Abstract: An underdrain block for a filter media system for supporting the filter media and distributing air and water backwash. The underdrain block comprising a top wall, a bottom wall and two side walls forming an open ended interior. Interior walls, known as lateral separation walls, divide the interior of the block into primary and secondary laterals. When the blocks are laid end to end, the laterals form a common distribution conduit for the transport of air and water backwash. The lateral separation walls can be angular or curved. Curved walls and specifically angled walls increase the area of the laterals. The spacing of the blocks can be approximately 18 inches and the height approximately 8 inches. The underdrain blocks are interlocking to form conduits of up to 40 feet in length.