An apparatus 10 for the treatment of a fluid comprises an array of ultraviolet light emitting diodes (UV-LED) 13 provided about a transparent pipe 16 wherein UV light rays from the LED are passed through a lens such that the lens collimates light in the direction of a longitudinal axis of the pipe and concentrates light in a cross section of the pipe. An apparatus for cooling LED(s) in a system wherein the apparatus is configured to controllably transfer heat from the LED to the fluid being treated. An apparatus comprises an array of UV-LED’s about a transparent pipe, one or more UV transmittance sensors, and a treatment monitoring device and/or software configured to turn the UV-LED on and off in response to the transmissivity of a fluid. A method comprising introducing a fluid into a fluid chamber comprising a central axis, directing light emitted from a UV-LED(s) into the fluid wherein at least a portion of the UV light emitted from the UV-LED(s) is collimated such that the UV light rays are parallel in a first plane parallel to the central axis.
