Abstract: A method and apparatus for treating a liquid using ultraviolet radiation. A thin flow of liquid in a supercritical flow is irradiated by at least one radiation source external to the thin flow. The flow may desirably have a Froude number greater than 3 and a velocity greater than 0.8 m/s. The apparatus for treating a liquid including may have a reservoir supplying liquid though a slot into an open channel to generate a thin flow of liquid having a supercritical flow along the channel with at least one radiation source external to the thin flow to irradiate the thin flow.