A process and device are disclosed for purifying liquids, in particular water, contaminated with inorganic and/or organic impurities. The impurities are transformed and/or precipitated through the use of the catalytic and photocatalytic properties of semiconductors, if necessary with the addition of an oxidizer. For that purpose, a semiconductor in the form of at least one semiconductor layer, for example a porous layer, is applied on an equally porous substrate (17), in order to form a filter capable of being activated by the light of at least one lamp (22); the liquid flows therethrough and is thus purified. The light stimulus starts redox reactions at the surface of the semiconductor layer that lead to the transformation of harmful chemical substances, or to the precipitation of dissolved noble metals.