Abstract: An advanced oxidation process system and associated method includes a device with a housing having a wall with a first side exposed to water of the swimming pool or spa and a second side opposite from the first side. The device additionally includes an ultraviolet lamp within the housing. In some embodiments, an electrical ballast for an ultraviolet generator may be attached to the second side of the wall and in thermal contact with the wall. In certain embodiments, an end cap may be attachable to and removable from the housing, and the ultraviolet lamp may be supported by the end cap and positionable within the water chamber.