4.4.6.3 Long-Term Effectiveness and Permanence -- Alternative 6 The long-term effectiveness and permanence of this alternative is assessed in terms of the concentration of PCBs in treatment residuals. However, performance data for pilot- or full-scale applications of biological treatment of PCBs are virtually unavailable for CSPB and are very scarce for in situ biological treatment. The limited laboratory-scale data available are often reported in terms unrelated to concentrations of total PCBs. The lack of applicable performance data does not allow the effectiveness of biological treatment to be readily evaluated in terms of whether biological treatment can meet the PCB cleanup standard for the CD sites. The sections below discuss bioremediation performance, factors that influence performance, and system limitations of bioremediation. Bioremediation Performance Performance data for pilot- and full-scale applications are very scarce, and laboratory-scale data are often reported in terms unrelated to concentrations of total PCBs, which does not allow results to be directly compared to the PCB cleanup standards for the CD sites. However, performance data for several representative laboratory- and pilot-scale studies of anaerobic and aerobic PCB biodegradation are provided below. Anaerobic dechlorination of highly-chlorinated PCB congeners has been observed intrinsically in naturally-occurring anaerobic soils and sediments (Brown and others 1987) and in laboratory studies. During a 16-week laboratory study, from an initial concentration of 700 ppm Aroclor 1242, indigenous anaerobic microorganisms in river sediments reductively dechlorinated highly chlorinated PCB congeners, resulting in the reduction of the proportion of tri- through hexachlorinated biphenyls from 91 to 12 percent, with a corresponding increase from 9 to 88 percent in the proportion of mono- and dichlorobiphenyls (Quensen, Tiedje, and Boyd 1988). During another laboratory-scale study, from an initial concentration of 17,000 ppm PCBs, dechlorination resulted in 46 to 99 percent reduction in tri-, tetra-, and pentachlorobipheyl congeners based on gas chromatograph (GC) peak heights, but no biodegradation was observed in samples with lower initial concentrations of 150 and 1,500 ppm PCBs after 9 months (Risatti 1992). Because the dechlorination process involves only the loss of chlorine atoms and not the destruction of biphenyl rings and because the PCB cleanup goals are expressed in terms of total PCBs, even under ideal dechlorination conditions, anaerobic dechlorination alone will not meet cleanup goals. However, dechlorination significantly reduces the magnitude of remaining risk associated with the dechlorinated materials because the less chlorinated congeners that would result from anaerobic biological treatment are much less toxic and less bioaccumulative than the highly-chlorinated congeners initially present in the Aroclors disposed of at the CD sites (PRC 1995e). Aerobic biodegradation of PCBs has been observed both in the laboratory and in pilot-scale field demonstrations. Performance data for three representative studies (two laboratory-scale and one pilot-scale) are summarized below. In a laboratory-scale study, from an initial concentration of 100 ppm Aroclor 1242, a dichlorobiphenyl congener decreased 93 percent and trichlorobiphenyl congeners decreased 47 to 49 percent in 70 days based on GC peak heights (Brunner, Sutherland, and Focht 1985). In a laboratory-scale study, from an initial concentration of 10 ppm Aroclor 1242, dichlorobiphenyl congeners decreased 95 to 100 percent, trichlorobiphenyl congeners decreased 60 to 100 percent, and tetrachlorobiphenyl congeners decreased 0 to 95 percent based on GC peak heights (Bedard and others 1987) . In a pilot-scale in situ field demonstration, total PCBs decreased from 50 to 14 ppm after 73 days (Harkness and others 1993). The results of these studies provide evidence that sequential anaerobic-aerobic treatment can reduce the levels of PCBs in contaminated materials, but do not demonstrate that the site- specific PCB cleanup standards can be achieved. Although the contaminant source is not immediately removed during implementation of this alternative, the contaminant levels should be gradually reduced as biodegradation proceeds and the potential for migration will not be increased if the clay layer is not breached. However, the CD requires that the entire clay layer underlying the lagoon be treated. Treatment of the entire clay layer by in situ biological treatment would require shearing the clay and mixing it with the sludge to increase the bioavailability of PCBs in the clay and increase contact between microorganisms and amendments. Because the clay layer underlying the lagoon provides a natural barrier to leachate migration, treatment of the entire clay layer by in situ biological treatment would increase the risk of exposure to the community caused by potential groundwater contamination. Shearing the clay would involve removing it, thereby eliminating the protective barrier it provides. However, the integrity of the clay layer would be maintained, and the clay layer would continue to provide protection to the groundwater during implementation if only the clay contaminated with PCBs above the cleanup standard were scraped up and treated. In 1983, PCB contamination was detected in only three of eight clay core samples collected from the lagoon at concentrations of 3, 9, and 15 ppm (ISDH 1994). Although more current data are needed, results indicate that if the proposed TSCA Spill Cleanup Policy industrial cleanup standard of between 10 and 25 ppm PCBs is applied, the integrity of the clay layer would most likely be maintained if only clay exceeding the industrial cleanup standard requires treatment. Adequate controls such as runoff collection and leachate collection included in this alternative should also prevent leachate migration. Long-term institutional controls with this alternative could include groundwater monitoring and land-use restrictions. Factors that Influence Performance The factors that influence the effectiveness of biological treatment include contaminated material characteristics, use of amendments, and use of indigenous microbial populations versus use of inoculations with isolated exogenous strains. These factors are discussed below. Contaminated Material Characteristics Contaminated material characteristics that influence the effectiveness of biological treatment include the types of contaminants, the degree of chlorination of PCB congeners, the concentration of total PCBs, the bioavailability of contaminants, and the amount of solid waste and heterogeneities in the contaminated materials treated. In general, biological treatment is well established for certain classes of petroleum hydrocarbons but is still emerging for most other classes of VOCs and SVOCs. Biological treatment has not been demonstrated for treatment of dioxins and furans. Although microorganisms cannot destroy metals, they are capable of altering the reactivity and mobility of the metals. However, applying this to biological treatment of metal-contaminated wastes has not been proven (NRC 1993). A high degree of chlorination of PCB congeners may prevent aerobic bacteria from metabolizing the congeners, but anaerobic bacteria are capable of dechlorinating highly chlorinated congeners. During reductive dechlorination, anaerobic bacteria remove chlorine atoms, which are replaced by hydrogen atoms, while the biphenyl rings remains intact (Mohn and Tiedje 1992). The lightly chlorinated PCB congeners that result from anaerobic dechlorination are amenable to destruction by aerobic microorganisms. Therefore, full-scale biological treatment of highly-chlorinated PCBs most likely requires sequential anaerobic-aerobic biological treatment (Abramowicz and others 1993; Alexander 1994; Bedard and others 1987; NRC 1993; and Quensen, Tiedje, and Boyd 1988). The maximum concentration of PCBs at which microbial life can no longer be sustained has not been determined. In 1993, the American Chemical Society conducted a study of biodegradation of high concentrations of PCBs in the Hudson River sediment. The study showed that the microbial activity, which was measured as the rate of anaerobic dechlorination of PCBs, plateaued at about 800 ppm and remained at that level of activity as the PCB concentration was increased to 1,500 ppm (Abramowicz and others 1993). These results indicate that microorganisms are capable of surviving and degrading PCBs at concentrations of up to 1,500 ppm, which is higher than the average PCB concentration in the contaminated materials at the CD sites. However, an absolute upper limit to the PCB concentration at which microorganisms can survive is unavailable because microorganism survival is also impacted by other factors that are site- and waste-specific. In the laboratory, anaerobic dechlorination has been observed to occur faster and to a greater extent at higher initial total PCB concentrations (Abramowicz and others 1993; Quensen, Tiedje, and Boyd 1988; and Risatti 1992). This dependence of dechlorination on PCB concentration may be related to PCB bioavailability. Bioavailability is the degree to which contaminants are available for degradation by microorganisms. Higher total PCB concentrations result in higher solution concentrations in pore water according to partitioning equilibria, and probably only PCBs in solution are available for uptake by the dechlorinating microorganisms (Quensen, Tiedje, and Boyd 1988). At the same time, however, higher total PCB concentrations also result in higher concentrations of PCBs resistant to biodegradation because they are not bioavailable (PRC 1995e). One mechanism that reduces bioavailability is sorption to solids, which, for nonpolar compounds such as PCBs, is largely dependent on the organic fraction of the soils or sludges (Alexander 1994). Aerobic degradation appears more limited by bioavailability than does anaerobic dechlorination, possibly because anaerobic dechlorination processes occur slower than aerobic processes, which allows more time for desorption (PRC 1995e). Because PCBs are present in the lagoon sludges at concentrations of up to about 5,000 ppm and because these sewage treatment lagoon sludges most likely have a high organic content, bioavailability may be a primary factor determining whether the cleanup standard can be achieved within the allowable time. Solid wastes and heterogeneities in contaminated materials create preferential flow paths that limit contact of the amendment solution and oxygen with microorganisms, causing transfer limitations. In addition, capacitors contain liquid PCBs that cannot be effectively biodegraded. For these reasons, the effectiveness of this alternative depends on shredding or crushing the biodegradable solid wastes and removing the nonbiodegradable solid wastes and capacitors from the contaminated materials. Additional materials handling and pretreatment may also be required to shear any excavated pieces of clay or other tightly bound soils. Amendment distribution piping and aeration piping can be spaced more closely if transfer limitations are not resolved by additional materials handling and pretreatment. The lagoon sludges at the Winston-Thomas Sewage Treatment Plant site are not expected to contain solid wastes and capacitors, and material mixing should adequately homogenize the sludges. Amendments Biodegradation of PCBs may require amendment of the contaminated material with organic substrates and inorganic nutrients. PCB-degrading bacteria able to metabolize PCBs may be unable to use PCBs as a carbon source for growth or as an energy source; therefore, enrichment of the PCB-contaminated material using a nonhazardous PCB analog as an amendment may be needed to support the growth of PCB-degrading bacteria (Brunner, Sutherland, and Focht 1985). In particular, amendment with biphenyl has been shown to have a co-metabolic effect on PCB degradation in laboratory-scale studies (Anid, Ravest-Webster, and Vogel 1993; and Brunner, Sutherland, and Focht 1985) and in field pilot-scale studies (Harkness and others 1993). In addition to an organic carbon and energy source, bacteria require other inorganic nutrients for growth. In certain environments, nitrogen, phosphorous, or both may be the limiting nutrients, and addition of these nutrients may stimulate biodegradation (Alexander 1994). During one in situ pilot-scale field study, the number of anaerobic PCB-degrading bacteria increased by at least six orders of magnitude in response to the addition of ammonia-nitrogen, phosphate, and oxygen (Harkness and others 1993). Use of Indigenous Microbial Strains Vs. Inoculations of Isolated Exogenous Strains In laboratory-scale studies, strains of PCB-degrading bacteria have been isolated, grown on a PCB-analog substrate, and inoculated into PCB-contaminated samples. Certain strains were successful in the laboratory at enhancing PCB biodegradation (Bedard and others 1987; and Brunner, Sutherland, and Focht 1985). During field experiments, however, exogenous strains have been shown to survive poorly when introduced to natural environments (Harkness and others 1993). The number of sites at which known PCB-dechlorinating microorganisms have been found suggests that PCB dechlorination may be the result of a common reductive pathway present in many different anaerobic microorganisms (Abramowicz 1994). Although the lagoon sludge has most likely been under anaerobic conditions for several years and the PCBs appear to have undergone very little dechlorination (Brown 1994), anaerobic bacteria capable of dechlorinating PCBs are most likely present but require stimulation using a PCB-analog substrate such as biphenyl or 2,6-dibromobiphenyl (PRC 1995e). System Limitations In general, limitations of bioremediation include the factors below. Bioremediation of PCBs has not been proven at full scale. A large amount of space is required for CSPB. Excavation of contaminated soils is required. Treatability testing should be conducted to determine the biodegradability of contaminants and appropriate oxygen and nutrient loading rates. Similar batch sizes require more time to complete cleanup using CSPB than using slurry phase processes. Low bioavailability of contaminants may result in high residual concentrations. The presence of co-contaminants such as heavy metals, oil and grease, or other unknown compounds that may be toxic or inhibitory to the microorganisms may inhibit bioremediation. Most of these limitations may be overcome, but doing so adds to the total cost and problems associated with managing the biotreatment system.