Each cap of a pair of end caps mounted on opposed ends of a conventional radial flow cylindrical filter element of a filter cartridge includes a circular sleeve extending axially from the filter element for penetrably receiving a supporting stud and for maintaining an O-ring seal therebetween. The area of one of the end caps radially outwardly of its circular sleeve is greater than the corresponding area of the other end cap to cause fluid to be filtered to pressure bias the filter cartridge toward the other end cap and to positionally retain the filter cartridge within its housing. A positioning member extending into the filter element from one of the end caps receives and stabilizes sleeve for a u/v lamp disposed within the filter element to kill any microorganisms present and to oxidize any organic matter subjected to the emitted ultra-violent radiation.