Goal: The intent of this project is to demonstrate a passive, in-situ, treatment strategy for the bioremediation of ground water contaminated with waste mixtures. The project, incorporating laboratory and field components, focuses on the design, implementation and optimization of a permeable barrier reactor capable of pentachlorophenol (PCP) and naphthalene degradation.
Rationale: In-situ bioremediation schemes often fail because a suitable substrate, the contaminant and viable microorganisms lack adequate mixing in the subsurface. In an effort to improve contaminant removal, minimize cost and maximize process control, a down-hole permeable reactor was developed. In-situ groundwater treatment is achieved using a large diameter well and a permeable biological reactor installed within a screened interval of the contaminated aquifer. The reactor assembly is equipped with sensors, nutrient delivery, and mixing systems for the support of a subsurface biological population. Reactor environmental conditions are controlled from the surface and allow the operation of three unique (e.g. anaerobic, aerobic) biological treatment zones. Biodegradation of the aqueous phase organic compounds occurs over the length of the reactor.
Approach: Biological degradation of PCP generally occurs under the reduced conditions favored by anaerobic systems. PCP degradation by anaerobic reductive dechlorination is rapid and often results in complete mineralization. However, overall rates of PCP mineralization can be increased if the dechlorinated intermediates produced under anaerobic conditions are exposed to an oxidizing aerobic environment. Complimentary to increased rates of PCP removal, naphthalene degradation is also favored by aerobic conditions. An active wood treating facility with PCP and naphthalene ground water contamination was chosen to demonstrate the effectiveness of an in-situ permeable barrier treatment system operating under sequential anaerobic/aerobic treatment zones.
Status: During 1999, the reactor installed within the permeable barrier at the L.D. McFarland facility was operated under a variety of environmental conditions. Concentrations of the electron donor and electron acceptors (sulfate and oxygen) were varied resulting in different EH regimes within the reactor over time. Pentachlorophenol removal was evaluated under anaerobic conditions and in the presence of imitation vanilla flavoring as the electron donor. Intermediates included 2,3,4,5-tetrachlorophenol, 3,4,5-trichlorophenol, 3,4-dichlorophenol and 3,5-dichlorophenol. Complete removal of pentachlorophenol as well as these intermediates was observed. Parallel laboratory and field experiments were conducted to evaluate the effect of supplemental electron donor and electron acceptor concentrations on chlorophenol removal. Complete pentachlorophenol removal was observed in the laboratory and in the field at imitation vanilla flavoring concentrations of 10 mg/L as COD. Sulfate also was injected into the permeable barrier reactor. At influent concentrations of 100 mg/L, sulfate had no apparent effect on pentachlorophenol removal. A series of coordinated laboratory and field studies were conducted to evaluate the simultaneous removal of pentachlorophenol and naphthalene under sequential anaerobic (for pentachlorophenol reductive dechlorination) and aerobic (for naphthalene oxidation) conditions. Laboratory studies were conducted in serum bottles containing naphthalene, groundwater, and associated biomass from the permeable barrier reactor. Complete naphthalene removal was observed when aerobic serum bottles were amended with 2 to 3 mg/L naphthalene (removal required 100 - 500 hours). Napthalene removal was evaluated in the field by adding hydrogen peroxide to the mixing wells prior to aerobic biological zones in the permeable barrier reactor. Removal of naphthalene in the aerobic treatment zones was 86% within 12 days and 90% within three weeks of treatment (based upon a comparison of influent and effluent concentrations). Continued complete removal of pentachlorophenol was observed (PCP continued to be present at levels below our detection limit). The simultaneous removal of pentachlorophenol and naphthalene under sequential anaerobic and aerobic conditions indicates that potential usefulness of a permeable barrier technology for the removal of mixtures of contaminants in groundwater.