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
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
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 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.
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.