There have been two segments on Ask This Old House where they showed an emergency sump pump that ran off of city water. When the float gets high enough (higher than would cause the regular sump pump to engage) then city water would flow through the pump and outside. For every gallon of city water that went through the pump it would suck out 4 gallons of water from the sump (if I remember my numbers correctly.)
Does anyone out there actually have one of these? If so how does it work for you?