Titanium dioxide TiO2-P25 is the most commonly used semiconductor as photocatalyst for photooxidation, in the heterogeneous photocatalytic system for the degradation of pollutants in water. The reason for its use is reflected in the great power of photogeneration of pairs of electrons and cavities, low cost, high activity and similar. However, the problem limiting the application of TiO2 photocatalyst is the absorption of radiation of wavelengths exceeding 400 nm, as well as the problem of removing the photocatalyst suspension after the photocatalytic oxidation process has been completed and its re-use. Apart from the above, there is no simple mathematical model describing a heterogeneous photocatalytic process which also possesses a rate constant of degradation of organic matter independent of the geometry of the photoreactor. The mentioned model could be used for scaling up process. Accordingly, in this paper, a heterogeneous photocatalytic process was investigated in order to find a solution to the stated problems which limit the application of TiO2 photocatalyst. Salicylic acid was used as a model pollutant. The research showed that with the method of reduction of silver with chitosan the silver-doped photocatalyst can be synthesized, which exhibits photocatalytic activity (kvis = 0.0037 min^-1) under the radiation of wavelengths > 400 nm, while it exhibits 1.4 times higher activity under artificial solar radiation than undoped suspended TiO2-P25 under the same conditions. By using iron salt it is possible to improve the photocatalysis process and to carry out the separation of the photocatalyst suspension by coagulation (after 60 min 89.55% TiO2-P25 is removed), and if the iron salt is added for coagulation purposes only then the separation is more pronounced (after 95 min 95.05% TiO2-P25 is removed). Chitosan as a coagulant also showed satisfactory results of the TiO2-P25 suspension removal (after 60 min is removed> 95% TiO2-P25). The research showed that when the separation of TiO2-P25 photocatalyst is based on coagulation, whether iron salt or chitosan, it is not possible to use a precipitated photocatalyst in a new photocatalytic cycle. However, the research has shown that by photocatalyst immobilization can be solved the problem of reusing photocatalysts in a new cycle. The results showed that no decrease in activity occurs when TiO2 is immobilized on the reactor wall and applied to multiple salicylic acid degradation cycles. The research was conducted on 6 cycles of photocatalysis (1 cycle lasted 60 min). The rate constant of the immobilized photocatalyst on the reactor wall considering the mass of the photocatalyst is 6.3 times higher than the process with suspended TiO2-P25. In addition to the glass wall, TiO2 immobilization was also carried out on a hydroxyapatite carrier. Although the activity of said immobilized photocatalyst showed the same activity as suspended TiO2-P25, due to carrier decomposition during mixing, it proved to be ineffective. A mathematical model for a system with immobilized TiO2 on a reactor wall has been developed. The kinetic model includes a radiation emission model and a rate constant independent of the reactor geometry describing the degradation of salicylic acid, 2,5-dihydroxybenzoic acid, 2,3-dihydroxybenzoic acid and a group of compounds consisting of catechol, resorcinol, hydroquinone, aliphatic acids and other organic compounds which are, as a group organic by-products, followed by the total organic carbon method.