A filter system for filtering particles from a liquid includes an underdrain assembly having an underdrain defining at least one underdrain chamber and a media retainer assembly. The media retainer assembly includes an upper filter media retainer plate defining a plurality of hopper-shaped cavities and a lower filter media retainer plate defining a plurality of hopper-shaped cavities corresponding to the plurality of hopper-shaped cavities on the upper plate. The plates are in a nested arrangement such that the hopper-shaped cavities on the upper plate are in alignment with the corresponding plurality of hopper-shaped cavities on the lower plate to define a plurality of passages therebetween and allow for the passage of the liquid while preventing the passage of a filter media. During a backwash operation, the upper and lower plates are relatively moveable such that the backwash fluid releases any particles lodged in the passages between the plates.