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This simple devices offers effective protection against the backflow of sewage and stormwater into homes and businesses. Under normal operation, the lightweight flapper opens and allows waste water to exit to sewer line. During a backflow,the flapper seals closed, stopping the flow from reaching interior drains entering the living space. Backwater valves (BWV)are recommended on all branch lines serving low level appliances, leaving the main line open to discharge and vent. Any drain openings behind the BWV are protected from backflows. A BWV can also be installed on the main line before it enters a building.