By creating this site our goal is to give you an appetizer for what in situ bioremediation is all about. It is beyond the scope of this site to attempt to cover all aspects of this technology in details. Please check it out, get curious and let us know what you think!
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"In situ" means in place. In relation to bioremediation of contaminated soil and groundwater the meaning of the words in situ refers to in the subsurface where the contaminant is actually located. Bioremediation is a treatment technology designed to control subsurface conditions to make naturally occurring bacteria thrive and grow. Some of these bacteria are capable of biodegrading contaminants adversely effecting environmental quality.
If growth conditions are optimal to degrade the contaminant "the more degrading bacteria, the faster bioremediation occurs". Potentially the contaminants can be reduced into the final end products: CO2, water and cell biomass!
Reduction of contamination levels by in situ bioremediation of petroleum contaminated sites (link) has successfully been carried out for decades. Researchers are now trying to extend the use of this technology to more reluctant contaminants such as halogenated hydrocarbons.
In situ bioremediation offers several potential advantages over conventional technologies when it can be successfully applied:
Consideration of both the advantages and disadvantages of different remediation technologies is important for every site. Sometimes in situ bioremediation is not applicable or other technologies are more cost effective.
Contamination of soil and groundwater by Dense Non-Aqueous Phase Liquid, (DNAPL), is a huge problem. These compounds displace water and sink deep into the subsurface. A typical migration route of a DNAPL like TCE is visualized on figure 1.
Figure 1: DNAPL migration through subsurface.
A DNAPL travels through the vadoze zone leaving behind residual blobs until it hits the watertable. In the saturated zone it displaces water and moves deeper into the aquifer leaving behind dissolved and captured residuals in the soil pores in its wake. Upon hitting an impermeable barrier the DNAPL pools in depressions on top of this barrier. Once these depressions are filled, the DNAPL spills over the edge and continuos down slope and through cracks in the barrier under the force of gravity, sometimes in different direction than the groundwater flow !
Because the slope and cracks in the subsurface are not generally known, these transport phenomena make detecting DNAPL difficult. DNAPL’s are sinkers, hence they can contaminate deeper aquifers, used for drinking water supply.
Strong capillary forces cause the described capture of residual DNAPL blobs in the soil pores. The DNAPLs has very low solubility and the dissolved phase have strong affinity for organics which bind them tightly to the soil. The small amount of DNAPL that partition into the groundwater are quickly adsorbed to soil particle then a smaller fraction of the this dissolved and adsorbed part desorps into the groundwater again and the adsorption desorption cycle continues. Hence, the DNAPL plume moves very slowly constantly releasing small amounts of contaminant into the groundwater. This makes the DNAPLs virtually impossible to pump out of the ground.
As shown on figure 1 captured pools of DNAPL act as reservoirs that slowly bleed the dissolved contaminant into groundwater for a long time to come!
Learn more about transport of NAPLs in the subsurface, problems related to monitoring and remediation of these elsewhere in this primer. Our page focuses on in situ bioremediation of trichloroethylene (TCE), a common DNAPL found in contaminated soil and groundwater. This halogenated hydrocarbon has been proven to be carcinogenic, mutagenic and toxic even in low concentrations[4]. You may have been exposed to TCE by drinking contaminated groundwater or inhaling volitalized TCE in the shower.
Check out these sites to learn more about health concerns related to TCE and other toxic contaminants:
Environmental
Protection Agency
Centers for Disease
Control and Prevention
World Health
Organization
National Technical
Information service
Not many years ago it was believed, that the subsurface below the root zone was a deadland with no microbial life. Today it is well known, that the subsurface soil is a habitat for a big variety of bacteria, many of which are capable of degrading organic contaminants. The photographs below shows significant evidence that bacteria thrive on solids, such as soil. See the big difference between figure 2 of a solid surface without microbial growth and figure 3 of the same type of solid surface with microbial growth.
The photographs have generously been provided with the courtesy of Dr. R.C. Hoehn of Virginia Tech. The original photographer is Dr. Ken Dickson, Institute of Applied Sciences, Denton, Texas.
Figure 2: Solid without
microorganisms
Figure 3: Solid with growth of microorganisms
The combined microbiological metabolic activity of the interacting suite of indigenous subsurface bacteria is the engine that powers the bioremediation process. It is absolutely essential to understand how this engine functions in the subsurface to determine which growth factors are important for the contaminant degrading bacteria. From this understanding originate the desire to try to alter and control chosen environmental parameters and keep the contaminant degrading bacteria happy!
For many years chlorinated aliphatics were not considered biodegradable. Recently, however a wide variety of different biochemical pathways have been proposed including biodegradation performed by both aerobic, anaerobic and methanotrophic microorganisms. Only biodegradation of methanotrophic microorganisms will be considered here.
Unlike biodegradation of petroleum hydrocarbons, which can serve as a growth substrate for bacteria, no microorganism has yet been found that is able to grow on TCE as the sole carbon source. However, several pathways for co-metabolism TCE-degradation have been proposed. Identifying new pathways is an ongoing process.
Methanotrophs are an example of aerobic organisms capable of transforming chlorinated aliphatics, including TCE by co-metabolism. Although they need methane as a carbon and energy source these organisms can be stimulated to degrade TCE and CO.
The trick the microbes use to break down these fairly
persistent compounds is monooxygenises (MMO) where they
produce an enzyme (monooxygenase)
that initiates the transformation. Figure 4 illustrates
the series of events to perform the break down of TCE by
methanotrophs. 
Figure 4: Monooxygenesis [2]
The steps in MMO are:[5]
After the MMO initiates the degradation process of TCE the intermediates will be subject to further degradation. In a mixed culture containing heterotrophs as well as methanotrophs, the intermediates formate, CO and chlorinated aliphatic acids, is broken down to the basic constituents of CO2 and cell biomass by commonly occurring bacteria. The chlorinated aliphatic acids produced by monooxygenasis resemble natural occurring acids and can be utilized as growth substrate by many natural occurring soil bacteria. If not limited by toxicity, slow desorption rates, lack of oxygen, methane, nutrients or other growth conditions the TCE will be nearly totally degraded: 90-95% has been observed.
Now that we are familiar with the way the microbiological engine works, a tool to choose the important parameters for a successful bioremediation, "the fuel and maintenance", can be identified.
To determine if and how in situ bioremediation can be applied at a specific site the system engineer and the microbiologist need to team up with a third field of expertise; the hydrogeologist! The ability to visualize the geological setting in the subsurface is of great importance in all phases of the life cycle of the bioremediation;
Without hydrogeological expertise it will not be possible to effectively bring the bacteria in contact with the contaminants and the growth factors provided.
Bioremediation offers a huge challenge to the project manager of creating a strong cooperation and coordination between all three fields of knowledge and experience:
To attempt to design a bioremediation system without all off the above three fields integrated, will be like groping around in the dark without a flashlight!
The following gives a suggested general procedure approach. The general approach is followed by introductory checklists of things to consider during the different phases of the process.
The overall design process for in situ bioremediation is presented below:
Figure 5: Flow chart for design procedure
Are the hydogeological and microbiological site conditions conducive to bioremediation?
Conditions essential to in situ bioremediation:
Are methanotrophic bacteria present?
This can be investigated observing the degradation in
simple labatory microcosms using native soil and the
bacteria within it.
Where are the existing conditions relative to optimum
for the specific MMO bioremediation process?
Specific conditions essential to MMO:
What conditions can we operationally alter and control
to achieve and maintain optimal conditions for growth?
The parameters that can possibly be controlled in
practice are:
How can a bioremediation system be designed for the specific site?
To make the site specific system for MMO biodegradation, the engineer must investigate the hydrogeology, to design a grid of injection and extraction wells, that will insure sufficient transport of nutrients, methane and oxygen. This is not an easy task especially when dealing with low permeable subsurface conditions like clay or if the subsurface have many different layers and contaminant levels. The more heterogeneous the subsurface at the site is, the more difficult is the task of creating a sufficient grid of wells. Installing and operating wells are expensive, so at some point the bioremediation might lose out to some other more appropriate technology, if the geology is too complex.
What options do we have during the operation?
During the operation of the system altering pumping directions can decrease the risk of only supplying along preferential pathways.
Pulsing of the gasses or fluids that is injected into the subsurface can create dynamic conditions. The environment will then change between rich and poor conditions, which can sometimes be utilized to trick bacteria to degrade the contaminant. This will be illustrated in an example later!
How can we be sure that the remediation is proceeding as planned?
For this purpose a sufficient monitoring program must be designed for the specific site. It is difficult and costly to prove that the actual biodegradation is taking place. Indirect indicator are that the contaminant level is decreasing or that soil gasses indicating microbial growth are increasing. It is also possible to count the increase in bacteria number or measure intermediate degradation product levels. All of these methods have the common problem of not being able to prove that these changes are actually occurring due to biodegradation of the contaminant and not caused by other processes such as adsorption or volatilization. They are indicators that something is going on, but not a final proof!
How do we decide the future operations?
Frequent evaluation of the efficiency of the system can lead to the conclusion that another technology may be more cost effective in a later phase. If the system is functioning there will come a time when the rate at which the contaminant level is dropping is so low or the total contaminant level is so low, that continuos operation of the system has no meaning.
After the system has been shut down, intrinsic bioremediation can still go on. Contaminant continues to desorb from clay layers or dissolves from residual reservoirs. Monitoring for an extended period after end of system operation is therefore essential!
An example of a MMO system design in operation is shown on the animation below:
An example of a MMO system design in
operation is shown on the animation here.
Figure 6: Operating MMO bioremediation system
Methane (green) and air mixed with nutrients (red) is pumped in pulses allowing the methanotrophic bacteria to establish and maintain a population. When there is enough methane present, the bacteria produce a lot of the MMO enzyme. During periods of starvation, when the concentration of their preferred substrate, methane, is used up, the enzyme reacts with the TCE instead, visualized by the shrinking plume. This way the bacteria is tricked to degrading TCE, even though they do not gain anything from the process!
This is only one of many examples of how to design a system for in situ bioremediation. New experiences with innovative remediation technologies like in situ bioremediation are continuously being generated. To keep in pace with the rapid development in these technologies, the following Web sites are good places to start:
Technology
Innovative Office
From this home page you will find links to;
Ground Water
Remediation Technologies analysis Center
Case studies of
Superfund Site clean up
In situ bioremediation by MMO is often
integrated with vapor extraction, VES. A soil and
groundwater bioremediation project employing this type of
dual technology system was installed at the Department of
Energy at Savannah River Site. The system proved
effective for TCE remediation at this specific site [5].
Learn more about this system at:
Savannah River Site Technology Center
An additional advantage of the VES system is that it helps to remove excess off gases from the soil that could accumulate in utility trenches and enter surface structures to cause explosions!
In situ bioremediation is only one of many remediation technologies. The most cost effective and environmentally sound solution may very well be to combine several remediation technologies described in this primer either simultaneously or in different stages of the remediation process.
Containment and intrinsic bioremediation is also worth considering when prioritizing how to spend remediation dollars, since it is seldom cost effective to clean contaminated sites down to virgin land quality, before redevelopment. Risk assessment based on health criteria is gaining more importance in handling of the tremendous challenge of prioritizing and targeting the remediation efforts.
Combination of treatments, containment and intrinsic remediation in so called " treatment trains" will probably be one of the major challenges in the future remediation of contaminated soil and groundwater. These treatment trains are in the present one of the more promising approaches, since so far no remediation process has proven effective applied to the diversity of the specific sites Learn more about other remediation technologies in other pages in this primer.
Comments, criticism or questions are very velcome !
This page is created by:
Eric Elmer Anderson, erander2@.vt.edu
Ph.D. candidate, Environmental Design and Planning,
Virginia Tech, Blacksburg, Virginia, USA.
Rikke Granum Andersen, rianders@.vt.edu
Visiting scholar at Virginia Tech, Environmental
Engineering
Graduate candidate, Environmental Engineering,
Aalborg University, Denmark.
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Send comments or suggestions to:
Student Authors: Eric Anderson, erander2@.vt.edu and
Rikke Andersen, rianders@.vt.edu
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998
