Novel fluidized-bed bioreactors with density-graded carriers for the bioremediation of nitrate in uranium industry effluents

This study presents an innovative bioreactor system that employs density-graded floating carriers to effectively remediate complex uranium-contaminated effluents generated by the nuclear industry. By combining the advantages of fixed-bed and fluidized-bed reactors, our system utilizes floating carriers to create a stratified biofilm environment, optimizing biomass retention and mass transfer. Controlled redox potential (ORP) enhances the removal of uranium and associated contaminants, especially in effluents with high-nitrate concentrations. The fluidized-bed configuration, with a high carrier load, minimizes biofilm-induced clogging, ensuring sustained performance. Carriers were synthesized with acrylate polymers in different compositions: HEMA 50%/0 AA, HEMA 50%/25% AA and HEMA 50%/50% AA w/w to obtain different hydrodynamic properties. The particle terminal velocities and drag coefficients of carriers were 3.14 × 10⁻⁶ m s⁻¹, 5 × 10⁻⁵ m s⁻¹, and 2 × 10⁻⁴ m s⁻¹ and 661 976, 20 734, and 26 221, respectively. The system achieved nitrate and COD removal efficiencies of up to 90% and 84%, respectively, at a hydraulic retention time of 23.9 h and with low energy consumption. The system behaved like a fluidized bed with a high carrier load similar to the PBBR, showing piston flux and variable column fluidization based on carrier densities. Frictions and collisions prevented clogging due to biofilm formation, ensuring sustained performance.