The ground water then travels through two columns, connected in series, that are filled with ion exchange resin. After passing through both ion exchange columns, the treated ground water is discharged to a nearby ditch. The two ion exchange columns contain 30 ft3 of resin each. Process hardware consists of a batch tank, pump, sand filter, ion exchange columns, and valving and piping for treated water. The HCRU also contains a salt tank, pump, and separate piping for regenerating the resin.
Regeneration is done counterflow to the process flow direction.
During normal processing, the effluent ground water is sampled in between the columns for hexavalent chrome. When the level between the two columns exceeds 11 ppb, the upstream column is regenerated and placed back in service as the downstream column. In this way the freshly regenerated resin serves as the back-up column. The resin used is a strongly basic Type I quaternary ammonium anion exchange resin with a styrene-divinylbenzene copolymer gel matrix. This resin was tested at LLNL using ground water similar to that at TFC in a bench-top test before selection for the field.