An apparatus for treating fluid comprises a fluid chamber 10 with a central axis 19, an ultraviolet (UV) light emitting diode (LED) 13 and a UV light directing element 12. The directing element is configured to collimate at least a portion of light rays emitted from the UV LED such that the collimated light rays are parallel in a first plane coincident with or parallel to the central axis. In another aspect, the UV light rays are parallel only in a first direction. In a further aspect, the apparatus comprises an array of UV LEDs provided radially or peripherally around the outside surface of the chamber, wherein the chamber is a transparent pipe 16. Each UV LED is mounted at a focal point of a reflector (26, Fig. 4). The reflector collimates light in the direction of a longitudinal axis of the pipe and concentrates or scatters reflected rays through a cross section of the pipe. In a further aspect, the UV light from each UV LED is passed through a lens which collimates light. The cooling of a light emitting d
