Biological treatment, or bioremediation, is a developing technology that uses microorganisms to degrade organic contaminants into less hazardous compounds. Bioremediation is most effective for dilute solutions of explosives and propellants. TNT in the crystalline form is difficult to treat biologically.

TNT degrades under aerobic conditions into monoamine-, diamino-, hydroxylamine-DNT, and tetranitro-azoxynitrotoluenes. RDX and HMX degrade into carbon dioxide and water under anaerobic conditions. Researchers have not identified any specific organisms that are particularly effective for degrading explosives waste; an indigenous consortium of organisms usually affects the degradation.

DOD currently is developing or implementing six biological treatments for explosives-contaminated soils: aqueous-phase bioreactor treatment; composting, land farming, phytoremediation, and white rot fungus treatment, which are solid-phase treatments; and in situ biological treatment.

Aqueous Phase Bioreactor Treatment: DOD is considering two types of aqueous-phase bioreactors for the treatment of explosive contaminants. The first is the lagoon slurry reactor, which allows contaminants to remain in a lagoon, be mixed with nutrients and water, and degrade under anaerobic conditions. The lagoon slurry reactor is still in the development stage. The second is the aboveground slurry reactor, which is either constructed on-site or purchased as a package plant.

Aqueous-phase bioreactors provide good process control, can be configured in several treatment trains to treat a variety of wastes, and potentially can achieve very low contaminant concentrations. A drawback of bioreactor treatment is that, unlike composting systems which bind contaminants to humic material, bioreactors accumulate the products of biotransformation. In addition, bioreactors have been shown to remediate explosives only at laboratory scale, so the cost of full-scale bioreactors will have to incorporate a variety of safety features that will add to their total cost.

Composting: DOD has been evaluating composting systems to treat explosives waste since 1982. To date, composting has been shown to degrade TNT, RDX, HMX, DNT, tetryl, and nitrocellulose in soils and sludges. The main advantage of this technology is that, unlike incineration, composting generates an enriched product that can sustain vegetation. After cleanup levels are achieved, the compost material can be returned to the site. Another advantage is that composting is effective for a range of wastes. The cost of composting can be limited, however, by the level of indigenous organisms in contaminated soil and the local availability of amendment mixtures. In addition, composting requires long treatment periods for some wastestreams, and composting of unfamiliar contaminants potentially can generate toxic byproducts.

Composting methods fall into three categories: static-pile composting; mechanically agitated, in-vessel composting; and windrow composting. In static-pile composting, contaminated material is excavated, placed in a pile under protective shelter, and mixed with readily degradable carbon sources. The pile undergoes forced aeration to maintain aerobic and thermophilic (55 to 60 ° or 131 to 140 °) conditions, which foster the growth of microorganisms. Bulking agents, such as cow manure and vegetable waste and/or wood chips, can be added to enhance biodegradation. In mechanically agitated in-vessel composting, contaminated material is aerated and blended with carbon-source materials in a mechanical composter. These devices have been used at municipal sewage treatment facilities and applied to explosives waste. Windrow composting is similar to static-pile composting except that compost is aerated by a mechanical mixing vehicle, rather than a forced air system.

Landfarming: Landfarming has been used extensively to treat soils contaminated with petroleum hydrocarbons, pentachlorophenol (PCP), and polycyclic aromatic hydrocarbons (PAHs), and potentially could be used to treat low to medium concentrations of explosives as well. In land farming, soils are excavated to treatment plots and periodically tilled to mix in nutrients, moisture, and bacteria. In one pilot study at an explosives waste site in Hercules, California, land farming failed to achieve the target cleanup levels of 30-ppm TNT, 5-ppm DNT, and 5-ppm DNB. However, the study achieved a 30 to 40% contaminant degradation.

Phytoremediation: The U.S. Army Environmental Center (USAEC) is developing technologies to effectively clean up contaminated soil with residues of explosives like TNT, RDX, HMX, and DNT. One potential treatment alternative is phytoremediation using constructed wetlands. Phytoremediation is a process which uses plants to degrade, not uptake, explosives. Once this process is proven in constructed wetlands, it could be applied in natural wetlands to remediate explosives-contaminated ground water. Constructed wetlands have already proven to be effective for treating acid mine drainage and municipal waste waters. Wetlands phytoremediation is a technology that is relatively self-sustaining and cost-effective to maintain. In addition, this technology, unlike GAC, does not produce secondary waste streams.

In bench-scale testing, the EPA National Exposure Research Laboratory in Athens, GA, has identified a plant nitroreductase enzyme shown to degrade TNT, RDX, and HMX in concert with other plant enzymes. An immunoassay test has been developed that identifies nitroreductase activity in a wide variety of aquatic and terrestrial plants. This opens a door to a variety of potential applications for cleanup using plants in wetlands.

White Rot Fungus Treatment: White rot fungus, Phanerochaete chrysosporium, has been evaluated more extensively than any other fungal species for remediating explosives-contaminated soil. Although white rot has been reported in laboratory-scale settings using pure cultures (Berry and Boyd, 1985; Fernando et al., 1990), a number of factor increase the difficulty of using this technology for full-scale remediation. These factors include competition from native bacterial populations, toxicity inhibition, chemical sorption, and the inability to meet risk-based cleanup levels.

In bench-scale studies of mixed fungal and bacterial systems, most of the reported degradation of TNT is attributable to native bacterial populations (Lohr, 1993; McFarland et al., 1990). High TNT concentrations in soil also can inhibit growth of white rot fungus. One study suggested that Phanerochaete chrysosporium was incapable of growing in soils contaminated with 20 ppm or more of TNT. In addition, some reports indicate that TNT losses reported in white rot fungus studies can be attributed to adsorption of TNT onto the fungus and soil amendments, such as corn cobs and straw.

In Situ Biological Treatment: In situ treatments can be less expensive than other technologies and produce low contaminant concentrations. The available data suggest, however, that in situ treatment of explosives might create more mobile intermediates during biodegradation. In addition, biodegradation of explosive contaminants typically involves metabolism with an added nutrient source, which is difficult to deliver in an in situ environment. Mixing often affects the rate and performance of explosives degradation. Finally, effectiveness of in situ treatment is difficult to verify both during and after treatment.