, Research Paper
Bioremediation of Explosives in Contaminated Soil
Abstract
TNT is not the kind of substance that most people think of composting, but it can be done! At several U.S. Army depots, the water used in processing explosives was disposed of through evaporation from unlined lagoons. This has resulted in sediments and soils that are contaminated with TNT (2,4,6- trinitrotoluene) and its derivatives RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) and HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetraazocine). One way of cleaning up these sediments is by incinerating them. A less expensive and more environmentally friendly method is “bioremediation,” or use of natural biological and chemical processes to degrade the contaminants. That’s where composting comes in. TNT, RDX, and HMX are complex organic compounds made up of carbon, nitrogen, oxygen, and hydrogen. When combined with more conventional compost ingredients such as manures, sawdust, straw, and fruit and vegetable processing wastes, the explosives become broken down into harmless chemical forms. The Army is using composting to clean up munitions processing wastes at several of its ammunition plants, including ones in Louisiana, Wisconsin, and Oregon. At the Oregon site, composting is projected to save 2.6 million dollars compared with incinerating the contaminated soils. In addition to saving money, composting will also avoid the need for burning fossil fuels and will produce an end product usable for backfilling, landscaping, or erosion control.
Introduction
Whether people know it or not explosives, such as tnt, are a very important part of their lives. Since World War I, munitions have been manufactured in the United States using a variety of energetic materials, including propellants, explosives, and pyrotechnic (PEP) materials. (Noyes, 1996) Many manufacturing sites contain explosives. As a result they have contaminated soil because of prior and existing operations. These contaminated soils are can be very toxic and mutagenic. It is estimated that the U.S. Army has 40 sites requiring cleanup of explosives-contaminated soils.(Noyes, 1996) Currently, regulatory agencies only approve incineration and composting as decontamination technologies. Incineration was commonly accepted by the public in the late 1980’s, but the acceptance of this is now declining. Composting costs can also be high due to the necessary things needed for the composting process. For these reasons the Army has invested in developing and demonstrating other biotreatment technologies. The Army has been searching for alternatives to treat soil and groundwater contaminated with explosives. Contamination of soils has occurred at Army installations where explosives were produced and handled according to practices that were considered standard at the time. These installations require cost-effective techniques to treat large volumes of explosives-contaminated soils. The U.S. Army Environmental Center is using bioremediation – boosting the activity of naturally occurring microorganisms – to eliminate explosive compounds from the soil.(Noyes, 1996) A number of processes are being demonstrated to validate different bioremediation technologies and obtain critical performance information transfer applications to other Army installations. Bioremediation uses nature s processes with simple technology, costs less than incinerations, and has widespread public acceptance.(Smith, 1999)
Research
A natural process called bioremediation is now available as an alternative cleanup remedy for explosives in contaminated soils. Bioremediation boosts the activity of naturally occurring microorganisms to degrade hazardous substances in soil or sediment into nontoxic materials.
Composting uses naturally occurring microorganisms to degrade organic wastes. This process is similar to that used for household yard waste. Increased temperatures from heat produced by microorganisms speed their metabolism and degradation of the organic materials in the waste. (Smith, 1999)
Because of the modest equipment and monitoring requirements, windrow composting is a cost-effective technology, with a high degree of treatment effectiveness for explosives-contaminated soils at a low process cost.
Windrow composting mixes the soil with compost in long piles known as windrows. To facilitate the microbial growth, things such as wood chips, manure, straw, alfalfa, and other agricultural products are added. Windrows are used to facilitate aeration of the compost and are turned periodically using a compost turner. Moisture content, windrow oxygen level, and temperature are easily monitored. Windrow composting was used in the cleanup of a Superfund Site at Umatilla Army Depot, OR. TNT reductions were as high as 99.7% and removals of RDX and HMX were 99.8% and 96.8%.(Smith, 1999)
For sites requiring greater process control, more complete degradation, or where the cost of importing compost ingredients is prohibitive, soil slurry biotreatment or “bioslurry” is a bioremediation option. The contaminated materials are mixed into a slurry to allow contact between the microorganisms and the contaminants. Because conditions are optimized for the microorganisms, slurry processes are faster than many other biological processes. The treated slurry is suitable for direct land application, similar to finished compost.
Aerobic bioslurry is a destruction technology that achieved ninety-nine percent plus removal of TNT, HDX and RDX at Joliet Army Ammunition Plant, Illinois. Molasses was added as a microbial food source. Simple intermittent aeration permitted reactor conditions to cycle between aerobic and anoxic states, thus optimizing total explosives degradation. Metabolic fate studies showed virtually complete breakdown of the explosives molecule to CO2, simple organic acids, and carbon fragments in the biomass.
The Army is conducting a field test of a proprietary anaerobic bioslurry process on explosives-contaminated soils at Iowa Army Ammunition Plant, Iowa. (Smith, 1999) The field test will provide performance and cost information to evaluate this process in relation to other bioremediation technologies. Based on results of field experiments, USAEC has developed conceptual designs and cost estimates to enable users to apply these technologies full-scale. (Smith, 1999)
Numerous Army installations have sites where past industrial operations, testing, or training activities have left chemical substances in the soil, surface water, groundwater, etc. The Restoration Technology Team supports the Army’s cleanup efforts at these sites by demonstrating, validating, and transferring cleanup technology that can be used there. Current team activities include demonstrations of composting and bioslurry technologies to treat soil that contains explosives, advanced oxidation technology to remove residues of explosives from groundwater, and soil washing/leaching technology to remove heavy metals like lead.
Past production and handling of conventional munitions has left explosives in soils at many Army installations. Depending on the concentrations of explosives mainly trinitrotoluene (TNT), cyclonite (RDX) and cyclotetramethylene (HMX) the affected soils can pose reactivity and toxicity hazards. Because these explosives can migrate from the soils into groundwater, the affected soils should be treated to eliminate any threat to human health or the environment.
Incineration is the traditional proven cleanup technology, but it is costly and not readily accepted by regulators. The Army has searched since the 1980s for alternatives to incineration. Extensive tests have shown that bioremediation the use of living organisms to remove pollutants from soil or water could be a cost-effective treatment.
The U.S. Army Environmental Center (USAEC) has field tested several bioremediation methods, including soil slurry reactor treatment and windrow composting. USAEC recently demonstrated windrow composting at Umatilla Army Depot Activity, Oregon, validating the technology as a way to treat explosives in soil. The test worked so well that regulators approved windrow composting as the cleanup method for the Superfund site at Umatilla Army Depot Activity. Results from the treatment continue to exceed expected levels and the cleanup is proceeding ahead of schedule.
Soil slurry bioremediation also proves to be promosing. The technology starts with an aqueous mixture of water and the contaminated soil or sludge. This slurry is pumped into a reactor, where conditions are enhanced to promote the growth of microorganisms that already live in the soil. This increased microbial activity can degrade the explosives into environmentally acceptable byproducts.
After treatment, the the watere is taken out of the soil and the soil can be returned to the site. The reactor water can be recycled and used in the process again.
In 1995, USAEC led a soil slurry bioremediation test at Joliet Army Ammunition Plant in Illonois. The demonstration, conducted by Argonne National Laboratory, looked to prove that degradation of explosives-contaminated soil in a bioreactor is both possible on a large scale and an affordable alternative to incineration. Joliet AAP was an ideal test site because of its history as a manufacturer and handler of munitions. The bioslurry treatment reduced TNT concentrationa at the Joliet site by ninety-nine percent. (Smith, 1999)
Researchers conducted the field test in three phases. First, they determined the operating characteristics of the bioslurry system. Second, they enhanced the microbial population in the soil. Then they operated the system to gather the data necessary to determine operational efficiency and reliability. The bioslurry process effectively degraded the explosives and their metabolic byproducts, posting a 99.6 percent removal rate. Metabolic fate studies at the end of the treatment showed substantial amounts of the explosives had been broken down to carbon dioxide and carbon fragments in the cell mass.
Soils contaminated with explosives must be considered reactive unless research has shown them not to be. Determining the actual level for primary explosives will allow remediation managers to protect the workers while conserving resources for the remediation. Scientists conducted tests at various primary explosive concentrations and moisture levels, establishing a safety threshold reactivity level, and developing a database at higher confidence levels.
Composting has been developed as a cost-effective method for reducing explosives in the soil. However, composting does not achieve complete mineralization of the explosive material causing questions about its effectiveness. Trinitrotoluene (TNT) transformation products appear to be strongly bound to compost material and are unextractable. Studies are needed to determine if plants will extract these transformation products of explosives from the composted soil before composted soil is returned to a site.
Composting explosives-contaminated soil costs about 60% as much as incineration cost to cleanup contaminated sites. Numerous installations are considering composting as a cleanup technology. However, the question of TNT mineralization keeps the technology from being accepted without some reservation. Even though the transformation products are not readily extractable, there is concern that plants and long-term exposure to weather may eventually release these products. (Smith, 1999)
The Plant Uptake project consisted of five elements: shipping finished compost from Umatilla Army Depot Activity to TVA, producing control compost from soil and amendments shipped from Umatilla to TVA, developing and testing analytical methods, conducting greenhouse studies, and conducting long-term weathering studies. All testing was conducted at TVA s Facility in Muscle Shoal, Alabama. (Akhavan, 1998)
Analytical methods exist for calculating percent of explosives in soil and water, but the suitability of these methods to detect transformation products in plant tissue extracts is not certain. Personnel from Cold Regions Research Engineering Laboratory (CRREL) and U.S. Army Waterways Experiment Station (USAWES) assisted chemists from USAEC and TVA to determine the suitability of existing methods.
A total of nine plants were tested with Umatilla compost and control compost. The vegetable crops tested included radish, kale, bush beans, tomatoes, and chives. The range crops tested include alfalfa, sorghum, red top, and winter barley. Roots, stems and leaves, fruit, and soil around the root ball were analyzed.
Finished compost from Umatilla was used in long-term weathering studies to determine what happens to compost when exposed to sunlight, weather, and soil microbes. Different mixtures of compost and soil were placed in large pans outside and exposed to the elements. Leachate was collected and analyzed along with compost/soil samples over a three year period. The compost/soil mixtures were not manipulated in any manner during the weathering study. (Akhavan, 1998)
The test plan and safety plan have been prepared and approved. The finished compost, compost amendments, and uncontaminated soil have been shipped from Umatilla to Muscle Shoal, Alabama. Chemists from TVA, USAWES, USAEC, and CRREL have identified the appropriate analytical methods. The weathering studies have been completed. The control compost has been prepared and lab/greenhouse testing has been completed. (Akhavan, 1998)
Geokinetics has pioneered the use of electrokinetic techniques to dramatically enhance bio-remediation of TNT and other “difficult” organic contamination. (Westing, 1985)
Soil contaminated with TNT from a German World War I munitions factory (which exploded in 1918) was treated by electrokinetically enhanced bioremediation. Laboratory studies were undertaken using pilot scale batch units to determine optimal full scale operating conditions. The “Pool Process” provides the ideal means of adding nutrients,
water, extra micro-organisms and heat. Full scale remediation was undertaken using larger batch scale
units. Anode and cathode compartments were filled with
water and fitted to anolyte and catholyte circulation loops. (Westing, 1985)
During treatment, nutrients were added to the electrolytes and migrated into the contaminated soil to maintain optimal conditions for bio-remediation. In addition As, Pb and other ionic contaminants were migrated out of the soil and recovered from the electrolytes. During the process the soil was heated to 25-30′C as a result
of Joule heating. This was sufficient to enhance the bio-digestion process but conservative enough not to have an impact on the TNT. (Westing, 1985)
This process was carried on until the TNT levels were no longer detected, which took about three months. They accomplished in 3 months that which 72 years of natural
bioremediation had failed to achieve. (Westing, 1985)
Conclusions
The contamination of soils and sediments at Army ammunition plants and Army depots has occurred in areas where explosives and propellants were produced and handled. (Barth, 1994) One source of explosives-contaminated soils is lagoons and sedimentation basins used to receive the water from explosives manufacturing and washout operations. These practices resulted in contamination of soils and sediments with various explosives including 2,4,6-trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), and -methyl-n-2,4,6-tetranitroanaline (tetryl). (Barth, 1994) These contaminated soils are can be very toxic and mutagenic. Degrading these harmful explosives is a necessary job in order to keep those sites safe. There are many different techniques to degrade explosives such as composting, electrokinetically enhanced bio-remediation, Bioslurry, and Anaerobic Soil-Slurry. It is estimated that the U.S. Army has 40 sites requiring cleanup of explosives-contaminated soils. Currently, regulatory agencies only approve incineration and composting as decontamination technologies. Incineration was commonly accepted by the public in the late 1980’s, but the acceptance of this is now declining. (Barth, 1994) Composting costs can also be high due to the necessary things needed for the composting process. For these reasons the Army has invested in developing and demonstrating other biotreatment technologies. The Army has been searching for alternatives to treat soil and groundwater contaminated with explosives. Contamination of soils has occurred at Army installations where explosives were produced and handled according to practices that were considered standard at the time. These installations require cost-effective techniques to treat large volumes of explosives-contaminated soils. The U.S. Army Environmental Center is using bioremediation – boosting the activity of naturally occurring microorganisms – to eliminate explosive compounds from the soil. A number of processes are being demonstrated to validate different bioremediation technologies and obtain critical performance information transfer applications to other Army installations. Bioremediation uses nature s processes with simple technology, costs less than incinerations, and has widespread public acceptance.
Literature Cited
Chemical Weapons Destruction and Explosive Waste/Unexploded Ordnance Remediation
Robert Noyes / Noyes Data Corporation/Noyes Publications / September 1996
Environmental Availability of Chlorinated Organiated Organics, Explosives, and Heavy Metals in soils
Raymond C. Loehr (Editor),Bradley Smith (Editor),William C. Anderson (Editor) / American Academy of Environmental Engineers / February 1999