basin floor the illustrated underdrain unit 10 is a folded sheet metal unit typically formed of stainless steel and having a multiplicity of perforations indicated as slits as an example at least in its two sides 12 and they can also be included in the top surface 14 what is illustrated here is a trapezoidal folded underdrain unit which is preferred although the unit could be rectangular if desired these folded metal underdrain units are shaped essentially as shown in prior u s pat nos 6 090 284 6 569 328 6 691 413 and 6 797 166 the perforations for passing filtrate into the interior of the unit and for holding back filter medium can be formed in accordance with u s pat nos 6 569 328 and 6 691 413 both owned by the applicant corporation as described above the underdrain filtration unit 10 is formed with side ducts 16 inside the interior of the trapezoidal filtration unit these are air and water distribution ducts for cleaning the filter medium the ducts 16 are secured to the walls of the underdrain plenum by welding rivets or other suitable efficient securing means with a top horizontal part 18 of the duct structure preferably having a lip 20 that extends down at a slightly oblique angle to match the angle of the side walls 12 of the structure a vertical wall 21 extends down from the horizontal plate 18 as shown with the parts 18 20 and 21 all being an integral folded metal component each unit 10 preferably has a floor.