Abstract: A filter underdrain apparatus comprises plate means forming a plurality of horizontal distributor conduits, the conduits being in juxtaposed, laterally-spaced relationship and constructed and arranged to defined alternating conduits and troughs of a filter underdrain. At least some of the horizontal distributor conduits include interior partitions extending along such conduits and dividing the interior of each such conduit into a liquid passage and a gas passage. Liquid metering orifices communicate the interior of the liquid passage with the exterior of the distributor conduit for substantially evenly distributing backwash liquid flows from the liquid passage and for passing filtered liquid flows into the liquid passage. Gas metering orifices communicate the interior of the gas passage with the exterior of the distributor conduit for substantially evenly distributing gas from the gas passage when connected with a pressurized gas source.