Abstract: A process and apparatus for cleaning contaminated raw water, in which raw water and a flocculator are introduced into the reaction space of a pressure vessel, which vessel has such reaction space at the bottom thereof, a filter space above the reaction space and containing a filter made of particles of elastic, granular material, and a clean water space above the filter space so that the raw water passes through the three spaces, whereby the pollution material contained in the raw water becomes enclosed by the cells of the floccules of the flocculator and the floccules become absorbed by the filter particles. Accumulated pollution material is removed from the filter particles by regenerating the filter with a fluid flowing from the clean water space through the filter space into the reaction space. The required extent of purity is achieved by varying the velocity of the raw water introduced into the reaction space, to thereby adjust the percentage of the voids in the filter.