Enhanced in situ bioremediation (EISB) is increasingly being used for the remediation of chlorinated solvents and recalcitrant chemicals (e.g., perchlorate, RDX) in groundwater. Often, the repeated addition of nutrients creates conditions within the injection well screen and the surrounding filter pack that favor microbial growth and biofilm formation, which in turn results in a loss of well efficiency. When well efficiency declines below an acceptable level, physical well rehabilitation coupled to aggressive chemical shock treatment is typically deployed. Well rehabilitation is labor-intensive and can diminish the feasibility of EISB compared to other technologies by significantly increasing operating costs. The objective of this project was to evaluate several biofouling control agents, which would minimize well efficiency losses and the need for well rehabilitation and improve the feasibility of EISB. A technical report was developed to review the current state of biofouling control agents, including the applicability of conventional biofouling controls for in-situ application, case studies, regulatory acceptance, and a comparative analysis of the relative advantages and disadvantages of each control.
Biofouling control agents include oxidizing biocides that inhibit or inactivate biofilm-producing bacterial populations; physical means, such as jetting or ultrasound, that displace the biomass; and surfactants, dispersing agents, or chelating agents that destabilize the biofilm matrix. The aim with biofouling control is the regular addition of the biofouling control agent to minimize biofouling formation and the downtime associated with well rehabilitation, without causing negative impacts to either the downgradient water quality or to the bioremediation process itself.
In this project, a white paper was developed that discussed biofilm/biofouling theory and identified a host of biofouling control agents. The biofouling control agents were evaluated for field testing by considering their likely effectiveness, ease of handling and implementation, commercial availability, cost, and regulatory acceptance. Based on this evaluation, chlorine dioxide, glycolic acid, hydrogen peroxide, and sodium hypochlorite were selected for field study.
Available case studies discussing the effectiveness of biofouling controls also were summarized in the white paper. Several case studies demonstrated that chlorine dioxide can be an effective biofouling control for injection wells used in bioremediation applications. In general, however, there were few detailed case studies in the literature, and many were unverified case studies provided by vendors. This survey provided additional evidence that further objective testing of biofouling controls in the field is needed.
Considerable time was spent in locating an appropriate site for this demonstration. A shallow but wide trichloroethene (TCE) or perchlorate plume was required to keep well installation costs down and to provide sufficient separation of each of the biofouling control treatments, which would be evaluated in parallel. An extraction well that would yield more than 25 gpm to supply each of the injection wells with more than 5 gpm of flow was also desired. OT-20 at Robins Air Force Base in Georgia was identified as an appropriate site, because it met these requirements.
Unfortunately, the project was terminated prematurely due to unexpected site conditions identified during baseline sampling and hydraulic testing and before the biofouling controls were added to the injection wells. Therefore, the effectiveness of the biofouling controls and their effect on the aquifer geochemistry and desired biodegradation processes could not be evaluated.
Given the hundreds of perchlorate- and volatile organic compound (VOC)-impacted Department of Defense (DoD) and related sites expected to employ EISB in upcoming years, the development of safe and reliable preventative biofouling controls that eliminate or minimize the need for well rehabilitation is expected to represent cost savings in the $10 millions to DoD. Consequently, a demonstration that will generate cost and performance data for multiple biofouling controls that may be readily implemented at DoD facilities with varying geochemical and infrastructure conditions is still needed. (Project Completed – 2007)