Comparing the Efficiency of Oxidation, Sparging, Surfactant Flushing, and Thermal Treatment at Different Scales (Batch, Column, Metric Pilot)

Abstract

Several remediation techniques are available to treat source zones with Non-Aqueous Phase Liquids (NAPLs). Despite hundreds of treated sites, it is still quite difficult to estimate the potential efficiency of a treatment technique under a given context. In the literature, numerous papers deal with the potential limitations of one remediation technique at a laboratory scale or in the field, but only a few studies compared two or more different techniques. Thus, the objective of this chapter is to compare different treatment techniques in the same solid matrix and under the same conditions at three different experimental scales: batch, columns, and metric pilot tank experiments. The chosen techniques are among the most common ones, i.e., oxidation, sparging, surfactant flushing, and low-temperature thermal treatment. These four techniques were applied first at the batch and column scale in order to assess their potential efficiency under well-controlled conditions. These experiments are mandatory for choosing the best oxidant, the activating agents for this oxidant, the best concentration, and the determination of the limiting factors which can be encountered. All these lab experiments allowed a selection of favorable conditions for testing four techniques at the pilot scale. These treatment techniques were tested in the same configuration, in tanks of one cubic meter volume, filled with sand and emplaced sources containing a toluene/decane mixture as representative of a NAPL phase. The major findings of this study are: (1) Thermal treatment was the most efficient remediation technique; (2) Dissolution issues reduced the efficiency of the three other techniques; (3) Heterogeneity in the tanks caused a decrease in the removal efficiency for all treatments compared to efficiencies obtained at the column scale; (4) Density effects, which were not encountered at the column scale, caused problems for oxidant injection in the tanks; (5) Removal techniques that are equal for mass-based removal goal were quite different in lowering the dissolved contaminant flux from the system. Therefore, the ranking of techniques based on mass removal may not be appropriate and be replaced by a flux reduction selection; and (6) A need to perform experiments at three experimental scales is evidenced: batch studies allow sufficient variations in experimental conditions, the column experiments permitted the optimization of surfactant and oxidant injection, whereas tank experiments allow studies under heterogeneous flow conditions.