The invention provides an injector device for a water treatment apparatus, and a method of use. The injector device comprises a first coupling for fluid connection to a source of liquid to be treated; and a second coupling for fluid connection to at least one liquid treatment vessel arranged to expose liquid in the vessel to ultraviolet radiation in an advanced oxidation process reaction. The device comprises at least one injection port for injecting at least one gas into a liquid flowing through the injector device. The injector device is at least partially formed from a material that is transmissive to ultraviolet radiation. In another aspect, a water treatment apparatus defines a plurality of parallel flow streams between the inlet of the apparatus and the at least one liquid treatment vessel. The injector device comprises an injection port for each of the plurality of parallel flow streams.