Method of washing an upflow filter and filter bed employed in said filter

A method of washing an upflow filter between service runs, said upflow filter including a filter layer through which influent to be filtered is directed in an upward direction during each service run for causing floc in said influent to be retained in said layer, said method of washing including the steps of:

• (a) directing a combination of air and liquid in an upflow direction through the filter layer with the velocity of the liquid being less than the minimum fluidization velocity of the filter layer and being in the range of approximately 5-20 gallons per minute/sq.ft. and with the rate of air flow being in the range of approximately 1-9 standard cubic feet per minute/sq.ft.;
• (b) discontinuing directing said combination of air and liquid in an upflow direction after a period of time for disrupting only some floc retained in said layer during a previous service run, while leaving some floc attached to said particulate media in said filter layer and with the headloss through the filter layer being at least fifteen percent greater than the headloss through said layer when said layer is free of flock;
• and thereafter
• (c) directing only liquid in an upflow direction through the filter layer with the velocity of the liquid being less than the minimum fluidization velocity of the filter layer and being in the range of approximately 5-20 gallons per minute/sq.ft. for removing disrupted floc from the filter while leaving in said layer floc attached to said particulate media of said filter layer.