NOVELTY - A solar-activated photochemical fluid treatment system comprises: a fluid vessel having at least one opening and comprising an at least partially sunlight transmissive portion; at least one enclosure positioned inside of the fluid vessel, the enclosure comprising material that allows fluid and sunlight to pass into the enclosure; an at least partially sunlight-transmissive fiber substrate contained within the enclosure; and a semiconductor photocatalyst coupled to the fiber substrate.    USE - The solar-activated photochemical fluid treatment system is useful for purifying a fluid (claimed).    ADVANTAGE - The solar-activated photochemical fluid treatment system can be efficiently and effectively used in the purification of a fluid, such as water, and more particularly to the removal, reduction and/or detoxification of contaminants in the fluid, such as organic chemicals, inorganic chemicals, heavy metals, microorganisms and others through sunlight-activated photochemical means.    DETAILED DESCRIPTION - A solar-activated photochemical fluid treatment system comprises: a fluid vessel having at least one opening and comprising an at least partially sunlight transmissive portion; at least one enclosure positioned inside of the fluid vessel, the enclosure comprising material that allows fluid and sunlight to pass into the enclosure; an at least partially sunlight-transmissive fiber substrate contained within the enclosure; and a semiconductor photocatalyst coupled to the fiber substrate; where the fluid vessel is configured to contain a fluid such that, responsive to solar radiation passing through the at least partially sunlight transmissive portion of the fluid vessel and into the enclosure and to the semiconductor photocatalyst, photochemical modification of contaminants and living organisms in the fluid occurs. An INDEPENDENT CLAIM is a method for purifying a fluid by introducing the fluid into a photochemical treatment system, comprising: providing at least one non-porous fluid vessel with at least one porous enclosure position within the vessel; providing a semiconductor photocatalyst coupled to an at least partially sunlight-transmissive fiber substrate that is confined within the enclosure; introducing a fluid into the fluid vessel such that the introduced fluid is in contact with at least a portion of the semiconductor photocatalyst; and admitting solar radiation into the fluid vessel to illuminate at least a portion of the fluid and at least a portion of the photocatalyst within the enclosure to induce photochemical modification of contaminants in the fluid.