Optimal filtration conditions were evaluated for the ultrafiltration process planned for pretreating high-level waste (HLW) sludge in the Hanford Waste Treatment Plant. This sludge must be filtered in the pretreatment process to remove sodium and, consequently, reduce the number of canisters for storage. The evaluation, which was based on Hanford HLW slurry test data, was performed to identify the optimal pressure drop and crossflow velocity for filtration at both high and low solids loading. Results from this analysis indicate that the actual filtration rate achieved is relatively insensitive to these conditions under anticipated operating conditions. The maximum filter flux was obtained by adjusting the system control valve pressure to between 400 kPa and 650 kPa while the filter feed concentration increased from 5 wt% to 20 wt%. However, operating the system with a constant control-valve pressure drop of 500 kPa resulted in a reduction of less than 1% in the average filter flux. Also, allowing the control valve pressure to swing as much as 20% resulted in less than a 5% decrease in filter flux. This analysis indicates that a back pressure setting of 500 kPa100 kPa will give effectively optimal results for the system of interest.