TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred System: In the system, the enclosure comprises a porous bag that contains the fiber substrate and the photocatalyst and allows the fluid and solar radiation to pass into the porous bag, where the enclosure is not attached to the fluid vessel and can move within the fluid relative to the fluid vessel. The enclosure has an overall density of less than or equal to a density of the fluid, such that the enclosure floats or rises toward the top of the fluid in the fluid vessel. The enclosure comprises a buoyant material such that the enclosure floats or rises toward the top of the fluid in the fluid vessel. The enclosure is foldable or rollable such that is insertable through the opening in the fluid vessel and unfurlable within the fluid vessel. The enclosure comprises a polymeric mesh, where the mesh is at least partially sunlight transmissive. The fluid vessel comprises a flexible polymeric material. The system further comprises at least one filter positionable in the opening for filtering the fluid to prevent at least some particulate or other contaminants from entering or exiting the fluid vessel. The fluid in the fluid vessel comprises a dye that is an indicator of the purification of the fluid, where the fluid vessel comprises at least one handle or strap for supporting the vessel during operation or transportation. The combined volume of the photocatalyst and the fiber substrate is less than 5% of the volume of the fluid within the fluid vessel. The specific surface area of the photocatalyst within at least one portion of the fluid is greater than 100 square meters per liter of the fluid. The system as well comprises a solar radiation absorber positioned external to the enclosure, the solar radiation absorber being operable to convert absorbed solar radiation into heat and to thus heat a portion of the fluid adjacent to the solar radiation absorber and thus drive convective circulation of the fluid within the fluid vessel. The fluid vessel has first and second major surfaces of a material that is substantially transmissive to sunlight. Alternatively, the solar-activated photochemical fluid treatment system comprises: a fluid vessel; an at least partially sunlight-transmissive fiber substrate contained within the fluid vessel; a semiconductor photocatalyst coupled to the fiber substrate; and means for driving convective circulation of the fluid within the fluid vessel; where the fluid vessel is configured to contain a fluid in contact with the semiconductor photocatalyst such that the fluid treatment system, responsive to solar radiation applied to the semiconductor photocatalyst, is configured to induce photochemical modification of contaminants in the fluid. The means for driving convective circulation of the fluid within the fluid vessel comprises a solar radiation absorber that is configured to absorb incident solar radiation that has first passed through the photocatalyst. The means for driving convective circulation of the fluid within the fluid vessel comprises optically absorbent material attached to the exterior of the fluid vessel. Preferred Method: Purifying the fluid by introducing the fluid into the photochemical treatment system further comprises modifying the photocatalyst after it is coupled to the fiber substrate. A metal is deposited onto the photocatalyst by an electroless process after it is coupled to the fiber substrate.