A purification device for water is provided, which operates according to the principle of reverse osmosis and has a plurality of pipes essentially traveling in parallel, wherein first self-supporting common parts are provided, which have a plurality of recesses for force-fitting accommodation of the pipes and the common parts can be stacked one on top of the other. Through the formation of common parts, it is possible to erect large water treatment plants made from modules in a simple, cost-advantageous manner.