Bioremediation Of Explosives In Contaminated Soil Essay

, 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.