Devices and methods associated with these devices for significantly reducing or eliminating liquid (e.g., water) in a water or wastewater treatment unit above the underdrain from moving downwardly into the underdrain during an air only wash cycle of the treatment unit. The treatment unit can take many forms including but not limited to an upflow filter and a downflow filter. The treatment unit may include a filter bed having one or more layers of filter media supported by gravel above the underdrain. The treatment unit may also include an underdrain having a porous plate, slotted plate or slotted upper structure that obviates the need for gravel. Positive pressure is provided to the underdrain to overcome the downward force of the driving head in the treatment unit to significantly reduce or prevent liquid present in the treatment unit above the underdrain from moving downwardly into the underdrain.