The most energetic part of the solar radiation spectrum can be used in the presence of a catalyst (Photo Catalysis) or in Photo Fenton techniques to treat chemically or biologically contaminated water This process requires an efficient optics to collect and deliver solar radiation to the liquid to be treated. Non Imaging Optics provides the most efficient way to collect solar radiation and in the present paper we describe one optical solution for absorbing surfaces holding the catalyst and immersed in the water to be treated. The water circulates in UV and blue transparent glass tubes and the absorber is a flat fin parallel to the optical axis of the system. An optical solution is derived, achieving maximum concentration without rays being geometrically rejected. However it is shown that this concentration falls short by a small margin of the maximum concentration predicted by ideality in Non Imaging Optics because of caustic formation inside the dielectric. In the paper we present an example developed for the investigation of photocatalysis in the treatment of biologically contaminated waters.