The Fluoride feeder runs out of feed solution. The rapid mix tank is connected to a 5 km long distribution pipe. The flow rate into the tank mix tank is 0.44 m?/s and the volume of the tank is 2.50 m?. The velocity of the distribution pipe is 0.17m/s. If the Fluoride concentration in the tank is 1.0 mg/L when the feed stops how long until the concentration is reduced to 0.1 mg/L at the end of the distribution pipe? The fluoride may be considered non-reactive.