A bed of granular media is supported in a vessel to process liquid flowed in through both the upper and lower faces of the bed and out of the bed at a location intermediate the upper and lower faces. A drain structure for the processed liquid is mounted at the location intermediate the two bed faces with the capacity to withdraw the quantity of liquid flowing into both of the bed faces. A closed circuit is provided to transfer residual material in the drain structure back to the inlet system of the bed after backwash of the bed. A holding vessel is connected to the vessel in which the media bed is supported to temporarily receive the media for service periods of the bed vessel.