Backflow is the reversal of the direction of non-potable water flow in to a potable water system. For example, backflow can occur through a cross-connection (cross-connection is any connection between the public water supply and a source of contamination). This process can contaminate drinking water supplies. There are two types of backflow--back pressure and back siphonage.
Back-pressure occurs as the result of the downstream pressure being greater than the potable water supply pressure. This reduction in supply pressure occurs whenever the amount of water being used exceeds the amount of water being distributed, this can be created by pumps, temperature increases in boilers, breaks in water mains, etc.
Back siphonage is backflow caused by a negative pressure (i.e., the reversal of normal flow in a system caused by a vacuum or partial vacuum within the water supply piping) in a public water system or consumer's potable water system. Back siphonage can occur when there is a high velocity in a pipe line, or a stoppage of water supply due to nearby fire fighting, a break in a water main, etc.
Backflow can be prevented with the installation of the backflow devices.
Below is a list of a Lincoln Water System approved backflow preventer means (https://www.lincoln.ne.gov/city/pworks/water/backflow/faq.htm)
Approved air gap
An air gap is a vertical, physical separation between the end of a water supply outlet and the flood-level rim of a receiving vessel. This separation must be at least twice the diameter of the water supply outlet and never less than one inch. An air gap is considered the maximum protection available against back pressure backflow or back siphonage, but is not always practical and can easily be bypassed.
Double check backflow assembly (DC)
A DC is a testable mechanical backflow preventer that consists of two independently acting, spring-loaded check valves. It includes shutoff valves at each end of the assembly and is equipped with test cocks (required to be tested annually by a State Certified Grade 6 Operator) against back pressure backflow and back siphonage, but should be used only on non-health hazards.
Reduced pressure principle backflow assembly (RP)
An RP is a testable mechanical backflow preventer that consists of two independently acting, spring-loaded check valves with a hydraulically operating, mechanically independent, spring-loaded pressure differential relief valve between the check valves and below the first check valve. It includes shutoff valves at each end of the assembly and is equipped with test cocks (required to be tested annually by a State Certified Grade 6 Operator). An RP is effective against back pressure backflow and back siphonage, and may be used on health or non-health hazards.
Pressure vacuum breaker assembly (PVB)
A PVB is a testable mechanical backflow preventer that consists of an independently acting, spring-loaded check valve and an independently acting, spring-loaded air inlet valve on the discharge side of the check valve. It includes shutoff valves at each end of the assembly and is equipped with test cocks. A PVB may be used to isolate health or non-health hazards, and is to be installed 12-inches above highest downstream water discharge. It is effective against back siphonage only and is most commonly used on lawn sprinkler systems.
Atmospheric Vacuum Breaker (AVB)
An AVB is a non-testable mechanical backflow preventer with a gravity opening poppet air opening, designed to admit atmosphere into the downstream sides of the unit under a no flow condition to prevent back siphonage. It must be installed 6-inches above highest downstream water discharge. There shall be no valves or reduction of pipe size on its downstream side. (Note: This device is installed on certain equipment that uses potable water by the manufacturer or contracted installer. Examples of this equipment are dishwashers, soap dispensers, faucets and deep sinks, etc.) It is up to the City of Lincoln that has jurisdiction to determine which type of backflow protection is required based on the degree of hazard that the property represents to the potable water supply.