The material is fed into a prewash tank at the bottom of the prewash screw and pressed under water by paddle rollers. The material is then conveyed to the top by 1-2 screw conveyors. During transport, the material is in heavy motion, so contaminants such as stones, sand, glass and metals can easily sink. In addition, the adhesions on the plastic soaked so that they can be removed more easily. Depending on the type of contamination, different discharge systems can be used. Simple, small contaminants such as stones or glass are discharged via a time-controlled slide system. In the case of larger or long contamination such as wire, or else larger quantities of contamination scraper chain conveyors or screws are used for contamination discharge.