A horizontal rotary drum in which the materials to be treated are introduced through a feed hopper together with a water flow admitted through treatment nozzles so as to form a suspension of water and materials is described. The horizontal rotary drum has inclined paddles that move forward the heavy materials deposited on the bottom of the drum towards an exit end where an extractor conveys them towards concentric screening means, a stop ring being arranged transversally inside the drum so as to leave only a peripheral annular passage having a height lower than the level of the suspension of water and materials present on the bottom of the drum so as to stop the lighter organic contaminants that are in the upper part of the suspension. Means for slightly inclining the axis of the rotary drum within a small angular interval, as well as with clod-breaking teeth for treating clayey materials are also described.