NOVELTY - The solar-activated photochemical fluid treatment system comprises a module comprising a partially sunlight-transmissive porous fiber substrate (160) and a semiconductor photocatalyst coupled to the substrate, floats coupled to the module such that the module floats at or near a surface of an open body of liquid such that photochemical modification of contaminants and living organisms in the liquid occurs responsive to solar radiation applied to the semiconductor photocatalyst, and connectors coupling the modules together to form a group of interconnected modules.    USE - The solar-activated photochemical fluid treatment system is useful for remote applications such as passive remediation of surface waters over extended treatment periods, where the fluid includes water from lakes, ponds, rivers and pools.    ADVANTAGE - The efficient solar-activated photochemical fluid treatment system purifies environment bodies such as lakes, ponds, rivers and pools with the removal of contaminants such as organic chemicals, inorganic chemicals, heavy metals and microorganisms, and has improved photocatalytic performance and high mass transfer efficiency.    DETAILED DESCRIPTION - The solar-activated photochemical fluid treatment system comprises a module comprising a partially sunlight-transmissive porous fiber substrate (160) and a semiconductor photocatalyst coupled to the substrate, floats coupled to the module such that the module floats at or near a surface of an open body of liquid such that photochemical modification of contaminants and living organisms in the liquid occurs responsive to solar radiation applied to the semiconductor photocatalyst, and connectors coupling the modules together to form a group of interconnected modules. The module: is configured to allow liquid and sunlight to be in present contact with the semiconductor photocatalyst; further comprises a liquid permeable housing (162), and a metal deposited onto the photocatalyst by an electroless process; and has first and second opposing ends. The fiber substrate is contained in whole or in part within the housing. The housing comprises a polymeric mesh that is partially sunlight transmissive. The floats comprise first and second floats, and are coupled to each module of the group of modules. The first float is coupled to the first end of the module. The second float is coupled to the second end of the module. A specific surface area of the photocatalyst within a portion of fluid (140) is greater than 100 square meters per liter of the fluid. A group of modules has an overall density of less than or equal to a density of the liquid such that the group of modules floats or rises toward an upper surface of the body of liquid. An INDEPENDENT CLAIM is included for a method for purifying a fluid.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic side view of a solar-activated photochemical fluid treatment system.    Sun (100)    Electromagnetic radiation (110)    Fluid (140)    Fiber substrate (160)    Housing. (162)