Abstract: In a modular plastic packing for the biological treatment of waste waters by percolation, of the type constituted by an alternate array of undulated sheets united to each other, so as to define a plurality of inclined channels, in such a way as to supply an intersecting flow of fed liquors, and in each one of said sheets the generatrices of the undulations being shaped as flat parallel crests directed in an inclined direction relatively to the plane on which the packing leans, on the surface of each sheet comprised between each pair of adjacent crests a plurality of bossages substantially flat and parallel to said supporting plane is provided.