A fluid steriliser comprising a fluid duct having a UV transmissive wall providing a surface area for irradiation, wherein the cross section of the duct is between 1×10−4 m2 and 5×10−2 m2 and the thickness of the duct defines the depth of fluid flow adjacent the UV transmissive wall of no more than 50mm a source of UV radiation arranged to irradiate fluid flowing in the duct through the UV transmissive wall such that the UV radiation incident on fluid in the duct has a UV power density a plurality of mixing stages configured to provide turbulent flow in the fluid and spaced apart along the length of the duct wherein the segments of the duct between the mixing stages are arranged to provide flow adjacent the UV transmissive wall 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 metre of the surface area for irradiation is provided to the fluid during the dwell time of the fluid in the duct.
