Organic synthetic dyes used in textile and food industries are major sources of the environmental contaminations due to high toxicity to aquatic species and negative impact on human health. Organic dyes can be converted to their intermediate products, such as aromatic amines, that can be toxic or carcinogenic compounds. Most organic dyes are considered to be basically non-degradable and common degradation processes, such as active sludge, physical and chemical treatment, are unable to completely remove dyes from wastewaters. Alternative methods for wastewater treatment have been developed, including advanced oxidation processes (AOPs), with heterogeneous photocatalytic oxidation process as the most promising technique. The aim of the study was to develop novel polymer nanocomposite photocatalysts with extended activity towards the visible light region to enhance the technology for wastewater treatment. Novel polymer nanocomposite photocatalysts were tested by the photocatalytic degradation of organic dyes Reactive Red 45 (RR45) and Acid Blue 25 (AB25) chosen as target pollutants. For this purpose the synthesis routes for conducting polymers, polyaniline (PANI) and polypyrrole (PPy), were studied in order to obtain the appropriate structures that can activatie the photocatalysts titanium dioxide (TiO2) and zinc oxide (ZnO) under the Sun light. Fine TiO2 photocatalyst, commonly in nanosize, is subjected to agglomeration that reduces its surface and reflects to its catalytic efficiency. There is also a problem concerning the separation and recovery of the photocatalyst at the end of the wastewater treatment. Fly ash is a waste material formed during the combustion process (coal, solid waste) that needs to be collected and recycled. As the fly ash is an aluminosilicate this makes it suitable material for use as a supporter of the catalyst. The waste fly ash (FA) material was subjected to chemical treatment with HCl at elevated temperature for a different time to modify its porosity. Modified FA particles with highest surface area and pore volume were further used as a support for TiO2 catalyst during FA/TiO2 nanocomposite preparation. The nanocomposite photocatalysts were obtained by in-situ sol–gel synthesis of titanium dioxide in the presence of FA particles. To perform accurate characterization of modified FA and FA/TiO2 nanocomposite photocatalysts, gas adsorption-desorption analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), UV/Vis and infrared spectroscopy (FTIR) were used. Photocatalysis has been carried out up to five cycles with the same catalysts to investigate their stability and the possible reuse. To achieve a specific morphology and high surface it is important to control the crystallinity, porosity and composition of the TiO2 catalyst. Therefore the physical/chemical properties of a catalyst may be quite different, depending on their phase structure and particle size. TiO2 catalyst has been prepared by different synthesis conditions including the starting chemicals, reaction duration and temperature as well as the duration of calcination. TiO2 catalysts were synthesized by three different methods and TiV, TiCh, TiB samples were prepared by sol-gel synthesis. TiO2 photocatalysts were characterized by FTIR, XRD, SEM and UV/Vis spectroscopy. Photocatalytic validation has been made by determining the degree of RR45 and AB25 removal. The results show that the photocatalytic efficiency depends on the crystal structure of TiO2. Furthermore, nanocomposites of TiO2 with conducting polymer PPy were prepared by chemical oxidation polymerization. Such composites possess enhanced photocatalytic activity under visible light in comparison with pure TiO2 photocatalyst. It is very important to optimize the synthesis conditions in order to obtain PPy/TiO2 composites with the optimal thickness of conductive polymer layer on TiO2 and minimal possible aggregation of particles. ZnO, as one of the best photocatalysts in terms of the efficiency, shows high photocatalytic potential under ultraviolet (UV) radiation. There are many efforts to modify ZnO and obtain a new photocatalysts, which absorb whole or the most of the sun's irradiation, not just the ultraviolet part. The synthesis of the composite polypyrrole/zinc oxide (PPy/ZnO) has been carried out by polymerization of pyrrole monomer in the presence of ZnO, with various duration of polymerization reaction (45, 60 and 75 minutes). Also, the polyaniline/TiO2 (PANI/TiO2) composite photocatalysts were prepared by the in- situ chemical oxidation of aniline (An) in the presence of TiO2 particles. For this purpose, photocatalysts with different amounts of PANI polymer were prepared and analysed. The 15PANI/TiO2 sample presented the highest photocatalytic efficiency under ultraviolet A (UVA) irradiation, in comparison to pure TiO2 by the formation of uniformly dispersed PANI on the TiO2 particles, which is responsible for the synergistic PANI-TiO2 effect. The main barriers for extensive ZnO applicability are the narrow light-absorption range, the charge-carrier recombination and the photoinduced corrosion–dissolution at extreme pH conditions. Therefore, polyaniline/zinc oxide (PANI/ZnO) composites were prepared from neutral media by in-situ chemical oxidation of aniline (ANI) in the presence of different amounts of diethylene glycol (DEG). The advantages of preparing PANI/ZnO composite photocatalysts in neutral media with diethylene glycol (DEG) additions are: (i) improved aniline solubility in neutral aqueous solution and more stable dispersion, (ii) the ability to avoid dissolution of ZnO during in-situ synthesis, (iii) the ability to enable photocatalysis under solar irradiation due to a diminished bandgap, and (iv) the ability to prevent corrosion-dissolution of ZnO during photocatalysis. Characterization of all prepared composite photocatalysts was done by FTIR spectroscopy, X-ray diffraction, thermogravimetric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), measurements of electric conductivity, UV-Vis spectroscopy and by specific surface area (SBET) measurements. The photocatalytic activity of photocatalyst composites was examined in the process of degradation of RR45 and AB25 dyes under UVA and simulated solar irradiation which was monitored by UV-Vis spectroscopy. The mineralization of RR45 and AB25 by photocatalysis was determined as a removal of total organic carbon (TOC).