Smouldering technology for aquifer remediation (STAR): a novel remediation technique for pooled and residual NAPL

Smouldering technology for aquifer remediation (STAR) is an innovative remediation technique that uses smouldering combustion to remediate land contaminated with non-aqueous phase liquids (NAPLs). Many NAPLs (e.g., petrochemicals, coal tar, most chlorinated solvents) are combustible and contain considerable chemical energy that can be released as heat during the smouldering process. Smouldering is a slow, flameless form of combustion, less energetic than flaming combustion. Within a porous media, the hot gases produced by the smouldering reaction are trapped, supplying energy to the continuing reaction. The heat generated by the reaction may be sufficient to progress the reaction through the porous media, making the reaction self-sustaining. If this phenomenon is achieved, the ignition source may be removed, making the process energy and cost-efficient. Air (oxidant) supply may assist propagation of the combustion front through the NAPL distribution in the subsurface, destroying the NAPL pool and making the process self-targeting. Once initiated, STAR can be manipulated by controlling and targeting the supply of air or oxidant in the subsurface. Results from laboratory and small-scale field experiments of STAR are very promising. A number of environmentally-relevant fuels have been studied, including coal tar, crude oil and other petroleum products and chlorinated solvents. Results suggest that smouldering reduces subsurface contamination significantly, with either trace or no detectable contaminant remaining. Modelling suggests that as the scale of the application increases, its efficiency will increase as well. Gaseous by-products, which are contaminant-specific, have been quantified and eliminated effectively via filtration. Field experiments are underway to test two potential configurations where STAR could be implemented. The first configuration is a direct scale-up of laboratory experiments where excavated, NAPL-contaminated soil is treated in a reactor, achieving an ex situ application. The second configuration replicates an in situ application. Soil that has been treated by this method has the potential to be left in place, avoiding the costly excavation/landfilling process while protecting the affected aquifer. This paper reviews the key factors affecting the success of smouldering combustion remediation revealed in the laboratory experiments, the scale-up of the process as revealed by the controlled field experiments, and issues associated with effective control of the process and treatment of generated by-products