A cartridge group of a filter assembly of a fuel, preferably diesel, filtration system of solid particles and water, wherein the filtration system comprises a head member to which the filter assembly is engageable and comprises a rod-shaped device, such as a purge rod and/or support rod for the water level sensor, wherein the cartridge group is housed in a filter body of the filter assembly and wherein the cartridge group extends along an axis (X-X) and comprises: - a filter septum crossable in an axial direction by the fuel and extending in height parallel to the axis (X-X) between an upper end and a lower end, wherein the filter septum has an annular shape and defines at the axis (X-X) a central cavity; - a water separation and support device supporting the filter septum to the filter body and engageable by the head member comprising: i) a base element engaging the lower end of the filter septum, wherein the base element has a main extension substantially radial to the axis (X-X) and wherein the base element has channels and/or holes through which the filtered fuel flows in output from the lower end; ii) a tubular element extending into the central cavity along the axis (X-X) from the base element, characterized in that the tubular element has an outlet duct with a substantially annular cross-section defined by an outer circumferential wall which engages the inner wall of the filter septum and by an inner circumferential wall which defines along the axis (X-X) a through central passage, wherein in said through central passage said rod-shaped device is suitable to extend at the axis (X-X), wherein the outer circumferential wall and the inner circumferential wall respectively comprise at the top an outer sealing edge and an inner sealing edge sealingly engaging the head member; and iii) a separator element crossable by the filtered fuel to be separated from the water, wherein said separator element is housed on the base element and/or on the inner circumferential wall of the tubular element so that in the outlet duct flows the fuel filtered by the filter septum and separated from the water by the separator element. - a filter septum crossable in an axial direction by the fuel and extending in height parallel to the axis (X-X) between an upper end and a lower end, wherein the filter septum has an annular shape and defines at the axis (X-X) a central cavity; - a water separation and support device supporting the filter septum to the filter body and engageable by the head member comprising: i) a base element engaging the lower end of the filter septum, wherein the base element has a main extension substantially radial to the axis (X-X) and wherein the base element has channels and/or holes through which the filtered fuel flows in output from the lower end; ii) a tubular element extending into the central cavity along the axis (X-X) from the base element, characterized in that the tubular element has an outlet duct with a substantially annular cross-section defined by an outer circumferential wall which engages the inner wall of the filter septum and by an inner circumferential wall which defines along the axis (X-X) a through central passage, wherein in said through central passage said rod-shaped device is suitable to extend at the axis (X-X), wherein the outer circumferential wall and the inner circumferential wall respectively comprise at the top an outer sealing edge and an inner sealing edge sealingly engaging the head member; and iii) a separator element crossable by the filtered fuel to be separated from the water, wherein said separator element is housed on the base element and/or on the inner circumferential wall of the tubular element so that in the outlet duct flows the fuel filtered by the filter septum and separated from the water by the separator element. i) a base element engaging the lower end of the filter septum, wherein the base element has a main extension substantially radial to the axis (X-X) and wherein the base element has channels and/or holes through which the filtered fuel flows in output from the lower end; ii) a tubular element extending into the central cavity along the axis (X-X) from the base element, characterized in that the tubular element has an outlet duct with a substantially annular cross-section defined by an outer circumferential wall which engages the inner wall of the filter septum and by an inner circumferential wall which defines along the axis (X-X) a through central passage, wherein in said through central passage said rod-shaped device is suitable to extend at the axis (X-X), wherein the outer circumferential wall and the inner circumferential wall respectively comprise at the top an outer sealing edge and an inner sealing edge sealingly engaging the head member; and iii) a separator element crossable by the filtered fuel to be separated from the water, wherein said separator element is housed on the base element and/or on the inner circumferential wall of the tubular element so that in the outlet duct flows the fuel filtered by the filter septum and separated from the water by the separator element.