Method of biologically processing pre-treated bathing water, and a swimming pond or biologically treated pool

Filter substrate (4) for biological treatment of pure water, comprises artificially produced substrate particles (7), on whose surface specific, predefined elements that are present in lacking amount in the pure water, are provided for biofilm structure in biologically available form. The elements which are application specific or the elements to be limited in the treated pure water, are not present in the substrate particles in biologically available form. An independent claim is also included for producing the filter substrate consisting of the substrate particles, for biological treatment of pure water using biofilm structure, comprising producing the substrate particles by spinning natural or synthetic aluminum silicates (rockwool), and subsequently activating by corroding with alkaline solutions including ammonia vapor, ammonia solution or solutions of alkali or alkaline earth metal hydroxides, or with acids including hydrofluoric acid, where loading of the substrate particles with the required lacking nutrients is carried out either directly during the corrosion process by incorporating cations of the etching agent, or subsequently by ion exchange process in a solution containing the required nutrient ions, the substrate particles are prepared in such a way that on its surface excess negative and/or positive charges are present, on whose positions predefined cations and/or anions are stored for the biofilm structure, and the substrate particles are produced using artificially produced zeolite powder.