The water sitting in a toilet bowl is a plug against sewer gas
That pool of water always waiting in a flush toilet has a job. The bowl drains through a pipe bent like an upside-down U, and the lip of that bend sets how high the water can stand. The trapped water blocks sewer gas from rising into the room, while a vent pipe carries the gas away.
A standard flush toilet is a ceramic pan fed from above by a cistern that refills quickly and emptied below into a drain. One arm of the U-bend acts as a siphon, longer than the water in the bowl is deep, which is what drags the contents away when you flush. Because conventional flushing eats up a large share of a person's daily water, designers now offer low-flush bowls, pressurised tanks, and dual-flush handles or buttons with a smaller dose for urine. Some homes plumb in greywater left over from laundry, dishes and baths.
Moving vehicles need a different trick. Trains, aircraft, buses and plumbed ships often use vacuum toilets that suck waste away with under a quarter of a litre per flush, or none at all. Using so little water saves weight and stops the bowl sloshing over as the vehicle sways. On boats the toilet is called the head, and it may flush with seawater.
Plenty of toilets skip plumbing entirely. Pour-flush toilets, common across Asia, are rinsed with a bucket holding as little as two to three litres. Twin-pit latrines alternate between two pits so one can rest until its contents turn into a soil-like material that can be dug out. Floating toilets on platforms serve people who live on water or face long floods, as in Cambodia. The old pig toilet sent waste down a chute into a sty and was common in rural China; fish-pond versions fed carp the same way.
Culture shapes design too. Squatting toilets are more usual in Asia, sitting ones in Europe and North America, and bidets are common in largely Catholic countries and in Greece and Finland. In prisons, inmates in solitary confinement have used the bowl as a crude telephone, talking through the pipes.
Source: Toilet
