A simple demonstration of backflow can be seen when a glass of soda is emptied by a person sucking on a straw placed in the glass.

A lawn sprinkler system installed without the proper vacuum breaker can constitute another hazard; contaminants could get back into the domestic drinking water supply.

In California, a man using a weedkiller attachment on his garden hose, took a drink from the hose after he had disconnected the weedkiller device. A short time later, he was dead from arsenic poisoning because a cross-connection existed between the drinking water supply and his weedkiller attachment. A momentary back-siphonage occurred, the arsenic was vacuumed out of the weedkiller container and into the hose.

Back-siphonage is a form of backflow caused by a negative vacuum or subatmospheric pressure.

A simple device called an atmospheric or vacuum breaker could have saved this gardener's life. A vacuum breaker stops the formation of a vacuum in the water supply piping by allowing air (atmosphere) to enter the piping.

Other types of backflow preventers are designed to prevent contamination or pollution of the potable water supply.

Flush Tank

Most residential water closets (toilets) are flushed by a means of a flush tank. If the waste outlet for the fixture bowl should become clogged and the bowl becomes filled to the rim with wastes, a vacuum on the water supply to the tank supply fitting could back-siphon the wastes from the bowl into the drinking water supply. A properly installed anti-siphon ballcock can prevent such a contamination from occurring.

Spray Hoses on Faucets

The hose and nozzle used on kitchen faucets to rinse dishes could become submerged in the dirty or soapy water during the dish washing process. Any vacuum on the supply line could siphon the sink water into the drinking water piping. The same condition could occur with hand-held personal shower hoses which hang into the bathtub well.