A flow-through fluid purification device comprising: a container arranged such that fluid to be purified can flow-through a volume of the container from an inlet to an outlet, wherein the container has an outer cylinder and an inner cylinder inserted into the volume of the outer cylinder, wherein the device has at least one end cap (20,21) engaged with an axial end of the outer cylinder, arranged to hold the inner cylinder in position in the outer cylinder and to fluid-tightly close the outer cylinder, and wherein the inner cylinder forms a receptacle for accommodating a radiation source in the form of a lamp, wherein an outer peripheral wall of the inner cylinder has an interface wall permeable for radiation with a wavelength in the UV-range, preferably between 150nm and 200nm, more preferably of 172 ± 8nm, and arranged to let radiation pass into the volume of the container where the fluid to be purified flows in operation; a plurality of baffle plates located in the volume of the container between the outer cylinder and the inner cylinder with an inter-baffle distance in the flow direction from the inlet to the outlet, wherein the number of baffle plates in the flow direction along the interface wall is 8 to 12, wherein the baffle plates are arranged to force the fluid flowing from the inlet to the outlet to flow substantially along the interface wall and through gaps between the interface wall and the baffle plates defining the shortest distance between the interface wall and the baffle plates, wherein the baffle plates each have a surface on the upstream side in the flow direction which is perpendicular to the interface wall, and wherein the inter-baffle distance between the upstream surfaces of two consecutive baffle plates, at least adjacent to the gap, is in the range from 4 to 30mm, preferably in the range from 10 to 20mm, more preferably 10mm. a container arranged such that fluid to be purified can flow-through a volume of the container from an inlet to an outlet, wherein the container has an outer cylinder and an inner cylinder inserted into the volume of the outer cylinder, wherein the device has at least one end cap (20,21) engaged with an axial end of the outer cylinder, arranged to hold the inner cylinder in position in the outer cylinder and to fluid-tightly close the outer cylinder, and wherein the inner cylinder forms a receptacle for accommodating a radiation source in the form of a lamp, wherein an outer peripheral wall of the inner cylinder has an interface wall permeable for radiation with a wavelength in the UV-range, preferably between 150nm and 200nm, more preferably of 172 ± 8nm, and arranged to let radiation pass into the volume of the container where the fluid to be purified flows in operation; wherein the container has an outer cylinder and an inner cylinder inserted into the volume of the outer cylinder, wherein the device has at least one end cap (20,21) engaged with an axial end of the outer cylinder, arranged to hold the inner cylinder in position in the outer cylinder and to fluid-tightly close the outer cylinder, and wherein the inner cylinder forms a receptacle for accommodating a radiation source in the form of a lamp, wherein an outer peripheral wall of the inner cylinder has an interface wall permeable for radiation with a wavelength in the UV-range, preferably between 150nm and 200nm, more preferably of 172 ± 8nm, and arranged to let radiation pass into the volume of the container where the fluid to be purified flows in operation; a plurality of baffle plates located in the volume of the container between the outer cylinder and the inner cylinder with an inter-baffle distance in the flow direction from the inlet to the outlet, wherein the number of baffle plates in the flow direction along the interface wall is 8 to 12, wherein the baffle plates are arranged to force the fluid flowing from the inlet to the outlet to flow substantially along the interface wall and through gaps between the interface wall and the baffle plates defining the shortest distance between the interface wall and the baffle plates, wherein the baffle plates each have a surface on the upstream side in the flow direction which is perpendicular to the interface wall, and wherein the inter-baffle distance between the upstream surfaces of two consecutive baffle plates, at least adjacent to the gap, is in the range from 4 to 30mm, preferably in the range from 10 to 20mm, more preferably 10mm.