A fluid steriliser comprising a fluid duct 1 having a UV transmissible wall 11 providing a surface area for irradiation wherein the cross section of the duct is between 1x10-4 m2 and 5x10-2 m2 and the thickness of the duct defines a gap 12 wherein the depth of the gap adjacent the UV transmissible wall is no more than 50mm, a UV radiation source 17 to irradiate fluid flowing in the duct through the UV transmissible wall such that the radiation incident on fluid has a UV power density, a plurality of mixing stages 18 spaced apart inside the duct and configured to provide turbulent flow wherein the segments of the duct between the mixing stages are arranged to provide flow adjacent the UV transmissible wall, and a flow control means arranged to control the linear speed of fluid flow along the duct based on the length of the duct and the UV power density so that at least 300 Joules of UV energy per square meter of the surface area for irradiation is provided to the fluid during the dwell time of the fluid in the duct. Ideally, the UV source is inside the duct. Further, an apparatus comprising a plurality of unit wherein each unit comprises a duct with a UV transmissible wall, is also disclosed.
