A cross connection is a physical connection between drinkable water and a liquid or gas that could make the water unsafe to drink. An example may make this more clear. Let’s suppose you have a water hose sitting in a swimming pool or in a puddle of water in your yard. If the city water supply pressure drops for some reason, then it is possible for the swimming pool water or the muddy water in the puddle to be sucked or siphoned backwards through the water hose and into your home’s water supply pipes, and possibly even into the city’s water supply system. This process of liquid flowing in the direction opposite of what is intended is called backflow. It is no different than siphoning gas out of a gas tank with a hose. Hopefully this explanation is clearer than mud.
Although a loss of pressure is not a common occurrence, it can and has happened many times over the years. When it does occur and backflow results, then flushing and decontamination of a home’s, and possibly the city’s, water distribution system is necessary.
Water hoses are the most common source of cross connections at homes. Simply putting your water hose into your gutters or in a downspout to clean them creates a cross connection. Attaching your hose to a spray wand that sprays soap or pesticide creates a cross connection. There are two methods to prevent cross connections with your water hose: one is free but less convenient to do in the long run, and the other should cost less than $100, but once done, you really don’t have to think about it again. The first method is to never submerge your hose into water or a rain gutter or connect it to a spray wand or to anything else that contains chemicals or other contaminants. In many ways, this makes your water hose less useful and convenient. The other method is to purchase and install hose bibb vacuum breakers on all of your outside hose bibbs (water faucets). Once this method is implemented, you can essentially forget about it.
Some other sources of cross connections in your home are dishwashers, washing machines, sinks/tubs, sprinkler systems, etc. I will explain how each of these can become a cross connection between wastewater and your home’s water supply system.
If there is water sitting in the bottom of your dishwasher and the city’s or your home’s water pressure drops low enough, then it is possible for the dirty water in the bottom of your dishwasher to be siphoned up through the drain hose (backflow) and enter your home’s water supply piping.
I’ve read a story ( https://inspectapedia.com/plumbing/Plumbing_Cross_Connections.php) about a washing machine drain hose being tightly connected to a pipe in the home’s drain/sewage system. Basically, the connection was airtight. During the night, the city’s sewage system had a backup. This back up caused sewage to back up into the home’s sewage system which was then pushed through the airtight-connected washing machine drain hose and into the washing machine. The family woke up to a washing machine full of sewage. Again, a rare occurrence, but it does happen. Although, this was not caused by a loss of water pressure and did not result in contamination of the home’s water supply system, it was caused by a cross connection and did result in the need for decontamination of the washing machine.
By today’s plumbing code, faucets for sinks and tubs must be higher than the flood rim (top of the tub or sink) so that the faucet can never be submerged in water when the tub or sink is filled to the top. However, there are some older homes with tubs and sinks where the faucet was installed lower than allowed today and will be submerged in water if the tub or sink is completely full. This creates a cross connection. This cross connection can allow water to be sucked out of the sink or tub and backflow through the faucet and into the home’s water supply system if water pressure is lost.
Your sprinkler system also creates a possible cross connection. If you have standing water in your yard (which may contain chemicals and certainly contains dirt and bacteria), then it is possible during a loss of pressure for this water to be sucked through a sprinkler head, through the sprinkler system piping, and into your home’s (and possibly the city) water supply system.
Preventing cross connections and backflow
It is possible to prevent these cross connections and the backflow that can result from them. As explained above, the best way to prevent a cross connection with your water hose is to install vacuum breakers on all of your hose bibbs. (In the photo to the right, the gold-colored device on the end of the hose bibb is the backflow preventer.)
To prevent a cross connection on a sink or bathtub, an air gap must be maintained. This means that the end of the faucet must be higher than the flood level of the tub or sink. Current plumbing codes require that the end of the faucet be installed at least twice as high above the top of the sink or tub as the diameter of the faucet. In other words, if the diameter of the faucet is 1 inch, then the outlet end of the faucet must be at least 2 inches above the top of the tub or sink.
To avoid a cross connection on your dishwasher, there must be a “high loop” on the drain hose under the sink. This simply means that the drain hose has to be mounted under the sink up high where it forms an upside-down letter “U.” This high loop will contain air which will break the siphon and prevent wastewater from being sucked backwards through the drain hose.
For your sprinkler system, backflow preventers are required to be installed on the system to prevent backflow from occurring. All sprinkler system installations are required to be performed by licensed irrigators, and all licensed irrigators should install a backflow preventer on the sprinkler system.
All building codes (plumbing, electrical, etc.) are put into place to protect people, and the codes regulating cross connections are no different. People have been injured, sickened, and even killed by water contaminated by sewage, pesticides, chemicals, etc. While this is a fairly rare occurrence, it is worth preventing. Preventing most cross connections is fairly cheap and just requires some common sense. Now that you understand cross connections and the dangers that they present, it should be a fairly simple task to prevent them at your own home.