OPERATION OF AN UPFLOW FILTER AND NON-BUOYANT FILTER BED IN SERLES WITH A DOWNFLOW FILTER AND FILTER BED

摘要

A method of washing an upflow filter (12) between service runs employs a filter bed (26) having a non-buoyant particulate filter layer (56) through which influent to be filtered is directed in an upward direction, and through which liquid employed in the washing operation is directed in an upflow direction. A two-stage washing operation includes the steps of first directing a combination of air and liquid in an upflow direction through the filter layer (56) with the velocity of the liquid being less than the minimum fluidization velocity of the filter layer (56), for disrupting only some floc retained in the filter layer during a previous service run, and thereafter directing only liquid in an upflow direction through the filter layer at a velocity less than the minimum fluidization velocity of the filter layer for removing disrupted floc from the filter layer while leaving some floc attached to said particulate media of the filter layer. The process most preferably is carried out with a filter bed (26) including a coarse, particulate media flocculation layer (52) for distributing the flow of influent and promoting flocculation.