A device (1) comprises a source (20) for emitting ultraviolet light, an inlet (30) for letting in fluid to the device (1), an outlet (40) for letting out fluid from the device (1), and means (51, 52) for performing a straightening action of a flow of fluid through the device (1). The flow straightening means comprise at least one flow straightening element (51, 52) having inlet openings for letting in fluid at one side and outlet openings for letting out fluid at another side, wherein each inlet opening is in communication with a plurality of outlet openings, and wherein the element (51, 52) comprises a maze of randomly arranged, interconnected holes. In such a structure, a water element that is moving from one side of the element (51, 52) to another side may take one of various paths, as a result of which variations in inlet conditions can be dampened.