a filling for a percolating filter for biological purification of waste water includes a plurality of flexible , irregularly touching , randomly arranged plastic strips up to 8 cm in width and several meters in length twisted together and suspended from a support over the height of the filter tank . the strips may have a maximum thickness of 30 - 100 μm and a minimum thickness of 10 - 60 μm , varying in such thicknesses over their width . the strips may be residue edge portion cuttings severed from the production of continuously produced sheet materials .