The Hanford Site, managed by the U.S. Department of Energy (DOE), contains 56 million gallons of legacy radioactive waste stored in underground tanks. Hanford Tank Waste Operations and Closure (H2C, formerly Washington River Protection Solutions) holds stewardship of the tank farms and operates the Tank-Side Cesium Removal (TSCR) system, which processes tank waste supernatant by removing suspended solids through filtration and extracting cesium via ion exchange before vitrification for long-term disposal. Pacific Northwest National Laboratory developed and operates a bench-scale Direct Feed Low-Activity Waste test platform that replicates TSCR and LAW vitrification unit operations, enabling evaluation of filtration, ion exchange, and vitrification using aqueous tank waste supernatant supplied by H2C. Effective dilution and filtration are essential to sustaining cesium removal efficiency and process reliability. The latest demonstration, initiated in 2024, involved diluting 8.4 L of AW-105 supernatant from over 6.4 M Na to approximately 5.7 M Na using Columbia River water, followed by reduced-temperature filtration at 16 °C with a Mott Media Grade 5 sintered steel dead-end filter. The filtration system was operated at a nominal flux of 0.065 gpm/ft² and a transmembrane pressure (TMP) below 2 PSI—mirroring the full-scale TSCR operation. Additionally, standard TSCR filter maintenance procedures, such as back-pulsing to reverse fouling when TMP reaches 2 PSI, chemical cleaning with 0.1 M NaOH, and clean water flux assessments before and after filtration, were replicated at bench scale. Key process variables including TMP, filter flux, and permeate dynamic viscosity were used to measure filter resistance throughout the filtration campaign. Four Hermia models – standard blocking, intermediate blocking, complete blocking, and cake filtration – were used to characterize membrane fouling behavior by fitting the filter resistance data to each model. Minimization of the root mean square error between the measured and modeled resistances showed intermediate blocking to be the predominate fouling mechanism in the filtration of AW-105 tank waste supernatant. Resistance behavior diverged from the model once settled solids were resuspended and introduced into the system as the fouling rate increased significantly, but fouling was effectively reversed upon back-pulsing the filter with permeate. While it was observed fouling rate can suddenly increase when settled solids in the AW-105 supernatant feed are disturbed, proper filter maintenance procedures demonstrated that fouling of the Mott Media Grade 5 stainless steel filter can be effectively reversed with standard TSCR back-pulsing protocol.
