Formatted Title

Cometabolic Treatment of 1,4-Dioxane and cVOCs Using an Isobutane-Fed Fluidized Bed Bioreactor: Bench Studies

Background/Objectives

1,4-Dioxane (1,4-D), a widely used stabilizer in chlorinated solvents, has emerged as an important groundwater contaminant throughout the United States, including at numerous U.S. Department of Defense (DoD) sites. Because of its application as a stabilizer, 1,4-D commonly occurs at sites in conjunction with chlorinated volatile organic compounds (cVOCs).  The US EPA classifies 1,4-D as a likely human carcinogen, with several states establishing drinking water and groundwater guidelines, while various cVOCs are regulated through federal national drinking water regulations.  1,4-D is poorly retarded in aquifers, resulting in the potential to create large contaminant plumes that threaten drinking water supplies distant from release sites. Traditionally, for such contaminated groundwater plumes, cVOCs are treated via ex situ air stripping, followed by 1,4-D removal using carbon or advanced oxidation processes (AOPs). Such multi-unit processes have relatively high capital costs and significant operational complexity.  Hence, the use of a single biological technology to treat both 1,4-D and cVOCs in groundwater potentially provides improved life-cycle treatment costs for the DoD at numerous contaminated sites.  This study aims to demonstrate and validate the use of an advanced bioreactor design, a fluidized bed bioreactor (FBR), for the treatment of these co-contaminants.

Approach/Activities

This demonstration evaluates the performance and cost of a biological FBR for treating the co-contaminants of 1,4-D and cVOCs in groundwater using an isobutane-mediated, cometabolic process. The FBR is an efficient fixed-film bioreactor with a high biomass concentration attached to a fluidized medium. Within the fluidized medium, biological treatment of the contaminated water occurs. The FBR is fed isobutane as a substrate and is seeded with an isobutane-oxidizing bacterium (Rhodococcus aetherivorans ENV493) that has been shown in laboratory batch studies to be capable of biodegrading 1,4-D and multiple cVOCs.  The FBR process is capable of treating typical groundwater concentrations of 1,4-D at 1-200 µg/L and cVOCs at 1-100 µg/L by 90%, and possibly to less than remediation goals of 1.0 µg/L and current potable requirements (e.g., TCE=5 µg/L and cis-1,2 DCE= 70 µg/L), respectively.

During this study, simple microcosm batch tests and a continuous flow bench-scale FBR study were performed.  The batch tests were conducted to demonstrate the efficacy of the microbial culture, using isobutane as a substrate, to co-metabolically treat the co-contaminants in the groundwater from a DoD facility.  The bench reactor studies focus initially on 1,4-D treatment alone, then in combination with cVOC addition. To maximize co-contaminant treatment rates, critical design parameters of hydraulic residence time (HRT), isobutane and oxygen addition rates, and reactor temperature/pH were monitored and optimized.

Results/Lessons Learned

The batch testing revealed that multiple isobutane-degraders, including ENV493, were capable of near complete co-contaminant treatment in site groundwater. Subsequently, with a bench-scale FBR inoculated with ENV493, the initial treatment testing is being conducted at an HRT between 90-110 minutes in laboratory water.  Testing for 1,4-D treatment alone is ongoing, but early results demonstrate a robust microbial biofilm is developing within the FBR media as isobutane addition occurs in optimized dosages. Early results have demonstrated near 50% removal of the 1,4-D during initial reactor start up and acclimation.  As 1,4-D treatment continues to improve, reduction in HRT to less than 60 minutes and the addition of cVOCs will occur.  These studies and results, as well as the next steps to advance this project forward to a field pilot-scale study at a DoD facility, will be further detailed.