The main advantage of ex situ treatment is that it generally
requires shorter time periods than in situ treatment, and there
is more certainty about the uniformity of treatment because of
the ability to homogenize, screen, and continuously mix the soil.
However, ex situ treatment requires excavation of soils, leading
to increased costs and engineering for equipment, possible
permitting, and material handling/worker exposure considerations.
Bioremediation techniques are destruction or transformation
techniques directed toward stimulating the microorganisms to grow
and use the contaminants as a food and energy source by creating
a favorable environment for the microorganisms. Generally, this
means providing some combination of oxygen, nutrients, and
moisture, and controlling the temperature and pH. Sometimes,
microorganisms adapted for degradation of the specific
contaminants are applied to enhance the process.
Biological processes are typically implemented at low cost.
Contaminants can be destroyed or transformed, and little to no
residual treatment is required. However, the process requires
more time and it is difficult to determine whether contaminants
have been destroyed. Biological treatment of PAHs leaves less
degradable PAHs (cPAHs) behind. These higher molecular cPAHs are
classified as carcinogens. Also, an increase in chlorine
concentration leads to a decrease in biodegradability. Some
compounds, however, may be broken down into more toxic
by-products during the bioremediation process (e.g., TCE to vinyl
chloride). An advantage over the in situ applications is that in
ex situ applications, these by-products are contained in the
treatment unit until nonhazardous end-products are produced.
Although not all organic compounds are amenable to
biodegradation, bioremediation techniques have been successfully
used to remediate soils, sludges, and ground water contaminated
by petroleum hydrocarbons, solvents, pesticides, wood
preservatives, and other organic chemicals. Bioremediation is not
yet commonly applicable for treatment of inorganic contaminants.
The rate at which microorganisms degrade contaminants is
influenced by the specific contaminants present; oxygen supply;
moisture; nutrient supply; pH; temperature; the availability of
the contaminant to the microorganism (clay soils can adsorb
contaminants making them unavailable to the microorganisms); the
concentration of the contaminants (high concentrations may be
toxic to the microorganism); the presence of substances toxic to
the microorganism, e.g., mercury; or inhibitors to the metabolism
of the contaminant. These parameters are discussed briefly in the
Oxygen level in ex situ applications is easier to
control than in in situ applications and is typically maintained
by mechanical tilling, venting, or sparging.
Anaerobic conditions may be used to degrade highly
chlorinated contaminants. This can be followed by aerobic
treatment to complete biodegradation of the partially
dechlorinated compounds as well as the other contaminants.
Water serves as the transport medium through which
nutrients and organic constituents pass into the microbial cell
and metabolic waste products pass out of the cell. Moisture
levels in the range of 20% to 80% generally allow suitable
biodegradation in soils.
Nutrients required for cell growth are nitrogen,
phosphorous, potassium, sulfur, magnesium, calcium, manganese,
iron, zinc, and copper. If nutrients are not available in
sufficient amounts, microbial activity will stop. Nitrogen and
phosphorous are the nutrients most likely to be deficient in the
contaminated environment and thus are usually added to the
bioremediation system in a useable form (e.g., as ammonium for
nitrogen and as phosphate for phosphorous).
pH affects the solubility, and consequently the
availability, of many constituents of soil, which can affect
biological activity. Many metals that are potentially toxic to
microorganisms are insoluble at elevated pH; therefore, elevating
the pH of the treatment system can reduce the risk of poisoning
Temperature affects microbial activity in the treatment
unit. The biodegradation rate will slow with decreasing
temperature; thus, in northern climates bioremediation may be
ineffective during part of the year unless it is carried out in a
climate-controlled facility. The microorganisms remain viable at
temperatures below freezing and will resume activity when the
temperature rises. Too high a temperature can be detrimental to
some microorganisms, essentially sterilizing the soil. Compost
piles require periodic tilling to release self-generated heat.
Temperature also affects nonbiological losses of contaminants
mainly through the volatilization of contaminants at high
temperatures. The solubility of contaminants typically increases
with increasing temperature; however, some hydrocarbons are more
soluble at low temperatures than at high temperatures.
Additionally, oxygen solubility decreases with increasing
temperature. Temperature is more easily controlled ex situ than
Bioaugmentation involves the use of cultures that have
been specially bred for degradation of a variety of contaminants
and sometimes for survival under unusually severe environmental
conditions. Sometimes microorganisms from the remediation site
are collected, separately cultured, and returned to the site as a
means of rapidly increasing the microorganism population at the
site. Usually an attempt is made to isolate and accelerate the
growth of the population of natural microorganisms that
preferentially feed on the contaminants at the site. In some
situations different microorganisms may be added at different
stages of the remediation process because the contaminants in
abundance change as the degradation proceeds. USAF research,
however, has found no evidence that the use of non-native
microorganisms is beneficial in the situations tested.
Cometabolism, in which microorganisms growing on one
compound produce an enzyme that chemically transforms another
compound on which they cannot grow, has been observed to be
useful. In particular, microorganisms that degrade methane
(methanotrophic bacteria) have been found to produce enzymes that
can initiate the oxidation of a variety of carbon compounds.
Treatability or feasibility studies are used to
determine whether bioremediation would be effective in a given
situation. The extent of the study can vary depending on the
nature of the contaminants and the characteristics of the site.
For sites contaminated with common petroleum hydrocarbons (e.g.,
gasoline and/or other readily degradable compounds), it is
usually sufficient to examine representative samples for the
presence and level of an indigenous population of microbes,
nutrient levels, presence of microbial toxicants, and soil
characteristics such as pH, porosity, and moisture.
Available ex situ biological treatment technologies include biopiles,
and slurry phase biological treatment.
These technologies are discussed in Section 4. Completed ex situ
biological treatment projects for soil, sediment, bedrock and sludge are
shown inTable 3-7 and additional information
on completed demonstration projects are shown on the FRTR Web Site.