A backwater valve is a device that is usually installed at the exit of the sanitary system from the house. This device can either be installed outside or inside. The purpose of this device is to prevent the backflow of raw sewage from the city sewer system, back into your dwelling. This can happen in certain areas during heavy rains or seasonal flooding. We recommend and install the Rector Seal brand backwater valve. JEM has a handful of them out and in service performing very well.