A system and method for photochemical treatment of liquid substances with ultraviolet light inside liquid conveying tubings. The system includes an elongated polymeric light guiding liquid conveyance tube having first and second open ends and an internal surface defining the interior and liquid conveying conduit of the tube, and a UV light source, which is optically connected to the interior and liquid conveying conduit of the tube. The light guiding tube includes one or multiple concentric layers of polymeric materials selected from a group that includes thermoplastic and thermosetting polymers, elastomers and composites. The tube has at least one UV light transmitting polymeric material layer having a lower refractive index value than the refractive index value of the liquid substance conveyed within the tube.