A sewer system that carries only sanitary sewage; no storm-water runoff. When a sewer is constructed this way, wastewater treatment plants can be sized to treat sanitary wastes only and all of the water entering the plant receives complete treatment at all times. Separate sewers mainly comprise of water sources that comes from surface runoff, and municipal wastewater. These sewer systems are designed to answer issues concerning volume of the wastewater that is brought and whether the other water sources can be reused once the treatment process is complete.