Relative impact on GAC usage rates of operating strategies for treatment of contaminated groundwater

The former Nebraska Ordnance Plant has groundwater contaminated with a mixture of trichloroethylene (TCE) and the high explosive RDX that is treated in a system of eight granular activated carbon (GAC) contactors. A mathematical model was utilized to simulate alternative operating scenarios along with variable influent concentrations. In this study, blending effluent from parallel columns resulted in the lowest GAC usage rate when the target effluent C/C₀ was >0.3. Lead-lag operation was best when the target effluent C/C₀ was <0.1. Only between a target effluent C/C₀ of 0.1 and 0.3 did the percent of the mass transfer zone (MTZ) contained in the column become important. Even then, selection was more sensitive to changes in the target effluent C/C₀ value than the percent of the MTZ contained in the column. Decreasing the TCE influent concentration prior to the GAC treatment system by air stripping may reduce the GAC replacement costs but has significant new capital costs. Because contaminant levels in the extraction wells vary, influent segregation has the potential to reduce GAC usage rates by up to 65% with a relatively low capital expenditure.