An apparatus for filtering water and/or wastewater including at least one underdrain lateral having a primary gas chamber and a secondary gas chamber where the secondary gas chamber serves to equalize pressure differences that may occur during the initial distribution due to velocity head changes and friction effects. Preferably, the underdrain lateral includes a plurality of chambers directing a washing fluid into the filter bed where at least one of the plurality of chambers directs a liquid and a gas simultaneously into the filter bed and at least one other chamber of the plurality of chamber directs only one of a liquid and a gas into the filter bed at the same time as that the combination of liquid and gas is directed into the filter bed.