Devices and methods associated with these devices for significantly improving the efficiency of an existing or new treatment unit having a granular media filter bed supported above an underdrain for removing impurities passing through the filter bed. The devices and methods are specifically designed to prevent clogging of the underdrain to improve the operation and efficiency of the treatment unit. The devices and methods are directed to providing trapping structures and methods that allow a washing fluid to pass through the trapping structures but traps impurities in the washing fluid that have the potential of partially or completely clogging the underdrain. Further, one or more sensors are provided to determine if the trapping structure is performing properly.