Abstract: An apparatus for the treatment of potable water having a substantially cylindrical pressure vessel which houses an ultraviolet light. The pressure vessel is in fluid communication with a filter stage which filters the water before treatment by the ultraviolet light. An ultraviolet lamp assembly sealed within a light permeable sleeve and having a single end cap provided with electrical terminals at one end only is disposed in watertight fashion within the pressure vessel. Downstream from an inlet to the pressure vessel is a deflector/baffle assembly provided to regulate the flow pattern and flow rate of the water within the pressure vessel thereby providing a uniform flow rate to achieve optimum kill rates of bacteria and other living organisms by exposure to the ultraviolet light. The ultraviolet lamp assembly is secured within the pressure vessel by a threaded closure nut which is easily secured or removed by hand.