An underdrain assembly for filtering particulates from a fluid having an upper structure connected to a bottom plate. At least one flow control vane is positioned between the upper structure and the bottom plate for directing and managing fluid flow through the assembly. The upper structure can have first and second filtration members, the second filtration member positioned between the first filtration member and the bottom plate. The second filtration member can have a shape that restricts fluid flow within the underdrain. The underdrain can have air and water inlets. The water inlet can have an end with a plate covering an upper portion thereof. The end can also include an opening having an angular cut forming an angle with respect to a longitudinal axis of the water inlet. Thus arranged, air is prevented from migrating out the water line. A resilient mounting arrangement for the underdrain is also disclosed.