The quality of the high purity water used in the production facilities of a research laboratory where special wafers and test wafers are produced is of paramount importance. The water required is used in the most varied steps of wafer production. The quality of usual high purity water is not sufficient for these special application criteria with regard to particles and harmful ions. As the number of the residual molecules contained in the medium can be too high, the purified water has to be further purified.