The hydraulic retention time for a single reactor or treatment unit can be calculated by dividing the volume of the reactor by the influent flow rate A gravity sand filter bed that is rectangular with horizontal dimensions 6x18 ft has an influent flow rate of 50 gpm and a hydraulic retention time of 216 minutes How deep is the sand bed Round your answer to the nearest hundredth hint first calculate the volume of the filter bed then use the known dimensions to calculate the missing one Y