Crossflow filtration (CFF) is the unit operation selected to separate solids from waste streams retrieved from the Hanford Tank Farms in the Low Activity Waste Pretreatment System (LAWPS) facility. It is also a planned unit operation in the Hanford Waste and Immobilization Treatment Plant (WTP) Pretreatment Facility. In contrast to the WTP Pretreatment facility, CFF in the LAWPS facility is expected to encounter solid concentrations in the waste streams that are relatively low: nominally ~1 wt% (and sometimes much less), with a maximum of ~3 wt%. Recent work with Pacific Northwest National Laboratoryâs bench-scale crossflow filtration system demonstrated that waste simulant slurries with solid concentrations in this range can produce sufficient amounts of filtrate at low transmembrane pressures, i.e. they did not significantly foul the filter. These waste simulant slurries were developed on the basis of chemical similarity to the waste and did not incorporate the entire array of possible solids species that might be encountered during LAWPS operations. Thus, more information was needed to address one of the major concerns with filtering low solids slurries, namely the potential for more significant irreversible fouling. The irreversible fouling (fouling that cannot be reversed except by chemical cleaning) may be more likely occur since there is much less solids inventory present to form a protective cake on the filter/fluid interface. Irreversible fouling could be caused by penetration of fines into filter pores, adhesion of solids onto the filter surface, in-pore precipitation events, or some combination of these factors. In past work at PNNL, irreversible fouling has been observed to be more significant when certain solid species are present, and these species were not part of the initial suite of waste simulants already tested.
To understand how different fouling species found in Hanford waste affect fouling at low solids concentration, additional testing was performed in PNNLâs bench-scale crossflow filtration system. Using a chemically representative supernate as a base solution, fouling species were added at low concentrations (typically 0.1 wt%). Even at these low concentrations, certain solid species can induce significant fouling in CFF systems that eventually require chemical cleaning. One important observation from the data is that fouling in CFF systems separating Hanford waste does not appear to be solely a function of particle size and concentration, but also depends on the specific composition of that solid species. In support of these observations, data from CFF testing at PNNL, as well as comparisons with historical crossflow filtration data of waste simulants, will be presented and discussed.
