Apparatus for uniformly distributing gas and/or liquid in an underdrain lateral system

An underdrain distributor apparatus having at least one elongated distributor member which is adapted to be positioned generally horizontally beneath a bed of granular media which must be periodically backwashed with fluids, including a backwash liquid and a backwash gas, said distribution apparatus including means for receiving a backwash liquid and a backwash gas therein, under pressure, which will produce a vertical plenum space of gas above said liquid, and being characterized in that said distributor member includes a non-planar porous, media retaining portion having a total open area per unit of length of said distributor member of a first predetermined size through which said backwash liquid and backwash gas can pass upwardly into said media bed during a backwashing operation;

• said distributor member further including an enclosed flow distributing portion which, when viewed in a cross-section taken normal to the axis of said distributor member, has an outer surface which contains a plurality of openings positioned at multiple vertical levels which are adapted both for use in a collection mode for collecting a liquid filtrate which has passed downwardly through said porous, media retaining portion and, in a backwash mode, for ejecting said backwash liquid and backwash gas outwardly therefrom and into contact with said media, said plurality of openings having a total open area per unit of length of said distributor member of a second predetermined size which is considerably less than the said total open area per unit of length of said first predetermined size of said porous, media retaining portion, said plurality of openings which are positioned at multiple vertical levels consisting of a first plurality of upper openings which are sized and spaced along the length of the distributor member for the relatively uniform ejection of the backwash gas and a second plurality of lower openings which are sized and spaced along the length of the distributor member for the relatively uniform ejection of the backwash liquid, said first plurality of upper openings having portions thereof which are open at multiple vertical levels, and at least some of which extend downwardly from an uppermost portion of said outer surface by a distance of at least one inch, said second plurality of lower openings being positioned at at least one vertical level which is located at or above the bottom of said outer surface, said first plurality of upper openings having a total open area per unit of length which is considerably less than the total open area per unit of length of said second plurality of lower openings, said distributor apparatus further comprising an unperforated baseplate.