How do airplane toilets work?

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Even in an age where flying is a fairly regular occurrence, there are some things that you never forget about your first journey on an airplane.That visceral thrill of being pushed back into your seat as the plane accelerates toward take-off, the jitters that accompany your first bout of turbulence, and the SLUUUUURP sound in the lavatory.
The alarming noise ensues when you press the 'flush' button in an airplane toilet and the bowl's contents are magically sucked away into oblivion. With the noise and essentially 'clean' bowl as a result, there's clearly some sort of vacuum cleaner-type effect at work.But how do aircraft toilets work?
Aerospace engineer Bill Crossley, the head of the School of Aeronautics and Astronautics at Purdue University in Indiana, explains to Popular Science that the answer is as simple as it is ingenious. The system relies on one simple fact.
'When you go up to high altitude and you're flying fast, the pressure outside the cabin is a lot lower than it is inside,' he says.
The laws of physics—and specifically the ideal gas law—dictate that the contents of a region with relatively high pressure will tend to flow into a region with relatively low pressure. Flushing the toilet on an airplane opens a valve between the pressurised cabin and a tank that remains at atmospheric pressure. That process creates exactly this sort of pressure differential.
In other words, 'when you flush the toilet, you're basically opening a valve to the outside, and the pressure differential sucks away whatever's in the bowl,' Crossley says.
The beauty of the system is that it doesn't require vacuum pumps or other complications to create the pressure differential. It simply makes use of the existing difference in pressure between the aircraft's interior and exterior at altitude. Of course, this requires that there is a pressure difference.
On the ground, Crossley says, the system uses a vacuum pump. The pump remains in use until the plane reaches an altitude where the atmospheric pressure allows the system to work without it, at which point it is switched off.
The idea itself is simple and has remained largely unchanged since it was first patented in 1975. However, retired aircraft engineer Nigel Jones explains that the complexity of the implementation still varies from aircraft to aircraft. Some planes' systems evacuate every toilet to a single tank, while others use multiple smaller tanks. Some use complex piping arrangements, while others opt for simpler configurations.
[ Related: How does a composting toilet work? Ditch the flush.]
Then there's the Lockheed TriStar, an airliner built between 1968 and 1984 that remains Jones' favorite to this day. The TriStar's toilets were famously arranged in a circle, and they were as idiosyncratic behind the scenes as they were to passengers.
'[The TriStar] had a logic system to manage flushing,' he says fondly. 'It had a tank at the front for the forward toilets and another for the five or six toilets at the back. The system had three, I think it was three toilet pumps, and it would flush pumps in turn that the logic box would if say toilet five wanted to flush, the logic box would say, 'Right, it's pump one's turn.' It was incredibly complicated, and all just to flush the toilet.'
Some might argue that this system sounds perhaps a little over-engineered. 'We did feel that way every time there was a problem and looking at the pump didn't fix it,' Jones concedes. 'If we had to go in and look at the logic box, we'd have to lie on top of the tank. This was… well, it was not pleasant.'
While such complexity may be overkill–and is rarely found in modern airliners–Jones notes that some degree of twisting and turning in the system's piping is important for one simple reason: it slows down the waste.
'It's not a straight run [from toilet to tank], and it can't be,' Jones says. 'The vacuum pressure means that the drawing speed is such that if there were no bends in the pipes, the waste would hit the wall of the tank with considerable force.'
How considerable? According to Jones, it would be 'enough that you could potentially break the tank.' Even without such a catastrophic outcome, he notes dryly, 'the noise would be most alarming.'
Prior to the advent of the vacuum-based flush, planes used chemical systems similar to those that remain in use today in portable toilets, where the bowl connects directly to a tank full of the dreaded blue liquid. This also explains the origin of the term 'blue ice', which refers to waste that somehow escapes the tank—at high altitude, any such waste freezes immediately, and remains frozen until it returns to the ground, at which point it reveals its true nature to anyone unfortunate enough to cross its path.
Such systems are vanishingly rare today, remaining in use only on aircraft large enough for aviation regulations to require the presence of a toilet, but small enough that a vacuum system is more trouble than it's worth. Crossley says that the only planes to fall into this category are small business aircraft, which are sometimes equipped with what he describes as a 'fancy port-a-potty.'
On larger planes, he says, the vacuum system really is the only game in town. 'It's everything you want an airplane system to be: first, it's safe; second, it's reliable; and third, it's lightweight.'
This story is part of Popular Science's Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you've always wanted to know? Ask us.