Bioremediation A Challenging Application of Biochemical Engineering Principles" L. E. ERICKSON, J. P. McDONALD, L. T. FAN, S. DHAWAN, A N D P. TUITEMWONG Department of Chemical Engineering Kansas State University Manhattan, Kansas 66506
INTRODUCTION Some of the greatest challenges for scientists and engineers are in the environmental arena. Biochemical engineers have the necessary background to significantly contribute to this arena through the development of bioremediation technology. Microorganisms transform numerous organic compounds in wastewater treatment plants, sanitary landfills, compost piles, agricultural fields, forests, and spill sites polluted by organic contaminants. Bioremediation is concerned with the biodegradation of organic compounds to nontoxic forms in order to improve the environmental quality of a site. Because of past practices, leaking underground storage tanks, and other spills, opportunities abound to clean up contaminated sites. Frequently, it is desirable to remediate such sites in situ to minimize environmental disturbance and the disruption of productive activities at the site. Other bioremediation alternatives include bringing the contaminated soil to the surface followed by land farming o r treatment in a slurry reactor. Some recent reviews on bioremediation are found in references 1-8. This work reviews biochemical engineering problems and opportunities associated with the biorernediation of contaminated soil and groundwater. Although the emphasis here is on biochemical engineering, it should be pointed out that a team effort is needed to investigate and remediate a contaminated site. Team members could include a soil scientist such as a soil chemist o r soil microbiologist, a civil engineer with specialization in hydrogeology and groundwater flow, a geologist with experience in site characterization, and a biochemical engineer.
aThis research has been funded in part by the United States Environmental Protection Agency (EPA) under Assistance Agreement No. R-815709 to the Hazardous Substance Research Center for EPA Regions 7 and 8 with headquarters at Kansas State University, but it has not been subjected to the Agency's peer and administrative review; therefore, it may not necessarily reflect the views of the Agency and no official endorsement should be inferred. The research was also partially supported by the Kansas State University Center for Hazardous Substance Research. 404
EIUCKSON et al.: BIOREMEDIATION
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BIOCHEMICAL ENGINEERING CHALLENGES Bioremediation frequently involves organic compounds with limited solubilities in water. Thus, the biochemical engineer must often consider two or more material phases in developing models of bioremediation and in designing technologies for it. The vadose or unsaturated zone consists of the soil, aqueous, and gas phases when the organic compounds do not form a second liquid phase. Nevertheless, it is common that these organic compounds or oil will appear as a separate phase; this oil phase can be either liquid or solid. The saturated zone often contains two liquid phases and one or more solid phases. With new developments in horizontal drilling technology, it is possible to supply air bubbles to the saturated zone as well.
Transport Phenomena
For biodegradation to occur, microorganisms need to possess the necessary genetic material to degrade the substrates, and all of the necessary nutrients must be available to these microorganisms. Transport phenomena play significant roles in bioremediation; because the concentrations of the organic contaminants are low and the oxygen and other inorganic nutrients are difficult to distribute in soil systems, the rates of transport of these substances can be the rate-controlling factors for bioremediation. Specific challenges that are transport-related include (1) transport and distribution of oxygen for aerobic biodegradation; (2) transport and distribution of inorganic nutrients; (3) transport and distribution of adapted cultures that have the ability to transform a particular substrate; and (4) transport of organic contaminants to the cell surface.
These transport problems can take different forms depending on the heterogeneity of the subsurface spill. The first example considered here is the problem of clay layers or other forms of soil aggregates in which an organic contaminant is dissolved in the aqueous phase and is also adsorbed to the soil surface. Although there may be convective flow through the macrovoids that surround the aggregates, some investigators (including o u r s e l ~ e s ) ~ have ~ ~ - l reported ~ that the transport within the aggregates is largely by diffusion. Brusseau and RaoI2 have reviewed sorption and transport in porous media including diffusion within aggregates. In the second example considered, an organic phase is present and most of the contaminants reside as droplets in pores in the soil. For numerous hydrocarbons with limited solubilities, a significant portion of the biodegradation occurs at the surfaces of the oil drop^.'^-*^ Because oxygen requirements are large for the biodegradation of hydrocarbons, oxygen diffusion through the aqueous phase is necessary for oxygen transfer to the oil-water interface where microorganisms slowly consume the organic contaminants. For hydrocarbons lighter than water, the second liquid phase frequently accumulates o n top of the water in the saturated zone; the oil phase is mixed and distributed into soil pores as the water table moves u p and down. More volatile
ANNALS NEW YORK ACADEMY OF SCIENCES
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compounds evaporate from the site, thereby leaving the less volatile, higher molecular weight fractions. This elevates the viscosity of the oil phase and transforms it into a tarlike material. AtlasI5 has reported that hydrocarbon liquids appear to be more available to microorganisms than hydrocarbon solids. Transport of hydrocarbons to the cell surface as well as transport into the cell appear to be facilitated by these hydrocarbon liquids. Chlorinated hydrocarbons and other chemicals denser than water pass down through the water to the bottom of the aquifer; in situ bioremediation of these chemicals is difficult because of hydrogeological hindrance as well as nutritional and genetic requirements.'"'* The transport phenomena of organic contaminants in saturated soils are highly complex because of the heterogeneity of soil; diffusivities in liquids are much larger than diffusivities in solids. Correlations to predict diffusivities in liquids depend on
1000
I.o X
f
0
E 0.8
100
3 \ 3
2 3 -
0.4
0
m 0.2
0 0
0.1
0.2
0.3
0.4
0.5
Water Content, cm3/crn3
FIGURE 1. Effect of solute concentration (mg/kg) and soil water content on relative solute flux (from Papendick and Campbell").
the viscosities of the 1iq~ids.I~ For fine clays and silts saturated with water, the appropriate diffusivity to use in modeling contaminant transport is difficult to determine because of the effects of viscosity, tortuosity, adsorption to surfaces, and multiphase complexity. Furthermore, Rao et dZ0 have shown that chemical solubility and adsorption phase equilibrium depend on the composition of chemical mixtures. Is mud a very viscous liquid suspension or a porous solid saturated with an aqueous phase? Diffusivities should be intermediate between those for the substance in water and those in a solid matrix. The experimental results of Ball and Roberts*l are within this range. It appears that the properties of the soil should be characterized for modeling contaminant transport by diffusion in the subsurface. The third example is concerned with the vadose zone; the diffusion of substrate is dependent on its concentration and the water c ~ n t e n tFIGURE . ~ ~ ~ 1, ~ from ~ the work
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of Papendick and Campbell,22illustrates how the diffusivity of a solute varies with the water content in the vadose zone for three different values of the solute concentration: the higher the water content, the greater the solute diffusivity; the lower the water content, the smaller the solute diffusivity. Nevertheless, oxygen transfer is more effective in the latter situation because more of the volume is available for the gas phase transport of oxygen. It is well known that the water table can be varied or lowered at any site. Consequently, the choice of remediation conditions corresponding to the vadose or saturated zone may be made by the engineer. Under certain circumstances, the optimal solution may be to move the water table cyclically based on a predetermined plan. Such a plan should take advantage of the fact that the oxygen transport is facilitated in the vadose zone and the nutrient and contaminant transport is facilitated under the saturated conditions. Biodegradability The review of grad^^^ covers essential background on the microbial, genetic, nutritional, and ecological aspects of biodegradation. It also covers methods for investigating biodegradability including the degree or extent of biodegradation and the kinetics of substrate utilization. Several aspects must be considered in conducting a treatability study. Exploratory investigation should be performed in such a way that the transport limitations mentioned earlier are minimized. This can be accomplished by adding sufficient water to the soil sample, whereby small soil particles remain suspended in the aqueous phase, and thus the formation of large soil aggregates is avoided. Agitation can be applied to maintain the soil particles in suspension. The extent of the reaction and the kinetics of growth can be studied under these ideal conditions; however, the necessary information must also be assembled to estimate the extent and rate of the reaction under conditions where transport limitations may be important. In some cases, it may be necessary or desirable to investigate treatability in soil microcosms with transport limitations. The measured extent of the reaction may b e somewhat different under these conditions. Because of the small volumes used for treatability studies, the experimental design must include controls in which losses due to volatilization can be evaluated. The resultant experimental data should b e analyzed in the light of mass balances. Thermodynamic information is required to model and design bioremediation processes. The solubilities of the contaminants in water and their phase equilibrium behavior should be known. BouchardZ5reports that the aqueous phase usually wets the soil surface when two liquid phases coexist. The phase equilibrium relationship determines the concentrations of the organic compounds at the interface between these two liquid phases. The Henry’s law relationship governs the gas phase concentration in equilibrium with the liquid phase. Frequently, phase equilibrium relationships are necessary for multicomponent systems because mixtures such as gasoline and diesel fuel are spilled. B o ~ c h a r d *has ~ reviewed the literature that reports that the equilibrium adsorption of hydrocarbons onto soil increases as the organic content of the soil increases. The free energy changes associated with the chemical reactions provide the thermodynamic driving force for biodegradation. The bioenergetics of the aerobic
408
ANNALS NEW YORK ACADEMY OF SCIENCES
metabolism of petroleum products has been investigated by various researchers. However, studies should be extended to low substrate concentrations. This is especially true for the multiphase problems commonly found in contaminated soil. Little quantitative information is available on the achievable limit or extent of biodegradation. Moreover, the minimum substrate concentration sufficient to maintain a culture in a soil system is not known for many substrates. Because contaminant mixtures, soils, and site hydrogeology are highly complex, the proper design of treatability studies to evaluate bioremediation alternatives requires input from expert professionals. Additional research on biotreatability methodologies for contaminated soil is required. It is imperative that an expert system be developed to assist those in the field with the design of treatability studies.
Mixed Culture Interactions and Microbial Ecohgy
Mixed cultures tend to flourish at field sites where bioremediation is applied; nevertheless, a variety of interactions contributing positively to biodegradation are not well understood. Microorganisms associated with the roots of plants and trees live in an enriched environment. Although it is known that the root environment enhances bioremediation, additional studies should be undertaken. The effects of protozoa, earthworms, and other organisms on bioremediation and the ecology of the site must be studied further.
MODELING BIOREMEDIATION IN AGGREGATED SOIL
The following example illustrates the application of biochemical engineering principles to bioremediation of contaminated soil and groundwater. Only a brief description is given because the details are available e l ~ e w h e r e . ~ J ~ ~ ~ ~ ~ ~ A model has been developed and simulated to estimate bioremediation time in heterogeneous soils containing aggregates. Transport within the aggregates is assumed to be solely by diffusion. The Monod model is adopted to represent the kinetics of biodegradation; oxygen and dissolved contaminants are included as substrates in the model. The kinetic model for biomass includes both growth and endogenous metabolism. Linear phase equilibrium relationships are assumed for the adsorption of the contaminants and microbial biomass to the solid surfaces of the aggregates. The model equations for an individual aggregate consist of a system of three nonlinear partial differential equations for organic contaminant, oxygen, and microbial biomass. Models for flow and reaction in the saturated macrovoids have been combined with the model for diffusion and reaction in the aggregates to simulate bioremediation for a wide variety of cases. Sensitivity analysis conducted by numerical simulation has demonstrated the effects of the partition coefficient, the aggregate radius, and the initial contaminant concentration on the time and mechanism of remediation. Results are summarized in TABLES1 and 2 for diffusion and reaction in a single aggregate (CAB model) and for aggregates of the same size with flow in the macropores described by a completely mixed flow model. For comparison, the time
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TABLE1. Effect of Partition Coefficient and Aggregate Radius on the Time Required for Bioremediation in the Bed Macropore Flow Model
CAB Model Initial Radius of Substrate Case Concentration Aggregate No. (R, cm) (qS0, mg/kg) 600 600 600 600 600 600 600
1 1
1 1 0 0.1 10
BiorePartition mediation Coefficient Diffusion Time Time (Tdrdays) (Thy days) (&sr cm3//g) 1.5 15 150 1500 15 15 15
46.0 407.1 4012.8 40073.8
7.3 17.4 24.7 41.3
-
-
4.1 40641.7
2.2 1660.9
Bioremediation Time (Th, days) 13.6 22.6 31.1 48.7 7.4 7.6 1650.3
required for the contaminant to diffuse out of the aggregate is listed. The diffusion time is obtained by solving the CAB model equations without biodegradation. For each case, the time corresponds to that necessary to reduce the contaminant concentration to below 1 ppb everywhere within the aggregates. In situ bioremediation is shown to be faster than pump-and-treat technologies that necessitate the contaminant to diffuse out of the aggregates. The time required for in situ bioremediation is smaller than that for the contaminant to diffuse out of the aggregates. Whenever both the biomass and the contaminant are in the aggregates, oxygen is transported into the aggregate to effect biodegradation; in general, the rate of oxygen diffusion is substantially greater than that of the contaminant. The time required for the contaminant to diffuse out of the aggregate is significant when the partition coefficient is large. When the contaminant is adsorbed strongly to the soil, its dissolved concentration is relatively small; under this situation, in situ bioremediation appears to be advantageous. The estimated bioremediation time is less than 1% of the diffusion time for two of the cases. Although soil aggregates in the field are not usually spherical, the radius of the model aggregate serves as a representative distance for diffusion within the aggreTABLE2. Effect of Initial Substrate Concentration on the Time Required for Bioremediation in the Bed Macropore Flow Model
CAB Model Initial Radius of Substrate Case Concentration Aggregate (R, cm) No. (qS0. mg/kg) 1 8 15 9 2 10 11
150 600 1500 15,000
1 1 1 1
Bioremediation Partition Time Coefficient Diffusion Time (Td, days) (Th, days) (&isI cm3/g) 15 15 15 15 15
287.6 354.4 407.1 421.26 488.1
4.0 6.5 11.4 31.2 83.9
Bioremediation Time (Th, days) 3.9 8.1 22.6 41.3
-
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ANNALS NEW YORK ACADEMY OF SCIENCES
gate. When the aggregate radius is 10 cm or larger, the estimated remediation time is much shorter if in situ bioremediation is utilized. The remediation is mainly effected by the diffusion of oxygen into the aggregates. When the aggregate radius is less than 0.1 cm, the flow rate of oxygen into the bed may be rate-limiting; the estimated time for diffusion of the contaminant out of the aggregate is on the same order of magnitude as that of bioremediation. For this case, pump-and-treat may be a suitable method. The results illustrate that soil aggregates in the subsurface may be remediated; when the aggregate size is large, it may be expedient to decrease the remediation time by size reduction. The estimated remediation time is much smaller for a completely mixed and homogeneous soil bed than for an aggregated bed. The results of simulation have demonstrated that the availability of oxygen frequently controls the rate of remediation.
REFERENCES
1. DAVIS,A. & R. L. OLSEN.1990. Predicting the fate and transport of organic compounds in groundwater. Hazardous Mater. Control 3(4): 18-37, L. E. & L. T. FAN.1988. Anaerobic degradation of toxic and hazardous wastes. 2. ERICKSON, In Handbook on Anaerobic Fermentations. L. E. Erickson & D. Y. C. Fung, Eds.: 695-732. Dekker. New York. 3. LEE, M. D., J. M. THOMAS, R. C. BORDEN, P. B. BEDIENT, C. H. WARD& J. T. WILSON. 1988. Biorestoration of aquifers contaminated with organic compounds. CRC Crit. Rev. Environ. Control 18: 29-89. 4. MACKAY, D. M. & J. A. CHERRY.1989. Groundwater contamination: pump-and-treat remediation. Environ. Sci. Technol. 23: 63c636. 5. OLSEN,R. L. &A. DAVIS.1990. Predicting the fate and transport of organic compounds in groundwater. Hazardous Mater. Control 3(3): 38-64. 1989. Bioremediation of Contaminated Surface 6. SIMS,J. L., R. C. SIMS& J. E. MA~THEWS. Soils. EPA/600/9-89/073. Robert S. Kerr Environmental Research Laboratory. Ada, Oklahoma. 7. SIMS,J. L., R. C. SIMS& J. E. MATIMEWS. 1990. Approach to bioremediation of contaminated soil. Hazardous Waste Hazardous Mater. 7: 117-149. 8. THOMAS, J. M. & C. H. WARD.1989.I n situ biorestoration of organic contaminants in the subsurface. Environ. Sci. Technol. 23: 760-766. 9. ERICKSON, L. E., L. T. FAN,S. DHAWAN & P. TUITEMWONG. 1990. Modeling, analysis, simulation, and bioremediation of soil and water. In Proceedings of the Conference on Hazardous Waste Research. L. E. Erickson, Ed.: 1746. Kansas State University. Manhattan, Kansas. 10. FAN,L. T., S. DHAWAN & L. E. ERICKSON. 1991. Modeling and simulation of biorestoration of contaminated aggregated soil. Paper presented at the Fourth World Congress of Chemical Engineering, Karlsruhe, Germany, June 1621,1991. 11. RAO, P. S. C., R. E. JESSUP& T. M. ADDISCOTT.1982. Experimental and theoretical aspects of solute diffusion in spherical and nonspherical aggregates. Soil Sci. 133: 342349. 12. BRUSSEAU, M. L. & P. S. C. RAO.1989. Sorption nonideality during organic contaminant transport in porous media. CRC Crit. Rev. Environ. Control 1 9 33-99. 13. ERICKSON, L. E., T. NAKAHARA & A. PROKOP. 1975. Growth in cultures with two liquid phases; hydrocarbon uptake and transport. Process Biochem. lO(5): 9-13. 14. SHAH,P. S., L. T. FAN,I. C. KAo & L. E. ERICKSON. 1972. Modeling of growth processes with two liquid phases: a review of drop phenomena, mixing, and growth. I n Advances in Applied Microbiology. Vol. 15. D. Perlman, Ed.: 367414. Academic Press. New York.
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15. ATLAS,R. M. 1981. Microbial degradation of petroleum hydrocarbons: an environmental perspective. Microb. Rev. 4 5 180-209. 16. SEMPRINI, L. & P. L. MCCARTY.1991. Comparison between model simulations and field results for in situ biorestoration of chlorinated aliphatics: Part 1. Biostimulation of methanotrophic bacteria. Ground Water 2 9 365-374. 17. GIBSON,D. T., Ed. 1984. Microbial Degradation of Organic Compounds. Dekker. New York. 18. ROCHKIND-DUBINSKY, M. L., G. S. SAYLER& J. W. BLACKBURN. 1987. Microbiological Decomposition of Chlorinated Organic Compounds. Dekker. New York. 19. PERRY,R. H. & D. W. GREEN,Eds. 1984. Perry’s Chemical Engineers’ Handbook. Sixth edition, p. 3-286 to 3-287. McGraw-Hill. New York. 20. RAO, P. S. C., L. S. LEE & A. L. WOOD. 1991. Solubility, Sorption, and Transport of Hydrophobic Organic Chemicals in Complex Mixtures. United States EPA Environmental Research Brief EPA/600/M-91/009. Robert S. Kerr Environmental Research Laboratory. Ada, Oklahoma. 21. BALL,W. P. & P. V. ROBERTS.1990. Slow diffusion of sorbing hydrophobic organic compounds in sandy aquifer material. Preprints of Papers Presented at the 199th ACS National Meeting. Vol. 30(1): 313-317. ACS Division of Environmental Chemistry. Washington, District of Columbia. 22. PAPENDICK, R. I. & G. S. CAMPBELL. 1981. Theory and measurement of water potential. In Water Potential Relations in Soil Microbiology. J. F. Parr, W. R. Gardner & L. F. Elliott, Eds.: 1-22. Soil Science Society of America. 23. RYAN,P. A. & Y. COHEN.1990. Diffusion of sorbed solutes in gas and liquid phases of low-moisture soils. Soil Sci. SOC.Am. J. 5 4 341-346. 24. GRADY,C. P. L. 1985. Biodegradation: its measurement and microbiological basis. Biotechnol. Bioeng. 27: 660-674. 25. BOUCHARD, D. C. 1989. Contaminant transport in the subsurface: sorption equilibrium and the role of nonaqueous phase liquids. In Intermedia Pollutant Transport. D. T. Allen, Y. Coben & I. R. Kaplan, Eds.: 189-211. Plenum. New York. 26. DHAWAN, S., L. T. FAN,L. E. ERICKSON & P. TUITEMWONG. 1991. Modeling, analysis, and simulation of bioremediation of soil aggregates. Environ. Prog. 10 251-260. 27. DHAWAN, S., L. E. ERICKSON & L. T. FAN.1991. Modeling and simulation of bioremediation of soil beds with aggregates. Ground Water. Submitted. 28. DHAWAN, S. 1991. Modeling and simulating bioremediation in aggregated soils. M.Sc. thesis. Kansas State University, Manhattan, Kansas.
INTRODUCTION Some of the greatest challenges for scientists and engineers are in the environmental arena. Biochemical engineers have the necessary background to significantly contribute to this arena through the development of bioremediation technology. Microorganisms transform numerous organic compounds in wastewater treatment plants, sanitary landfills, compost piles, agricultural fields, forests, and spill sites polluted by organic contaminants. Bioremediation is concerned with the biodegradation of organic compounds to nontoxic forms in order to improve the environmental quality of a site. Because of past practices, leaking underground storage tanks, and other spills, opportunities abound to clean up contaminated sites. Frequently, it is desirable to remediate such sites in situ to minimize environmental disturbance and the disruption of productive activities at the site. Other bioremediation alternatives include bringing the contaminated soil to the surface followed by land farming o r treatment in a slurry reactor. Some recent reviews on bioremediation are found in references 1-8. This work reviews biochemical engineering problems and opportunities associated with the biorernediation of contaminated soil and groundwater. Although the emphasis here is on biochemical engineering, it should be pointed out that a team effort is needed to investigate and remediate a contaminated site. Team members could include a soil scientist such as a soil chemist o r soil microbiologist, a civil engineer with specialization in hydrogeology and groundwater flow, a geologist with experience in site characterization, and a biochemical engineer.
aThis research has been funded in part by the United States Environmental Protection Agency (EPA) under Assistance Agreement No. R-815709 to the Hazardous Substance Research Center for EPA Regions 7 and 8 with headquarters at Kansas State University, but it has not been subjected to the Agency's peer and administrative review; therefore, it may not necessarily reflect the views of the Agency and no official endorsement should be inferred. The research was also partially supported by the Kansas State University Center for Hazardous Substance Research. 404
EIUCKSON et al.: BIOREMEDIATION
405
BIOCHEMICAL ENGINEERING CHALLENGES Bioremediation frequently involves organic compounds with limited solubilities in water. Thus, the biochemical engineer must often consider two or more material phases in developing models of bioremediation and in designing technologies for it. The vadose or unsaturated zone consists of the soil, aqueous, and gas phases when the organic compounds do not form a second liquid phase. Nevertheless, it is common that these organic compounds or oil will appear as a separate phase; this oil phase can be either liquid or solid. The saturated zone often contains two liquid phases and one or more solid phases. With new developments in horizontal drilling technology, it is possible to supply air bubbles to the saturated zone as well.
Transport Phenomena
For biodegradation to occur, microorganisms need to possess the necessary genetic material to degrade the substrates, and all of the necessary nutrients must be available to these microorganisms. Transport phenomena play significant roles in bioremediation; because the concentrations of the organic contaminants are low and the oxygen and other inorganic nutrients are difficult to distribute in soil systems, the rates of transport of these substances can be the rate-controlling factors for bioremediation. Specific challenges that are transport-related include (1) transport and distribution of oxygen for aerobic biodegradation; (2) transport and distribution of inorganic nutrients; (3) transport and distribution of adapted cultures that have the ability to transform a particular substrate; and (4) transport of organic contaminants to the cell surface.
These transport problems can take different forms depending on the heterogeneity of the subsurface spill. The first example considered here is the problem of clay layers or other forms of soil aggregates in which an organic contaminant is dissolved in the aqueous phase and is also adsorbed to the soil surface. Although there may be convective flow through the macrovoids that surround the aggregates, some investigators (including o u r s e l ~ e s ) ~ have ~ ~ - l reported ~ that the transport within the aggregates is largely by diffusion. Brusseau and RaoI2 have reviewed sorption and transport in porous media including diffusion within aggregates. In the second example considered, an organic phase is present and most of the contaminants reside as droplets in pores in the soil. For numerous hydrocarbons with limited solubilities, a significant portion of the biodegradation occurs at the surfaces of the oil drop^.'^-*^ Because oxygen requirements are large for the biodegradation of hydrocarbons, oxygen diffusion through the aqueous phase is necessary for oxygen transfer to the oil-water interface where microorganisms slowly consume the organic contaminants. For hydrocarbons lighter than water, the second liquid phase frequently accumulates o n top of the water in the saturated zone; the oil phase is mixed and distributed into soil pores as the water table moves u p and down. More volatile
ANNALS NEW YORK ACADEMY OF SCIENCES
406
compounds evaporate from the site, thereby leaving the less volatile, higher molecular weight fractions. This elevates the viscosity of the oil phase and transforms it into a tarlike material. AtlasI5 has reported that hydrocarbon liquids appear to be more available to microorganisms than hydrocarbon solids. Transport of hydrocarbons to the cell surface as well as transport into the cell appear to be facilitated by these hydrocarbon liquids. Chlorinated hydrocarbons and other chemicals denser than water pass down through the water to the bottom of the aquifer; in situ bioremediation of these chemicals is difficult because of hydrogeological hindrance as well as nutritional and genetic requirements.'"'* The transport phenomena of organic contaminants in saturated soils are highly complex because of the heterogeneity of soil; diffusivities in liquids are much larger than diffusivities in solids. Correlations to predict diffusivities in liquids depend on
1000
I.o X
f
0
E 0.8
100
3 \ 3
2 3 -
0.4
0
m 0.2
0 0
0.1
0.2
0.3
0.4
0.5
Water Content, cm3/crn3
FIGURE 1. Effect of solute concentration (mg/kg) and soil water content on relative solute flux (from Papendick and Campbell").
the viscosities of the 1iq~ids.I~ For fine clays and silts saturated with water, the appropriate diffusivity to use in modeling contaminant transport is difficult to determine because of the effects of viscosity, tortuosity, adsorption to surfaces, and multiphase complexity. Furthermore, Rao et dZ0 have shown that chemical solubility and adsorption phase equilibrium depend on the composition of chemical mixtures. Is mud a very viscous liquid suspension or a porous solid saturated with an aqueous phase? Diffusivities should be intermediate between those for the substance in water and those in a solid matrix. The experimental results of Ball and Roberts*l are within this range. It appears that the properties of the soil should be characterized for modeling contaminant transport by diffusion in the subsurface. The third example is concerned with the vadose zone; the diffusion of substrate is dependent on its concentration and the water c ~ n t e n tFIGURE . ~ ~ ~ 1, ~ from ~ the work
ERICKSON el al.: BIOREMEDlATlON
407
of Papendick and Campbell,22illustrates how the diffusivity of a solute varies with the water content in the vadose zone for three different values of the solute concentration: the higher the water content, the greater the solute diffusivity; the lower the water content, the smaller the solute diffusivity. Nevertheless, oxygen transfer is more effective in the latter situation because more of the volume is available for the gas phase transport of oxygen. It is well known that the water table can be varied or lowered at any site. Consequently, the choice of remediation conditions corresponding to the vadose or saturated zone may be made by the engineer. Under certain circumstances, the optimal solution may be to move the water table cyclically based on a predetermined plan. Such a plan should take advantage of the fact that the oxygen transport is facilitated in the vadose zone and the nutrient and contaminant transport is facilitated under the saturated conditions. Biodegradability The review of grad^^^ covers essential background on the microbial, genetic, nutritional, and ecological aspects of biodegradation. It also covers methods for investigating biodegradability including the degree or extent of biodegradation and the kinetics of substrate utilization. Several aspects must be considered in conducting a treatability study. Exploratory investigation should be performed in such a way that the transport limitations mentioned earlier are minimized. This can be accomplished by adding sufficient water to the soil sample, whereby small soil particles remain suspended in the aqueous phase, and thus the formation of large soil aggregates is avoided. Agitation can be applied to maintain the soil particles in suspension. The extent of the reaction and the kinetics of growth can be studied under these ideal conditions; however, the necessary information must also be assembled to estimate the extent and rate of the reaction under conditions where transport limitations may be important. In some cases, it may be necessary or desirable to investigate treatability in soil microcosms with transport limitations. The measured extent of the reaction may b e somewhat different under these conditions. Because of the small volumes used for treatability studies, the experimental design must include controls in which losses due to volatilization can be evaluated. The resultant experimental data should b e analyzed in the light of mass balances. Thermodynamic information is required to model and design bioremediation processes. The solubilities of the contaminants in water and their phase equilibrium behavior should be known. BouchardZ5reports that the aqueous phase usually wets the soil surface when two liquid phases coexist. The phase equilibrium relationship determines the concentrations of the organic compounds at the interface between these two liquid phases. The Henry’s law relationship governs the gas phase concentration in equilibrium with the liquid phase. Frequently, phase equilibrium relationships are necessary for multicomponent systems because mixtures such as gasoline and diesel fuel are spilled. B o ~ c h a r d *has ~ reviewed the literature that reports that the equilibrium adsorption of hydrocarbons onto soil increases as the organic content of the soil increases. The free energy changes associated with the chemical reactions provide the thermodynamic driving force for biodegradation. The bioenergetics of the aerobic
408
ANNALS NEW YORK ACADEMY OF SCIENCES
metabolism of petroleum products has been investigated by various researchers. However, studies should be extended to low substrate concentrations. This is especially true for the multiphase problems commonly found in contaminated soil. Little quantitative information is available on the achievable limit or extent of biodegradation. Moreover, the minimum substrate concentration sufficient to maintain a culture in a soil system is not known for many substrates. Because contaminant mixtures, soils, and site hydrogeology are highly complex, the proper design of treatability studies to evaluate bioremediation alternatives requires input from expert professionals. Additional research on biotreatability methodologies for contaminated soil is required. It is imperative that an expert system be developed to assist those in the field with the design of treatability studies.
Mixed Culture Interactions and Microbial Ecohgy
Mixed cultures tend to flourish at field sites where bioremediation is applied; nevertheless, a variety of interactions contributing positively to biodegradation are not well understood. Microorganisms associated with the roots of plants and trees live in an enriched environment. Although it is known that the root environment enhances bioremediation, additional studies should be undertaken. The effects of protozoa, earthworms, and other organisms on bioremediation and the ecology of the site must be studied further.
MODELING BIOREMEDIATION IN AGGREGATED SOIL
The following example illustrates the application of biochemical engineering principles to bioremediation of contaminated soil and groundwater. Only a brief description is given because the details are available e l ~ e w h e r e . ~ J ~ ~ ~ ~ ~ ~ A model has been developed and simulated to estimate bioremediation time in heterogeneous soils containing aggregates. Transport within the aggregates is assumed to be solely by diffusion. The Monod model is adopted to represent the kinetics of biodegradation; oxygen and dissolved contaminants are included as substrates in the model. The kinetic model for biomass includes both growth and endogenous metabolism. Linear phase equilibrium relationships are assumed for the adsorption of the contaminants and microbial biomass to the solid surfaces of the aggregates. The model equations for an individual aggregate consist of a system of three nonlinear partial differential equations for organic contaminant, oxygen, and microbial biomass. Models for flow and reaction in the saturated macrovoids have been combined with the model for diffusion and reaction in the aggregates to simulate bioremediation for a wide variety of cases. Sensitivity analysis conducted by numerical simulation has demonstrated the effects of the partition coefficient, the aggregate radius, and the initial contaminant concentration on the time and mechanism of remediation. Results are summarized in TABLES1 and 2 for diffusion and reaction in a single aggregate (CAB model) and for aggregates of the same size with flow in the macropores described by a completely mixed flow model. For comparison, the time
ERICKSON et al.: BIOREMEDIATION
409
TABLE1. Effect of Partition Coefficient and Aggregate Radius on the Time Required for Bioremediation in the Bed Macropore Flow Model
CAB Model Initial Radius of Substrate Case Concentration Aggregate No. (R, cm) (qS0, mg/kg) 600 600 600 600 600 600 600
1 1
1 1 0 0.1 10
BiorePartition mediation Coefficient Diffusion Time Time (Tdrdays) (Thy days) (&sr cm3//g) 1.5 15 150 1500 15 15 15
46.0 407.1 4012.8 40073.8
7.3 17.4 24.7 41.3
-
-
4.1 40641.7
2.2 1660.9
Bioremediation Time (Th, days) 13.6 22.6 31.1 48.7 7.4 7.6 1650.3
required for the contaminant to diffuse out of the aggregate is listed. The diffusion time is obtained by solving the CAB model equations without biodegradation. For each case, the time corresponds to that necessary to reduce the contaminant concentration to below 1 ppb everywhere within the aggregates. In situ bioremediation is shown to be faster than pump-and-treat technologies that necessitate the contaminant to diffuse out of the aggregates. The time required for in situ bioremediation is smaller than that for the contaminant to diffuse out of the aggregates. Whenever both the biomass and the contaminant are in the aggregates, oxygen is transported into the aggregate to effect biodegradation; in general, the rate of oxygen diffusion is substantially greater than that of the contaminant. The time required for the contaminant to diffuse out of the aggregate is significant when the partition coefficient is large. When the contaminant is adsorbed strongly to the soil, its dissolved concentration is relatively small; under this situation, in situ bioremediation appears to be advantageous. The estimated bioremediation time is less than 1% of the diffusion time for two of the cases. Although soil aggregates in the field are not usually spherical, the radius of the model aggregate serves as a representative distance for diffusion within the aggreTABLE2. Effect of Initial Substrate Concentration on the Time Required for Bioremediation in the Bed Macropore Flow Model
CAB Model Initial Radius of Substrate Case Concentration Aggregate (R, cm) No. (qS0. mg/kg) 1 8 15 9 2 10 11
150 600 1500 15,000
1 1 1 1
Bioremediation Partition Time Coefficient Diffusion Time (Td, days) (Th, days) (&isI cm3/g) 15 15 15 15 15
287.6 354.4 407.1 421.26 488.1
4.0 6.5 11.4 31.2 83.9
Bioremediation Time (Th, days) 3.9 8.1 22.6 41.3
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ANNALS NEW YORK ACADEMY OF SCIENCES
gate. When the aggregate radius is 10 cm or larger, the estimated remediation time is much shorter if in situ bioremediation is utilized. The remediation is mainly effected by the diffusion of oxygen into the aggregates. When the aggregate radius is less than 0.1 cm, the flow rate of oxygen into the bed may be rate-limiting; the estimated time for diffusion of the contaminant out of the aggregate is on the same order of magnitude as that of bioremediation. For this case, pump-and-treat may be a suitable method. The results illustrate that soil aggregates in the subsurface may be remediated; when the aggregate size is large, it may be expedient to decrease the remediation time by size reduction. The estimated remediation time is much smaller for a completely mixed and homogeneous soil bed than for an aggregated bed. The results of simulation have demonstrated that the availability of oxygen frequently controls the rate of remediation.
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