Abstract :
[en] Persistent micropollutants with unknown adverse effects on the environment and health overcome traditional wastewater treatment methods. They are flushed into Europe’s rivers daily. Their unpredictable molecular structures and minute concentrations make them challenging to destroy, and advanced oxidation processes (AOPs) are a realistic solution. Namely, ozone (O3), high energy UV light (UVC), photocatalysis and active carbon were successfully combined to destroy a cocktail of tens of persistent micropollutants with no external addition of chemicals.
In the beginning, a screening study on seven photocatalysts was performed. Materials were deposited as thin films through a sol-gel process and dip-coated on glass slides. Thin films and corresponding powders were characterized, checking the presence of the active crystalline TiO2 phase anatase and optimizing thicknesses.
Then, the efficiency of each photocatalyst was assessed through degradation tests on five pharmaceuticals that resisted O3 and UVC. Two organic syntheses (namely TiO2 doped with Ag nanoparticles, and TiO2 doped with Evonik P25) were selected for further use.
Ag-doped and P25-doped TiO2 photocatalysts were studied and optimized for performance and safety.
Several parameters were studied to improve the photoactivity: (i) the synthesis itself ; (ii) the amount of dopants ; (iii) the layer thickness ; (iv-v) the calcination duration and temperature and (vi) the type of substrate for the TiO2 layer. The numerous resulting samples were characterized and tested at laboratory scale on 22 micropollutants found in actual wastewater via an O3-UVC-TiO2 AOP. The most promising TiO2 photocatalyst was doped with 2% Ag and 10% P25 in a novel synthesis and could be deposited on
glass and steel without leaching nanoparticles in the treated water.
Then, the AOP was scaled up to treat 5 m3/h of the Duisburg wastewater treatment plant. At several scales (lab-scale, pilot-scale, industrial-scale), the process efficiency was assessed on different water matrices: laboratory-made water, industrial wastewater and municipal wastewater. Several parameters played a role: light intensity, flow rate, concentrations of micropollutants and the water matrix. The method of deposition
of TiO2 was changed from dip-coating to spray-coating to handle industrial-sized reactors. An adsorption step was added after a study on activated carbons. Practical performance indicators such as toxicity of the water and price were thoroughly studied. Conclusions were positive, as all micropollutants were at least partially degraded by the AOP, and activated carbon adsorbed the rest. The cost of operation is reasonable but not competitive yet: around 2 kWh/m3 per tenfold reduction of the concentration of the most resistant micropollutants.
To achieve full sustainability of the process, regeneration of activated carbon was finally studied: cylindrical monolithic carbon xerogels were produced by a sol-gel process, pyrolyzed and installed as cathodes in an electrically powered system. Under a steady current of 100 mA, the material produced 4 mg/L hydrogen peroxide (H2O2), which is sufficient to destroy adsorbed pollutants under UV light. The monoliths’ macrostructures and microstructures were optimized to enhance the diffusion of pollutants: synthesis
parameters were studied, and the shape of xerogels was tailored using 3D-printed dissolvable plastic molds. This 3D-printing step included a full study on the best type of plastic, mold design and printing parameters. The 3D-printed carbon xerogels adsorbed pollutants up to eight times faster and could be partially regenerated.
