filling material for percolating filters for the treatment of liquid and process for producing and assembling the material . especially in biological sewage treatment , filling material must provide the best possible hold for the biological filter film necessary for the purpose and must have uniform wettability . the disadvantage of known apparatuses meeting these requirements is that they either tend to clog or are difficult to produce . the new filling material will work effectively and reliably , while being simple to produce and assemble . to provide ideal growth surfaces for the biological filter film , the filling material has a plurality of plastic strips which are connected to one another by means of a single holding sheet . furthermore , the strips have a special shape , in particular corrugated edges and / or a structured neutral region . the filling material is especially suitable for biological treatment , particularly the denitrification of sewage or drinking water .