The objective of this demonstration was to evaluate the effectiveness and costs associated with cometabolic air sparging (CAS) for removal of chlorinated aliphatic hydrocarbons (CAHs) from groundwater. In CAS, the techniques employed by conventional air sparging (AS) are augmented by also injecting a gaseous cosubstrate. Specific objectives included: evaluating the distribution of oxygen and growth substrate (propane) in the subsurface; determining growth substrate acclimation requirements; determining uptake rates for oxygen and substrate after acclimation; evaluating the indigenous microorganisms' ability to degrade a variety of CAH compounds; making predictions of long-term process stability; estimating costs for CAS; and making cost comparisons to traditional air sparging and pump and treat (P&T) technologies.
The results showed that CAH degradation was successful in the saturated zone at McClellan Air Force Base. CAH removal rates were more rapid in the active site (air sparging with addition of substrate) than the control site (air sparging alone). Based on final CAH concentrations, the extent of CAH removal was also greater in the active site than the control site. Since CAH removal closely followed degradation of propane, it appears that CAH degradation and propane degradation were closely linked. This degradation continued after sparging stopped, suggesting that air stripping was not the sole removal mechanism. However, CAS technology was not completely successful, as it did not completely remediate the site's vadose zone without the use of soil vapor extraction equipment (with air stripping as the main driver). The technology may still be viable as a complete vadose and groundwater remediation tool at other sites.
Estimated CAS ($161/cubic yard) and conventional AS ($163/cubic yard) costs were very competitive at the selected scale for comparison, while P&T costs ($313/cubic yard) were much higher. Although the costs of CAS and conventional AS were similar at this scale, the relative costs of CAS would likely decrease per unit treated volume, while the cost of conventional AS would be relatively linear. CAS, unlike conventional AS, has several up-front costs that would decrease in proportion to the total project cost with increased scale. Such upfront costs include the microcosm studies (estimated at approximately $50K), pilot testing (approximately $36K), and groundwater and vadose zone monitoring. Monitoring costs could decrease significantly once the efficacy of the process at the site is confirmed.
CAS was effective for treatment of groundwater but ineffective in the vadose zone, resulting in incomplete contaminant removal from the site. It is not unreasonable to conclude that CAS could treat vadose zone contaminants at different sites. However, before initiating a field effort, microcosm work to determine if indigenous microbial life can catalyze CAH removal in both saturated and vadose zone soils should be conducted. Cost comparisons between CAS and conventional AS were competitive at this scale. However, normalized CAS costs could be reduced with an increasing volume of contaminated material. The regulatory compliance potential and acceptance of injecting a growth substrate into the treatment plot remains promising but will continue to require more investigation before ubiquitously accepted. (Project Completed - 2001)