A method for cooling at least one light emitting diode (LED) in a fluid treatment system by indirectly transferring heat from the at least one LED to the fluid being treated in the system. Preferably the indirect heat transfer is carried out by means of a coolant circuit. Ideally the method further comprises a step of controlling a light output of the at least one LED in response to a change in the temperature of the coolant or the transmissivity of the fluid. An apparatus for the treatment of a fluid comprising an array of ultraviolet light LEDs (UV-LED) provided about a transparent pipe, one or more transmittance sensors and a treatment monitoring device and/or software configured to turn on and off the UV-LEDs in response to changes in the transmissivity of the fluid. A method for the treatment of a fluid comprising the steps of introducing the fluid into a fluid chamber, directing light emitted from at least one UV-LED wherein at least a portion of the UV light emitted from the at least one UV-LED is collimated such that the UV light rays emitted from each ultraviolet light unit are parallel in a first plane parallel to the central axis.
