Assessment of Filter Conditioning During Cross-Flow Filtration of Simulated Tank Waste

Crossflow filtration is to be a key process in the treatment and disposal of approximately 60,000 metric tons of high-level waste stored at the Hanford Site in Richland, Washington. Pacific Northwest National Laboratory (PNNL) is assessing filter performance against waste simulant materials that mimic the chemical and physical properties of Hanford tank waste using a bench scale crossflow filtration system. Recent tests at PNNL with continuous filtration times of approximately 100 hours demonstrated that the filter flux does not appear to be approaching a steady-state value. This has important implications for filtration operations with actual waste, since this limits the throughput of the waste treatment system.

PNNL has performed additional testing to assess strategies for improving long-time filtration performance. The tests were designed to observe how operational parameters like backpulse period, number of backpulses, and shifts in transmembrane pressure affected the filter flux. It was found that the filter could be conditioned with backpulsing, after which the flux does not decay as it does when operated continuously. The flux was also found to converge to similar values (over several different tests) after approximately 4 to 6 hours despite the magnitude of the initial flux. This indicates that after a period of initial variability in the flux, a fouling mechanism that does not depend on initial conditions dominates the filtration performance (i.e. cake formation and maturation).