Below is a picture of a common backflow preventer, a Wilkins 720-A.
So... Why do I need a backflow preventer?
Because a lawn sprinkler system is considered a cross connection and a high hazard. This means there is a high possibility for contaminated water to enter the main water supply through the main sprinkler piping. Your lawn and landscape are filled with poisons or contaminants such as fertilizer, pesticides and animal feces just to name a few. Since the sprinkler heads are also located in these same spots, there is a possibility of dirty water entering through them.
Another common example is a garden hose that is attached to your irrigation piping. Sometimes we use the garden hose to fill a swimming pool or a bucket of water to wash the car in the driveway. If this hose is left in the pool or bucket, a back flow or back siphon-age may occur, and it would suck the chemicals from your pool or wash bucket into your drinking water. And not just your drinking water, but quite possibly the entire main water supply as well.
So... How can this contaminated water actually back flow into my drinking water?
It’s much easier than you think! Maybe there is a water main break down the street and water is gushing all over the place. This could cause water to be syphoned out of your home due to the low pressure from the break. Maybe the fire department is flushing the hydrants or connecting their booster pumps to their fire truck to extinguish a fire. These circumstances would also lower the water pressure at your home and the potential demand for water could be so high that it would actually syphon water backwards from any source that is connected to the main supply lines.
These are just a few instances of how back flow can occur and affect the safety and health of your family or neighborhood. Why take a chance?
Make sure your lawn sprinkler system has the proper back flow assembly installed and be sure to have it tested annually to ensure its working properly at all times.