A filtration system having an underdrain block and an underdrain wall adaptor. The underdrain wall adaptor includes a vertical end and horizontal base. The vertical end extends from a top wall to a bottom wall of an underdrain block and mates with an end portion of an underdrain block to secure the underdrain wall adaptor to the underdrain block. The horizontal base extends from the vertical end. The invention also includes a method of installing an underdrain wall adaptor. The underdrain wall adaptor controls air and water passage between the flume and the underdrain.