When you turn on a faucet you
expect clean water to come out. In North America we tend to
take our clean water supply for granted.
But in water supply systems there is a rare but dangerous
phenomenon called back-siphoning. This can only occur
when three conditions are present:
- A faucet (or a garden hose) is turned on.
- The faucet or hose is submerged in water that has
"left the system" and is therefore considered
- There is a sudden loss in water pressure in the water
supply system, perhaps caused by a break in a supply pipe
or simply the water being turned off for maintenance.
The sudden loss of pressure can allow dirty water to be
sucked into the supply pipes, which would contaminate
the water system. This could be just the plumbing in a single
house, if it was on its own well. Or it could be a large part
of a municipal water system.
A plumber once told me about a case where a man was
mixing pesticide in his backyard, and his garden hose
was submerged in a tank of water with the chemical.
There was a disruption in the supply pressure and
back-siphoning sucked the pesticide solution into the
city water system. What a mess!
(Imagine the guy in his backyard... "Hey,
where'd my insecticide go? That's weird! Who's messin'
around here? Dang kids..." That kind of
un-explainable event could drive a paranoid person to
extremes of behavior!)
Once the water system people finally noticed that the
tap water was tainted, it would have been necessary to
flush a lot of water from many hydrants and homes, and
perform tests on the water, and alert the public, and
deal with the news media, and many other headaches.
As a consequence, there are some elements of modern
plumbing codes that prevent back-siphoning:
- Faucets must be above the level of a basin, so that
a plugged basin cannot let dirty water into a faucet.
- Hose connections must have a special device called a
"vacuum breaker" that prevents the backwards
flow of water when pressure is shut off.
These things may seem like a hassle, but they are for the
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