Siphon power

Part two of my exploration of things I thought I understood but didn't, really:  the water closet or flush toilet.  Even before looking it up to write this piece, I guess I had a fairly decent understanding of the easily accessible drain-and-refill mechanisms located under the lid of the toilet tank.  It's a big tank with small, straightforward moving parts whose function is easily observed and understood.  The flush handle pulls a "piston" out of position at the bottom of the tank, allowing gravity to drain several gallons of tank water quickly through a large opening at the bottom.  As the water level falls, a float valve drops, operating another lever that opens a refill valve connected to the house's water supply.  (This resupply valve being a very small opening, the refill takes a minute or so while the draining operation takes only a few seconds.)  When the tank is full again, the float-ball lever should come back into position and shut off the refill valve.  Just in case, there's a little overflow pipe that lets excess refill water drain harmlessly down the drain, so that the worst that can happen is that you hear the refill water continue to run long after the flush cycle should have been completed.  That's called "The toilet won't stop running" or "why is the water bill so high this month?"

What was a little more mysterious to me was the bowl mechanism, which has no moving parts except water.  This part is a clever use of the siphon principle to provide waste-moving power with a built-in gas-trap seal.  While some commercial toilets use the building's water pressure to compress air gradually between flushes so that it can be used for moving power, most domestic water closets rely entirely on the potential energy stored in the raised water tank.  The clever part is the sideways-S-shaped wiggle built into the bottom of the bowl, which for some reason is called a "P-trap."  The S-shaped drain goes down from the bottom of the bowl, then up a few inches higher than the bottom of the bowl, then down again into the drainage system.  The high point of the bump-up is called a "weir," just like the dam-like affair that allows a pond to overflow when it reaches a certain level.  When the bowl is in stable "ready" mode, its water level is just high enough to fill the P-trap to the level of the weir, thus forming the gas seal that so contributes to our domestic comfort.  (Our sinks have P-traps for the same reason.)  If, at this point, we spill small amounts of liquid into the bowl, they will simply rise slowly over the weir and drain rather than flood the bathroom floor.  But if instead we dump several gallons quickly into the bowl, we activate a siphon that exerts a strong pull on the contents of bowl for several seconds until the siphon is broken again.  Assuming the drainpipe isn't clogged, we would be hard-pressed to pour enough water into the bowl quickly enough to make it overflow; it's a robust failsafe device.

No doubt many of us have learned that, when the water supply is temporarily off, the toilet can be made to flush pretty well simply by pouring a two-gallon bucket of water into it.  The bucket substitutes for the capacitor action normally performed by the water tank.  In normal action, though, a well-designed toilet tank adds two mechanisms that the emergency bucket does not duplicate:  it sends part of the water through "rim holes" near the top of the bowl, which wash down the sides, and it sends the rest through a siphon-jet hole near the bottom of the bowl, in order to jump-start the siphon mechanism.  Either way, unless the drainpipe is clogged (a melancholy prospect), all it takes to complete a flush is to introduce a couple of gallons of water quickly into the bowl.  To get this done in a pinch, we don't need water pressure, let alone a gas or electrical connection.  It's one of the things that works well in post-hurricane/zombie apocalypse conditions, if you've remembered to fill some bathtubs with water and the floodwaters outside remain lower than your bowl's weir.

All the finicky stuff inside the tank is just a much more convenient way of schlepping a couple of gallons of water into the bathroom for each flush, dumping it into the bowl quickly and without splashing when we want it to happen and not otherwise, and then refilling the tank slowly (while we leave the room and go on about our day) without much risk of overfilling the tank and flooding the floor.  Modern toilets do have extra wrinkles mandated by code, such as anti-siphon devices built into the refill water supply.  We're more careful now than we used to be about preventing back-flow from faucets or toilets into the house's clean water supply.  It's true that we're still using the potable water supply to transport our sewage, which is pretty primitive and wasteful, but that's another subject.

This video isn't bad except for its annoying sound effects; I recommend watching it with the sound off.