An ultraviolet "UV" liquid treatment apparatus comprising: a UV-transparent conduit; a conduit end having a liquid inlet and liquid outlet coupled to one end of the UV-transparent conduit; a UV-transparent internal inlet tube, positioned within the UV-transparent conduit, to direct liquid from the liquid inlet toward a second end of the UV-transparent conduit, wherein a gap between the UV-transparent internal inlet tube and the UV-transparent conduit is to direct the liquid toward the liquid outlet; a UV light emitting diode "LED" array holder external to the UV-transparent conduit positioned next to the second end of the UV-transparent conduit and separated from the liquid by a window, the array holder having a curved surface facing the window; a plurality of UV LED modules, including a central LED module and peripheral LED modules, all arranged on the curved surface of the array holder, wherein the central UV LED module is to illuminate incoming liquid flow within the internal inlet tube and the peripheral LED modules are located at the peripheral area of the surface facing the gap between conduit and internal tube and are tilted relative to the X-Y plane due to the concave structure of the surface to ensure that a majority of light emitted from the UV LED modules would enter the gap between the UV-transparent conduit and the internal inlet tube within an angle range that would enable the light to strike internal walls of the UV-transparent conduit at an angle above the critical angle for total internal reflection "TIR", wherein the UV LED modules are configured to generate a customized spatial light flux distribution within the conduit that matches liquid flow paths so as to obtain a desired UV dose distribution. a UV-transparent conduit; a conduit end having a liquid inlet and liquid outlet coupled to one end of the UV-transparent conduit; a UV-transparent internal inlet tube, positioned within the UV-transparent conduit, to direct liquid from the liquid inlet toward a second end of the UV-transparent conduit, wherein a gap between the UV-transparent internal inlet tube and the UV-transparent conduit is to direct the liquid toward the liquid outlet; a UV light emitting diode "LED" array holder external to the UV-transparent conduit positioned next to the second end of the UV-transparent conduit and separated from the liquid by a window, the array holder having a curved surface facing the window; a plurality of UV LED modules, including a central LED module and peripheral LED modules, all arranged on the curved surface of the array holder, wherein the central UV LED module is to illuminate incoming liquid flow within the internal inlet tube and the peripheral LED modules are located at the peripheral area of the surface facing the gap between conduit and internal tube and are tilted relative to the X-Y plane due to the concave structure of the surface to ensure that a majority of light emitted from the UV LED modules would enter the gap between the UV-transparent conduit and the internal inlet tube within an angle range that would enable the light to strike internal walls of the UV-transparent conduit at an angle above the critical angle for total internal reflection "TIR", wherein the UV LED modules are configured to generate a customized spatial light flux distribution within the conduit that matches liquid flow paths so as to obtain a desired UV dose distribution.