A Citizen’s Guide to Bioremediation

What is bioremediation?
Bioremediation is a treatment process that uses naturally occurring
microorganisms (yeast, fungi, or bacteria) to break down, or degrade, hazardous
substances into less toxic or nontoxic substances. Microorganisms, just like humans, eat
and digest organic substances for nutrients and energy. In chemical terms, “organic”
compounds are those that contain carbon and hydrogen atoms. Certain microorganisms
can digest organic substances such as fuels or solvents that are hazardous to humans. The
microorganisms break down the organic contaminants into harmless products mainly
carbon dioxide and water (Figure 1). Once the contaminants are degraded, the
microorganism population is reduced because they have used all of their food source.
Dead microorganisms or small populations in the absence of food pose no contamination
risk.
How does it work?
Microorganisms must be active and healthy in order for bioremediation to take
place. Bioremediation technologies assist microorganisms’ growth and increase microbial
populations by creating optimum environmental conditions for them to detoxify the
maximum amount of contaminants. The specific bioremediation technology used is
determined by several factors, for instance, the type of microorganisms present, the site
conditions, and the quantity and toxicity of contaminant chemicals. Different
microorganisms degrade different types of compounds and survive under different
conditions.
Indigenous microorganisms are those microorganisms that are found already
living at a given site. To stimulate the growth of these indigenous microorganisms, the
proper soil temperature, oxygen, and nutrient content may need to be provided. If the
biological activity needed to degrade a particular contaminant is not present in the soil at
the site, microorganisms from other locations, whose effectiveness has been tested, can
be added to the contaminated soil. These are called exogenous microorganisms. The soil
conditions at the new site may need to be adjusted to ensure that the exogenous
microorganisms will thrive. Bioremediation can take place under aerobic and anaerobic
conditions. In aerobic conditions, microorganisms use available atmospheric oxygen in
order to function. With sufficient oxygen, microorganisms will convert many organic
contaminants to carbon dioxide and water. Anaerobic conditions support biological
activity in which no oxygen is present so the microorganisms break down chemical
compounds in the soil to release the energy they need. Sometimes, during aerobic and
anaerobic processes of breaking down the original contaminants, intermediate products
that are less, equally, or more toxic than the original contaminants are created.
Bioremediation can be used as a cleanup method for contaminated soil and water.
Bioremediation applications fall into two broad categories: in situ or ex situ. In situ
bioremediation treats the contaminated soil or groundwater in the location in which it was
found. Ex situ bioremediation processes require excavation of contaminated soil or
pumping of groundwater before they can be treated.

In Situ Bioremediation of Soil

In situ techniques do not require excavation of the contaminated soils so may be
less expensive, create less dust, and cause less release of contaminants than ex situ
techniques. Also, it is possible to treat a large volume of soil at once. In situ techniques,
however, may be slower than ex situ techniques, may be difficult to manage, and are
most effective at sites with permeable (sandy or uncompacted) soil.
The goal of aerobic in situ bioremediation is to supply oxygen and nutrients to the
microorganisms in the soil. Aerobic in situ techniques can vary in the way they supply
oxygen to the organisms that degrade the contaminants. Two such methods are
bioventing and injection of hydrogen peroxide. Oxygen can be provided by pumping
air into the soil above the water table (bioventing) or by delivering the oxygen in liquid
form as hydrogen peroxide. In situ bioremediation may not work well in clays or in
highly layered subsurface environments because oxygen cannot be evenly distributed
throughout the treatment area.
In situ remediation often requires years to reach cleanup goals, depending mainly
on how biodegradable specific contaminants are. Less time may be required with easily
degraded contaminants.

Bioventing. Bioventing systems deliver air from the atmosphere into the soil above the
water table through injection wells placed in the ground where the contamination exists.
The number, location, and depth of the wells depend on many geological factors and
engineering considerations. An air blower may be used to push or pull air into the soil
through the injection wells. Air flows through the soil and the oxygen in it is used by the
microorganisms. Nutrients may be pumped into the soil through the injection wells.
Nitrogen and phosphorous may be added to increase the growth rate of the
microorganisms.

Injection of Hydrogen Peroxide. This process delivers oxygen to stimulate the activity of
naturally occurring microorganisms by circulating hydrogen peroxide through
contaminated soils to speed the bioremediation of organic contaminants. Since it involves
putting a chemical (hydrogen peroxide) into the ground (which may eventually seep into
the groundwater), this process is used only at sites where the groundwater is already
contaminated.

What Is An Innovative Treatment

Technology?

Treatment technologies are processes applied to the

treatment of hazardous waste or contaminated
materials to permanently alter their condition through
chemical, biological, or physical means. Innovative
treatment technologies are those that have been
tested, selected or used for treatment of hazardous
waste or contaminated materials but lack welldocumented
cost and performance data under a
variety of operating conditions.

A system of pipes or a sprinkler system is typically used to deliver hydrogen peroxide to

shallow contaminated soils. Injection wells are used for deeper contaminated soils.

In Situ Bioremediation of Groundwater

In situ bioremediation of groundwater speeds the natural biodegradation processes
that take place in the watersoaked underground region that lies below the water table. For
sites at which both the soil and groundwater are contaminated, this single technology is
effective at treating both. Generally, an in situ groundwater bioremediation system
consists of an extraction well to remove groundwater from the ground, an above-ground
water treatment system where nutrients and an oxygen source may be added to the
contaminated groundwater, and injection wells to return the “conditioned” groundwater
to the subsurface where the microorganisms degrade the contaminants.
One limitation of this technology is that differences in underground soil layering
and density may cause reinjected conditioned groundwater to follow certain preferred
flow paths. Consequently, the conditioned water may not reach some areas of
contamination. Another frequently used method of in situ groundwater treatment is air
sparging, which means pumping air into the groundwater to help flush out contaminants.
Air sparging is used in conjunction with a technology called soil vapor extraction and is
described in detail in the document entitled A Citizen’s Guide to Soil Vapor Extraction
and Air Sparging (see page 4).

Ex Situ Bioremediation of Soil

Ex situ techniques can be faster, easier to control, and used to treat a wider range
of contaminants and soil types than in situ techniques. However, they require excavation
and treatment of the contaminated soil before and, sometimes, after the actual
bioremediation step. Ex situ techniques include slurry-phase bioremediation and
solidphase bioremediation.

Slurry-phase bioremediation. Contaminated soil is combined with water and other

additives in a large tank called a “bioreactor” and mixed to keep the microorganisms
which are already present in the soil in contact with the contaminants in the soil.
Nutrients and oxygen are added, and conditions in the bioreactor are controlled to create
the optimum environment for the microorganisms to degrade the contaminants. Upon
completion of the treatment, the water is removed from the solids, which are disposed of
or treated further if they still contain pollutants. Slurry-phase biological treatment can be
a relatively rapid process compared to other biological treatment processes, particularly
for contaminated clays. The success of the process is highly dependent on the specific
soil and chemical properties of the contaminated material. This technology is particularly
useful where rapid remediation is a high priority.
Solid-phase bioremediation. Solid-phase bioremediation is a process that treats soils in
above-ground treatment areas equipped with collection systems to prevent any
contaminant from escaping the treatment. Moisture, heat, nutrients, or oxygen are
controlled to enhance biodegradation for the application of this treatment. Solid-phase
systems are relatively simple to operate and maintain, require a large amount of space,
and cleanups require more time to complete than with slurry-phase processes. Solid-
phase soil treatment processes include landfarming, soil biopiles, and composting.

Landfarming. In this relatively simple treatment method, contaminated soils are

excavated and spread on a pad with a built-in system to collect any “leachate” or
contaminated liquids that seep out of contaminant-soaked soil. The soils are periodically
turned over to mix air into the waste. Moisture and nutrients are controlled to enhance
bioremediation. The length of time for bioremediation to occur will be longer if nutrients,
oxygen or temperature are not properly controlled. In some cases, reduction of
contaminant concentrations actually may be attributed more to volatilization than
biodegradation. When the process is conducted in enclosures controlling escaping
volatile contaminants, volatilization losses are minimized.

Soil biopiles. Contaminated soil is piled in heaps several meters high over an air
distribution system. Aeration is provided by pulling air through the heap with a vacuum
pump. Moisture and nutrient levels are maintained at levels that maximize
bioremediation. The soil heaps can be placed in enclosures. Volatile contaminants are
easily controlled since they are usually part of the air stream being pulled through the
pile.
Composting. Biodegradable waste is mixed with a bulking agent such as straw, hay, or
corn cobs to make it easier to deliver the optimum levels of air and water to the
microorganisms. Three common designs are static pile composting (compost is formed
into piles and aerated with blowers or vacuum pumps), mechanically agitated in-
vesselcomposting (compost is placed in a treatment vessel where it is mixed and aerated),
and windrow composting (compost is placed in long piles known as windrows and
periodically mixed by tractors or similar equipment).

Will it work at every site?

Biodegradation is useful for many types of organic wastes and is a cost-effective,
natural process. Many techniques can be conducted on-site, eliminating the need to
transport hazardous materials.
The extent of biodegradation is highly dependent on the toxicity and initial
concentrations of the contaminants, their biodegradability, the properties of the
contaminated soil, and the particular treatment system selected.
Contaminants targeted for biodegradation treatment are non-halogenated volatile
and semi-volatile organics and fuels. The effectiveness of bioremediation is limited at
sites with high concentrations of metals, highly chlorinated organics, or inorganic salts
because these compounds are toxic to the microorganisms.

Where has it been used?

At the Scott Lumber Company Superfund site in Missouri, 16,000 tons of soils
contaminated with polyaromatic hydrocarbons (PAHs) were biologically treated using
land treatment application. PAH concentrations were reduced by 70%.
At the French Ltd. Superfund site in Texas, slurry-phase bioremediation was used
to treat 300,000 tons of lagoon sediment and tar-like sludge contaminated with volatile
organic compounds, semi-volatile organic compounds, metals, and pentachlorophenol.
Over a period of 11 months, the treatment system was able to meet the cleanup goals set
by EPA.