How would you like a tall cool glass of sewage? A spritz of herbicide from the garden hose? Maybe insecticide is more to your taste. Bathwater? How about a dilute solution of earthworms? You could end up drinking any or all of these cocktails if your plumbing system has a "cross connection."
What's a cross connection? It's an accidental link-up between the water supply pipes in your home and a potential source of contamination. Sounds like a pretty unlikely scenario, doesn't it? See if some of these examples don't change your mind.
If you happen to leave the nozzle of your garden hose lying in a puddle of water, that's a cross connection. If you use a hose-end garden sprayer, that's another. So is a flexible shower head hanging in a tub full of bathwater. Or a darkroom rinse hose resting in a sink. Or a laundry fill hose hanging into a laundry sink.
Why are all these situations dangerous? Ordinarily, they're not. But under backflow conditions, they can be. Backflow occurs when there's a pressure drop in your water lines. This can happen when you turn off the main valve to fix a dripping faucet.
It can happen when a water main breaks, or a car knocks over a neighborhood fire hydrant. It can happen if there's unusually high demand for water in your house, or even in houses near yours.
Whatever the cause, the result is the same. You get a partial vacuum in your water lines, and they start sucking up trouble. That hose lying in the puddle can suck earthworm soup back into your water lines. The insecticide in your hose-end sprayer can be sucked up, too. The flexible shower head siphons off bathwater. The darkroom rinse hose picks up chemicals.
What's the danger of that laundry hose hanging in an empty sink? Nothing--unless your sewer line clogs and backs up into the sink. Enough said.
What can you do about all this? Search out all possible cross connections and eliminate them. If you use hose-end accessories, don't leave them attached between uses. Go to the end of every water pipe in your house and make sure it cannot accidentally link up with a source of contamination.
Unfortunately, not all cross connections are as obvious as those mentioned above. One good example is a water softener hooked up to a drain line. So are old-fashioned sinks with faucets that extend down below the level of the overflow opening (see sketch).
Not all cross connections can be eliminated. An underground sprinkler system is a good example. In a case like this, make sure the line serving the possible cross connection is fitted with a special one-way vacuum breaker valve, which will sense a vacuum in the supply line and immediately seal to prevent contaminants from being pulled back into the line.