Abstract: An underdrain assembly in a wastewater filtration unit for providing even distribution of liquid and gas during filter backwashing operations and the uniform collection of filtered water during the normal operation of the filtration unit is provided. The underdrain assembly is provided with a flat, concrete plate with a perimeter matching the shape of the filtration unit. The concrete plate has a plurality of holes with individual PEX tubes in each hole extending from the concrete plate top surface through and protruding below the plate.