An apparatus for filtering water and/or wastewater including at least one underdrain lateral. The underdrain lateral is configured to receive a liquid during a filtration cycle and directs a washing fluid through a filter bed of one or more layers of filter media during a washing cycle. The filter can take any form including but not limited to an upflow clarifier, an upflow filter, a downflow clarifier, a downflow filter and a bi-flow filter. The filter media may be supported by one or more layers of gravel. Alternatively, a porous plate can be attached to the underdrain lateral in place of the gravel layers. The washing cycle can be a liquid only wash, a gas only wash, a combined liquid and gas wash or a combination of any of the aforementioned wash cycles. Preferably, the underdrain lateral includes 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 during a washing cycle 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. Preferably, the height of evacuation openings formed in at least one wall common to a gas wash chamber and at least one adjacent chamber are varied to compensate for velocity head or friction loss.