The disadvantage of chlorination of drinking water is the possible synthesis of toxic chlorinated fragments. In different cases UV can be an alternative to chlorination. The germicidal effectiveness of UV–radiation is in the 180–320 nm region with an optimum at 265 nm. Approximately 95% of the energy radiated by a low–pressure mercury arc is at the 253.7 nm line, so this source is the most effective one for germicidal applications. The germicidal effectiveness of a broadband source can be calculated. UV alone cannot decrease the concentrations of organic contaminants of the treated water. Quite promising are the systems where UV–radiation acts as a catalyst in oxidation reactions in order to decrease the organic contaminants.