Air/water distributor underdrain

In an apparatus including a filter bed supported above a floor defining an inlet chamber thereabove and an outlet chamber therebelow;

• an improved air/water underdrain distributor assembly comprising:
• a distribution member attached to the upper surface of said floor, said distributor member having a substantially horizontal surface spaced above said floor and a wall extending downwardly therefrom towards said floor so as to define a distributor chamber in communication with an opening extending through said floor;
• first and second spaced apart openings extending through said horizontal surface;
• a shroud extending downwardly from said first openings having a first end portion in fluid communication with said first openings and a second end portion in communication with an intermediate portion of said distributor chamber;
• and means for directing backwash water and gas into said outlet chamber upwardly through said opening in said floor into said distributor chamber such that gas collects in a gas compartment formed in an upper portion of said distributor chamber and is directed through said second openings upwardly through said filter bed and backwash water collects in a backwash water compartment formed in a lower portion of said distributor chamber and is directed upwardly through said shrouds and said first openings through said filter bed.