 1970s, that is used on all commercial aircraft, as this article explores.

The modern aircraft toilet

The toilet that we all know and use on aircraft today is a vacuum-based system, with a non-stick bowl. It was invented (and patented) in 1975 by James Kemper (you can view the US patent details here). The system is mostly water-free, clearing the bowl instead by suction.

This offers several advantages over a water-filled system, as is usual in toilet systems on the ground. It is much cleaner for a start (imagine the spillages during turbulence or sudden movements). But it is also lighter as there is no need for a large tank of water for flushing. If you think how many times a toilet can be used on a full 747 or A380, you can imagine the weight saving.

There hasn’t been much change to aircraft toilets since these were introduced, at least not the technology. Airlines frequently change the toilet designs and layouts of their bathrooms. Sometimes these changes are towards the large and luxurious, such as the spacious toilets and showers on some A380 aircraft, and sometimes towards the small and simple, such as space-saving versions installed on the Boeing 737 MAX (measuring just 24 inches from wall-to-wall).

Vacuum suction

So how does the vacuum toilet work? When the flush button is pressed, a valve at the base of the toilet bowl opens, a small amount of blue disinfectant liquid (known as Skychem) flushes through, then the contents are sucked out of the bowl by suction. This is the ‘whoosh’ sound you hear when flushing.

This suction works by pressure difference. The waste tank is kept at a lower pressure than the cabin (it is not pressurized like the cabin is). There is also a vacuum pump in the system which is used when there is not sufficient differential pressure (this explains how the toilets still work on the ground). Once the valve in the toilet bowl is opened, the pressure difference causes air from the toilet bowl to be sucked rapidly out.

The waste and Skychem solution then flows through pipes to the rear of the aircraft. It is stored in large sealed tanks and emptied once on the ground, either by tanker or pumped into an underground storage system

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