First proof of concepts test for the developed column inserts containing TiO2 (𝜔 𝑇𝑖𝑂2 = 0.1) were conducted with the decolorization reaction of Methylene Blue (MB) in an aqueous solution by UV light (λ = 365 nm). This reaction can easily be monitored with online UV/vis spectroscopy. Tests with the grid insert demonstrated the general feasibility to decolorize MB. Interestingly, an increasing photocatalytic activity was observed when the insert was reused (see Figure ). The initial reaction rate was more than tripled from 0.0075 nmol s -1 in the first experiment to 0.0275 nmol s -1 in the fourth experiment. A scanning electron microscopy (SEM) analysis of the inserts before and after irradiation with UV light revealed that the initially smooth polypropylene surface turned into a highly porous one (see Figure right). This was reasoned by a degradation of the polymer by OH-radicals formed at TiO2 in the aqueous media. Due to this degradation, more TiO2 particles are exposed, eventually increasing the activity. Besides this, it was observed that repeated use of the insert leads to coloration of the parts of the columns that were exposed to irradiation. This might be a result of the increased surface area, through which the adsorbed amount of MB increased and finally becomes visible with the naked eye.