Method and apparatus for increasing filter contaminant loading capacity
The filter contaminant loading capacity of a depth granular media filter is increased by imposing a so-called “back-slip” on the filter bed. At intervals between normal backwashing, a volume of liquid is passed upwardly through the media bed 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 filter vessel. Preferably, the filter bed comprises at least two media types of different but uniform particle sizes and densities, the media types each forming a distinct layer in which the effective particle size of the media in each successive layer decreases in a downward direction in the vessel.
1. 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.
2. A method as claimed in claim 1 wherein the volume of liquid passed upward through the media bed in step (c) is less than one filter void volume.
3. A method as claimed in claim 1 , wherein the volume of liquid passed upward through the media bed in step (c) is greater than the void volume of the finest media layer.
4. A method as claimed in claim 3 wherein the volume of liquid is passed upwardly through the media bed in step (c) under flow conditions selected so that contaminants loaded onto the filter media do not pass through the top surface of the uppermost media layer to a significant extent during the period in which liquid is passed upwardly.
5. A method as claimed in claim 1 wherein the duration of the period in which liquid is passed upward through the media bed in step (c) is chosen so that the uppermost media layer does not expand appreciably, but one or more of the lower media layers expands by an amount greater than 25% of its unexpanded height.
6. A method as claimed in claim 5 , wherein the duration of the period in which liquid is passed upward through the media bed in step (c) is less than 60 seconds.