ABSTRACT

INTRODUCTION

Bioremediation uses naturally occurring microorganisms to degrade various types of wastes. Like all living creatures, microbes need nutrients, carbon, and energy to survive and multiply. Such organisms are capable of breaking down organic contaminants to obtain food and energy, typically degrading them into simple organic compounds, carbon dioxide, water, salts, and other harmless substances. This technology has been proven to work on diverse waste streams, especially petroleum hydrocarbons, and more recently on recalcitrant chlorinated solvents.

Bioremediation has been shown to be an efficient and cost-effective treatment method for the cleanup of contaminated soils. As such, it has become one of the most promising technologies to consider in remediating contaminated sites in North America. In this paper, we highlight three different decontamination studies in North America that successfully used bioremediation to reduce petroleum hydrocarbon and chlorinated solvent concentrations to acceptable regulatory levels.

SITE HISTORIES

Petroleum Hydrocarbon Site: A former #6 fuel oil underground storage tank existed at a Central California winery, located in a desert climate. The fuel oil was used in the boiler operation at the winery. Over time, the fuel oil leaked through the wooden storage tank. Approximately 6,000 cubic yards of contaminated soil were impacted. The soil was excavated, removed and spread out on an adjacent 4-acre field. Twelve treatment cells were marked and constructed, each containing 500 cubic yards of soil one foot deep.

After treatment cell construction, soil sampling and chemical analysis was conducted to determine the initial petroleum-hydrocarbon concentration, pH, and moisture content.

Chlorinated Solvent Sites:

MATERIALS & METHODS

Petroleum Hydrocarbon Site: Soil samples were collected and analyzed using standard methods and State of California certified laboratories. Petroleum hydrocarbon concentrations were determined using EPA Methods 418.1 and 8015M (diesel standard). Moisture contents and pH were determined using Methods D2216-92 and 9045, respectively. Toxicity Characteristic Leaching Procedure (TCLP) analyses were determined using State of California Leaking Underground Fuel Tank Field Manual methods.

BioWorld Waste Treatment products consisting of bioenhancement liquid nutrients, vitamins, minerals and other compounds and selected, naturally occurring hydrocarbon digesting microorganisms were added to a 4,000 gallon water truck and evenly applied to the treatment cells. This procedure was repeated on a monthly basis during three summer months. Mechanical discing of the cells was conducted to provide soil mixing and aeration. Water was added to maintain reasonable moisture content in the cells.

Chlorinated Solvent Sites:

RESULTS & DISCUSSION

Petroleum Hydrocarbon Site: The contaminated soil consisted primarily of sandy loam. Initial petroleum hydrocarbon concentrations in the cells ranged from 2,260 to 4,680 mg/kg or parts per million (ppm) with an average of 3,200mg/kg (ppm). Initial moisture content and pH were 9.3% and 8.0, respectively.

After 1 month of treatment and prior to the second scheduled treatment, soil sampling and chemical analysis was conducted. The petroleum hydrocarbon concentration in the cells ranged from 940 to 2,600 mg/kg with an average of 1,820 mg/kg, or a 43% reduction.

Additional treatments, soil sampling, and chemical analysis indicated steady reduction in the total petroleum hydrocarbon (TPH) concentration. After 11 months the average TPH concentration was reduced to approximately 125 ppm. The target level for closure was 100 ppm. This is shown graphically in Figure 1.

When breaking down long-chain petroleum hydrocarbons using bioremediation, problems with recalcitrant organics can occur. We encountered interference with the infrared method (418.1) for determining the TPH concentrations. This method is non-specific for petroleum hydrocarbons since it measures the absorbance of the carbon-hydrogen stretch. Any compound containing carbon-hydrogen bonds, such as fatty acids and other break down products, will be included in the 418.1 analysis. To overcome this challenge, the gas chromatography-flame ionization detector (GC-FID) method (8015M) was used with diesel fuel as the standard. This gave better resolution since peak heights could be directly viewed and measured.

Using the GC-FID method, the analyses indicated a broad hump of unresolved substances at the end of the diesel range. This generally occurred at a carbon chain length of 25 or greater. These residues appeared to be stable, non-reactive, non-water soluble and non-biodegradable. Since the final TPH concentrations measured were slightly higher than the target level for closure, the question arose as to whether the residual substances remaining represented an environmentally acceptable endpoint. In order to answer this question, the leachability potential of the residual soil material was evaluated. Toxicity Characteristic Leaching Procedure (TCLP) extractions and analyses were completed to simulate the addition of winery wastewater to the residual soils.

The TCLP results indicated that 0.8 to 1.7% of the TPH as Diesel present in the soil had the potential to leach out by using aggressive extraction methods. These results agree favorably with the extensive research indicated in the Gas Research Institute report. Effective bioremediation can reduce hydrocarbon concentrations to levels where they no longer pose a threat to human health or the environment. Based on these findings, the regulatory agencies granted closure of the site.

Chlorinated Solvent Sites:

REFERENCES

1. US EPA, Understanding Bioremediation, EPA/540/2-91/002, February 1991.
2. US EPA, Bioremediation: Innovative Pollution and Treatment Technology, EPA/640/K-93/002, November 1993.
3. Miller, Michael, “Bioremediation on a Big Scale“, Environmental Protection, July 1995, pp. 15-16.
4. State of California Department of Health Services, “Biological Remediation of a Fuel Contaminated Soil Site in Carson, California”, March 1990.
5. Hardy, John J., and Mabbula, Sunand, “Close a Site for $3,000”, Soils, March 1994, pp. 30-35.
6. Baker, Katherine H. and Herson, Diane S., Bioremediation, McGraw-Hill Inc., 1994.
7. State of California Water Resources Control Board, Leaking Underground Fuel Tank Field Manual, October 1989.
8. Gas Research Institute, “Environmentally Acceptable Endpoints in Soil – Draft Report”, April 1995.

TABLE 1. FUEL OIL BIOREMEDIATION RESULTS

TOTAL PETROLEUM HYDROCARBON CONCENTRATIONS

BY SAMPLE DATE

(All concentrations are mg/kg or ppm)