Removal of TiO₂ nanoparticles from water by low pressure pilot plant filtration

Rising use of nanoparticles in manufacturing as well as in commercial products bring issues related to environmental release and human exposure. A large amount of TiO₂ nanoparticles will eventually reach wastewater treatment plants. Low pressure membrane filtration has been suggested as a feasible treatment of water streams. This study investigated first at laboratory scale the influence of: i) membrane material, ii) pore size and iii) water chemistry on nTiO₂ removal. TiO₂ retention was governed by the cake layer formation mechanism and significant retention of nanoparticles was observed even for filters having considerably larger pores than nTiO₂. PVDF showed a great potential for nTiO₂ rejection. Additionally, filtration pilot plant experiments were carried out using PVDF membranes (0.03 and 0.4μm pore size). The release of nTiO₂ in the pilot scale filtration system was always above the instrumental detection limit (>1.5μg/L) and in most cases below 100μg/L regardless of the pore size and applied conditions. The nTiO₂ membrane breakthrough predominantly occurred in the first few minutes after backwashes and ceased when the cake layer was formed. Ultrafiltration and microfiltration were comparable with rejection of nTiO₂ above 95% at similar permeate flow rates. Nevertheless, ultrafiltration is more promising than microfiltration because it allowed longer operation times between backwash cycles.