Experiments on Cake Development in Crossflow Filtration for High Level Waste

Abstract

Crossflow filtration is a key process step in many operating and planned waste treatment facilities to separate undissolved solids from supernatant slurries. This separation technology generally has the advantage of self-cleaning through the action of wall shear stress created by the flow of waste slurry through the filter tubes. However, the ability of filter wall self-cleaning depends on the slurry being filtered. Many of the alkaline radioactive wastes are extremely challenging to filtration, e.g., those containing compounds of aluminum and iron having particles whose particle size and morphology reduce cake permeability. Low filter flux can be a bottleneck in waste processing facilities such as the Salt Waste Processing Facility at the Savannah River Site and the Waste Treatment Plant at the Hanford Site. To date, increased rates are generally realized by either increasing the crossflow filter axial flow rate, limited by pump capacity, or by increasing filter surface area limited by space and increasing the required pump load. The Savannah River National Laboratory (SRNL) set up both dead-end and crossflow filter tests to better understand filter performance based on filter media structure, flow conditions, and filter cleaning. Using nonradioactive simulated wastes, both chemically and physically similar to the actual radioactive wastes, the authors performed several tests to demonstrate increases in filter performance. With the proper use of filter flow conditions, filter flow rates can be increased over rates currently realized today. This paper describes the selection of a challenging simulated waste and crossflow filter tests to demonstrate how performance can be improved by varied filter operation methods. Those methods were a slow startup to better develop the filter cake and scouring the filter wall. The results showed that for salt waste and metal oxide hydroxide sludges, the process of backpulsing is not necessary to maintain a good filter flux, and the process of periodically scouring the filter improves filter performance. The results also imply that initial filter operation is important to develop a filter cake that minimized pressure drop, the presence of a filter cake can lead to improved solids separation, and a well-developed cake with periodic scouring may allow a good filter flux to be maintained for long periods of time.