There is disclosed a method and UV reactor for improving the efficiency of treating fluids applied to a UV reactor (2) comprising a longitudinal flow chamber (4) having a longitudinal center axis (22), an input (6) for entry of fluid in the flow chamber (4), and an output (8) for fluid to exit the flow chamber (4), where at least the input (6) of the flow chamber (4) comprises an inlet pipe (10) followed by aninlet cone (12) which as a part of the flow chamber (4) increases the cross section of the channel from the inlet pipe (10) to a cross section of the longitudinal flow chamber (4) of UV reactor (2), said UV reactor (2) having at least one longitudinal UV-lamp (20) parallel to but not coinciding with the longitudinal center axis (22), and where the UV-lamp (20) is arranged such that fluid can flow along a flow path from the input (6) to the output (8) via the flow chamber (4), and so that fluid flowingalong the flow path can be exposed to UV radiation as it flows from the input (6) to the output (8) to receive a UV dose, which is characterized in, that the fluid applied to the UV reactor (2) via the input (6) of the flow chamber (4), when passing the inlet cone (12), is applied a uniform helical flow path in an extent that all particles in the fluid applied to the UV reactor (2), within the operation range of thecurrent UV reactor (2), at least passes at least one UV lamp (20) at a distance to receive at least a prescribed UV dose related to the current UV reactor (2), during passing of the fluid inside the UV reactor.
