| | Figure 1, Sewer Backflow Prevention Gate Valve |
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Backflow valve in sewer line: In most areas, the sewer system and rain drainage system are separate. It is not unusual, however, for floodwater to infiltrate the sewer system, causing it to back up into homes. These back-ups are not confined to homes in flood hazard areas, nor are sewer back-ups always caused by floodwater entering the system.
Most homes have interior drains that converge under ground near the house into a single 4-inch sewer line. The simplest, least expensive way to prevent backflow is with a flap valve installed on the single sewer line; it allows water from the home to flow to the sewer system but closes when water flows backward toward the house. Access must be provided to the valve so it can be cleaned at least semi-annually and if something obstructs the flap. (Figure 1, Sewer Backflow Prevention Gate Valve)
A ball or gate valve that is closed by hand is not subject to obstruction failures, but it won’t stop a backflow unless someone is home to close it. Automated or hydraulically operated backflow valves are considerably more expensive, but provide reliable protection at all times. (Figure 2, Flap Valve) (Figure 3, Ball Valve)