WHILE we may not want bugs crawling around our homes, some of them make remarkable little workers. Special micro-organisms that possess certain enzymes effectively degrade the pollutants in sewage at treatment plants. They also clean up soil contaminated by petroleum products. At closed gasoline stations, the large mounds of earth contaminated by leaking underground storage tanks are being composted by a variety of microbes that occur naturally in soils.
Microbial treatment of contaminated materials has become increasingly common. In 1989, Lawrence Livermore National Laboratory chemist John Knezovich and environmental scientist Jeffrey Daniels began a study with Professor Michael Stenstrom of the University of California, Los Angeles, an expert in wastewater treatment, to determine the feasibility of using similar, naturally occurring micro-organisms to treat materials contaminated with high explosives. The goal: to break down the hazardous compounds into nonhazardous units. The success of this inexpensive method could have wide ramifications. The Department of Energy's Pantex Plant in Amarillo, Texas, disassembles nuclear weapons as it reduces the size of the stockpile and generates high-explosive waste in the process. The Department of Defense must dispose of an even larger quantity of high explosives. Various international treaties require the demilitarization of large numbers of conventional weapons, all of which contain high explosives.
Both DOE and DoD use water and/or steam in the process of removing high explosives, resulting in large quantities of contaminated water. This water typically is run through activated carbon filters, which remove the high explosives and leave clean water, but the process contaminates the carbon filters. In fact, activated carbon laden with high explosives is considered a hazardous waste by the U.S. Environmental Protection Agency. To dispose of the carbon, the Pantex Plant has been burning it, but this disposal process is becoming unacceptable for environmental, health, and safety reasons. Some DoD facilities have been storing it, but this is only a short-term solution. The current best method for decontaminating the carbon is to heat it in a relatively expensive process called thermal regeneration, which requires shipment of the contaminated carbon to an offsite treatment facility.
Livermore's 1989 study, funded by the Laboratory Directed Research and Development Program, first demonstrated the feasibility of biologically treating the small quantities of high-explosive waste that are in wastewater produced at the Laboratory's high-explosives testing range at Site 300. The results of experiments were encouraging and led to the Livermore team developing, installing, and testing a pilot plant at Pantex in 1993 for the direct treatment of wastewater and the regeneration of contaminated carbon (Figure 1 above). Measurements at that plant have proved that naturally occurring micro-organisms can directly degrade RDX and HMX, the most commonly used high explosives within the DOE.
By late 1997, with funding from DOE and DoD, the project team will install another pilot plant at the Hawthorne Army Depot's Western Area Demilitarization Facility in Hawthorne, Nevada. That facility presently uses a steam-out process for removing high explosives from conventional ordnance, resulting in an annual production of up to 25 million gallons (95,000 kiloliters) of water contaminated with high explosives. Subsequent treatment of the wastewater by activated carbon filters generates approximately 120,000 pounds (130 metric tons) of contaminated carbon.

Bugs Need Help
Microbes that clean up sewage and gasoline-contaminated soil often directly feed on those materials. During the feasibility study, the project team learned that bugs do not "eat" the high explosives. Instead, a systematic process of supplying various nutrients showed that when the micro-organisms are fed ethanol and other simple compounds containing carbon in the presence of high explosives, they produce enzymes that degrade RDX and HMX.
Although the pilot plant at Pantex demonstrated that microbial treatment of water contaminated with high explosives was feasible, this process would not be efficient for the large quantities of wastewater that are generated at demilitarization facilities. Accordingly, the team developed a method that couples a chemical process that removes and degrades the high explosives on the carbon with subsequent biological treatment of the wastewater to render the by-products nonhazardous.
As illustrated in Figure 2, the procedure now in development involves first flushing the carbon-filled column with heated (80°C) alkaline water (pH greater than or equal to 12). This process, known as base hydrolysis, regenerates the carbon by removing the trapped high explosives and transforms the explosive materials into nonexplosive but easily degraded hazardous carbon compounds. As the wastewater flows from the carbon column to the "bioreactor" column, ethanol and other nutrients are added to the mixture. The microbial action in the bioreactor rapidly completes the breakdown process to nonhazardous products.






All high explosives contain nitrogen, in the form of nitro-groups, nitrates, or nitramines. The team analyzed the compounds in the effluent from the bioreactor column and found that essentially complete denitrification of those compounds occurs. Ideally, denitrification would reduce the compounds to nitrogen gas only. In this case, over 98% of the gas released by the micro-organisms is nitrogen, and the remainder consists primarily of oxygen, carbon dioxide, and hydrogen, none of which is a hazardous material. In addition, the majority of carbon-containing molecules have been converted to carbon dioxide, with the remainder converted to low-molecular-weight fragments that are below detection limits. Studies conducted with high-explosive compounds tagged with carbon-14 have verified that microbial action has converted all of the materials to their smallest possible units, which is the goal of any microbial treatment.
The pilot plant at the Pantex facility included pilot-scale biological treatment and will add pilot-scale chemical regeneration of carbon by the end of the year. Results from laboratory experiments for chemical regeneration of carbon and operation of the Pantex pilot plant indicate that the coupled chemical and biological method of regenerating activated carbon laden with high explosives is feasible, effective, safe, and efficient. Preliminary calculations also show that this process should be significantly less expensive than thermal regeneration and can be performed on site. Thermal treatment of 120,000 pounds of carbon per year (the amount produced annually at the Hawthorne facility) is estimated to cost about $0.79 per pound, while the coupled chemical/biological method should cost from $0.21 to $0.30 per pound.

The Next Step
The pilot plant at Pantex is relatively small because it was used to define the feasibility of the process and to determine the operating parameters for a larger treatment system. The bioreactor containing the micro-organisms is a column 1.7 meters (5.5 feet) high with an interior diameter of 0.18 meters (7 inches). The explosives-laden solution flows through the bioreactor just once at a rate of 60 milliliters per minute, staying in the bioreactor for about 8 hours.
A similarly sized pilot plant is planned for installation and operation at Hawthorne later this year. It will have a significantly greater capacity, however, because it will incorporate the carbon treatment via base hydrolysis, which is more efficient than biological treatment alone.
Knezovich, Daniels, Stenstrom, and their colleagues are expanding the process so that it could be applied to other high explosives. For example, ongoing research is addressing the treatment of TNT, in which the U.S. Army is particularly interested. Because TNT is more difficult for micro-organisms to degrade than RDX and HMX, the team is working to improve the base hydrolysis process to convert TNT to products that are more amenable to biological degradation. The results of this work will be used to optimize the treatment process at Hawthorne over the next two years. The team will also be looking at the feasibility of expanding these approaches for treatment of other high-explosive wastes.

-- Katie Walter

Key Words: decontamination, demilitarization, hazardous waste, high explosives, microbial treatment.

For further information contact John Knezovich at (510) 422-0925 (knezovich1@llnl.gov) or Jeff Daniels (510) 422-0910 (daniels1@llnl.gov).


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