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The transformation of both calcareous and siliceous Greek power station by-products (lignite ashes) into novel composite materials with photocatalytic properties for environmental application was investigated. Particularly, a comparison between the development of coated ceramic substrates and the modification of ash surfaces is attempted. Specifically, a) the sintering process (1000 °C, 2 h) of both fly and bottom ash (either calcareous or siliceous) for their conversion into compacted ceramic substrates coated with TiO2 slurry and then further thermally treated (500 °C, 1 h) to acquire TiO2 film consistency onto the ceramic substrate and b) the process of TiO2 precipitation on lignite ash surfaces in acidic solution after neutralization, and estimation of the TiO2 percentage, are compared. The microstructures obtained were examined by XRD and SEM-EDX analysis. Vickers microhardness was also determined for the ceramic microstructures, with satisfactory results (up to 356HV). The energy gap measurements of the coatings were found to be between 3.02eV and 3.17eV, which is located between the energy gap of anatase (3.23eV) and rutile (3.02eV). The coating mass was about 0.059 g/cm2. The photocatalytic activity under visible and UV irradiation was investigated in aqueous solutions of methylene blue and methyl orange organic dyes, with encouraging results. A main advantage of the processes proposed is the immobilization of TiO2 onto largely available secondary resources, which can lead to production of value-added ‘green’ photocatalysts for the treatment of industrial effluents in the framework of circular economy.