What structure supports the filter media and collects filtered water while distributing backwash water?

The underdrain is the correct answer because it plays a crucial role in both supporting the filter media and facilitating the collection of filtered water. This structural component is typically installed at the bottom of a filtration system, where it collects the water that has passed through the filter media and ensures efficient drainage. Additionally, during the backwashing process, the underdrain allows for the distribution of backwash water, which is essential for cleaning the filter media and maintaining the system's effectiveness.

By having an underdrain in place, operators can ensure that the filtered water is properly channeled away for further treatment or discharge and that the backwash can be effectively applied without disrupting the integrity of the filter bed. This dual function is essential for maintaining both the performance and longevity of filtration systems in water treatment facilities.