Patent ID: 7045067

Claim:
A method of operating a depth granular media filter comprising a filter vessel containing a bed of filter media, the method comprising the steps of: (a) passing a feed liquid downwardly through the bed during normal down-flow service of the filter for removing contaminants from the liquid by loading the contaminants onto the filter media; (b) periodically backwashing the media to remove contaminants from the filter media and displace the removed contaminants from the vessel; and, (c) at intervals between steps (a) and (b) passing upwardly through the bed a volume of liquid under flow conditions selected to displace at least some of the contaminants upwardly in the bed while essentially removing none of the contaminants from the vessel; wherein said filter bed comprises at least two media types of different but uniform particle sizes and densities, each media type forming a distinct layer in the vessel, the effective particle size of the media in each successive layer decreasing in a downward direction in the vessel; and wherein said volume of liquid is passed upwardly through the media bed in step (c) under flow conditions selected so that a lowermost one of said media layers expands by an amount greater than 25% of its unexpanded length while an uppermost media layer does not expand in length appreciably during the period in which liquid is passed upwardly through the media bed.