CROSSFLOW FILTRATION: EM-31, WP-2.3.6

Abstract

In the interest of accelerating waste treatment processing, the DOE has funded studies to better understand filtration with the goal of improving filter fluxes in existing crossflow equipment. The Savannah River National Laboratory (SRNL) performed some of those studies, with a focus on start-up techniques, filter cake development, the application of filter aids (cake forming solid precoats), and body feeds (flux enhancing polymers). This paper discusses the progress of those filter studies. Crossflow filtration is a key process step in many operating and planned waste treatment facilities to separate undissolved solids from supernate solutions. 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, which have particles whose size and morphology reduce permeability. Unfortunately, low filter flux can be a bottleneck in waste processing facilities such as the Savannah River Integrated Salt Disposition Process and the Hanford Waste Treatment Plant. Any improvement to the filtration rate would lead directly to increased throughput of the entire process. To date increased rates are generally realized by either increasing the crossflow filter feed flow rate, limited by pump capacity, or by increasing filter surface area, limited by space and increasing the required pump load. SRNL set up both dead-end and crossflow filter tests to better understand filter performance based on filter media structure, flow conditions, filter cleaning, and several different types of filter aids and body feeds. Using non-radioactive simulated wastes, both chemically and physically similar to the actual radioactive wastes, the authors performed several tests to evaluate methods to improve filter performance. With the proper use of filter flow conditions and filter enhancers, filter flow rates can be increased over rates currently realized today. Experiments that use non-radioactive simulants for actual waste always carry the inherent risk of not eliciting prototypic results; however, they will assist in focusing the scope needed to minimize radioactive testing and thus maximize safety. To that end this investigation has determined: (1) Waste simulant SB6 was found to be more challenging to filtration than a SRS Tank 8F simulant; (2) Higher solids concentration presents a greater challenge to filtration; (3) Filter cake is something that should be properly developed in initial filter operation; (4) Backpulsing is not necessary to maintain a good filter flux with salt wastes; (5) Scouring a filter without cleaning will lead to improved filter performance; (6) The presence of a filter cake can improve the solids separation by an order of magnitude as determined by turbidity; (7) A well developed cake with periodic scouring may allow a good filter flux to be maintained for long periods of time; and (8) Filtrate flux decline is reversible when the concentration of the filtering slurry drops and the filter is scoured.