In homes connected to a city water supply, there is a backflow device (also called a “backflow preventer”) installed to prevent water from flowing out of your home and into the water supply line. The purpose of the backflow device is to prevent contamination of drinking water in the event that pressure in the water lines changes. If backflow devices were not installed, it would mean that any time there is a significant change in the pressure within the water lines, water from the home (which might contain human waste, soap or detergent, chlorine, fertilizers or pesticides) would mix with the clean water people are using in their homes (which is to say that it would be a mess!). Essentially, backflow devices prevent this from happening by only allowing water to flow into the home and they stop water if it ever tries to go into the main water supply.