Sustainable ex situ bioremediation alternative to conventional pump-and-treat systems

Document Type

Conference Proceeding

Publication Date

12-1-2009

Abstract

Inaccessible DNAPL in bedrock at a Superfund site in the Northeastern U.S. continues to contaminate groundwater. Therefore, long-term groundwater extraction and treatment is necessary to prevent risks to down gradient receptors. A conventional 50-gpm pump-and-treat system is currently in use at the site. An innovative ex situ treatment system that employs anaerobic reductive dechlorination has been designed and successfully tested at the site. Data are presented that demonstrate the performance of each element of the alternative system over an 18-month pilot test period. At full-scale, the alternative system will provide more sustainable groundwater treatment at a significantly lower cost. The alternative process has the potential for use at other sites where it is necessary to pump and treat groundwater contaminated with chlorinated ethenes and ethanes. The existing pump-and-treat system employs conventional chemical and physical treatment processes including metals precipitation, filtration, air stripping, and carbon adsorption to remove metals, volatile organic compounds (VOCs) and PCBs from the extracted groundwater. This existing process requires substantial inputs of chemicals and energy, produces greater than 80,000 pounds per year of residuals for off-site disposal, and requires full-time operations personnel. The alternative ex situ biological treatment process provides the same high level of treatment while eliminating chemical inputs and reducing off-site residuals disposal by 75 percent, to 20,000 pounds per year. In addition, energy use and operations labor are reduced by approximately 30 percent. The alternative groundwater treatment system is a two-stage process with carbon adsorption followed by an anaerobic biofilter. The treatment system influent is processed through liquid-phase granular activated carbon to remove PCBs and commence anaerobic reductive dechlorination of VOCs. The influent to the carbon vessels is maintained in a reducing condition in order to prevent precipitation of iron and plugging of the carbon. The carbon adsorption system removes greater than 99 percent of the 10 μg/L of PCBs present in the influent. The effluent from the carbon vessels is processed through an anaerobic biofilter for destruction of VOCs. The biofilter is filled with sand-peat media, and water passes through it by gravity flow. The media is saturated with groundwater and an anoxic reducing environment is maintained throughout the biofilter. The approximately 300 μg/L of cis-1,2-dichloroethene and 200 μg/L of vinyl chloride present in the biofilter influent are reduced to less then 5 μg/L in the effluent, significantly below the NPDES discharge limits for the site. There is a corresponding increase in the ethene concentration in the biofilter effluent, thereby demonstrating dechlorination of the VOCs, as opposed to sorption of the VOCs onto the biofilter media. The anaerobic biofilter also provides polishing treatment for removal of PCBs in order to attain the very low (0.004 μg/L) discharge limit for PCBs.

Publication Source (Journal or Book title)

In Situ and On-Site Bioremediation-2009: Proceedings of the 10th International In Situ and On-Site Bioremediation Symposium

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