2 The 4 Technology Solutions
UNDERSTANDING THE LIMITATIONS OF IN SITU BIOREMEDIATION
An On-Line Version of a Column First Published in:
The National Environmental JournaJuly/Aug. 1993 Vol. 3 No. 4 Pages 22-23
by: David B. Vance
Remediating groundwater contaminated with hydrocarbons can be a long, difficult and expensive process. Starting in the late 1970s, and continuing to the present, in-situ bioremediation has been championed as a cost effective adaptation to the process. (Other New Processes)
Bioremediation is not fool proof, nor can it be guaranteed to be successful even in instances where properly applied . The technology has its limits, the purpose of this column is to put those limits in perspective.
It is first essential to understand the chemical systems to which bioremediation will be applied. Based on origin and susceptibility to microbial interaction hydrocarbons may be divided into two broad classes:
- Petroleum hydrocarbons that are largely associated with the production, storage, or use of fuels, lubricants and chemical feedstocks. Petroleum hydrocarbons have been demonstrated to be biodegradable by numerous species of bacteria. Over a period of 3.5 billion years bacteria have been able to evolve genetic resources that allow some of them to potentially use petroleum hydrocarbons as a source of food.
- Complex industrial hydrocarbons include chlorinated aliphatic and aromatic hydrocarbons, pesticides and herbicides, and polymers. It is only in the last 100 years that mankind has manufactured quantities of these new industrial chemicals. Bacteria have not had time to evolve the genetic information required to utilize them as a source of food. Due to recalcitrance to microbial attack these complex industrial chemicals are termed xenobiotic.
Successful in-situ bioremediation of groundwater has been demonstrated at sites impacted with petroleum hydrocarbons. The most successful method of application has been through stimulation of indigenous microbial species. Microbial stimulation is the process of insuring that environmental conditions, nutrient availability and requirements for an electron acceptor are adequate in the contaminated portions of the aquifer.
The most common cause for failure of saturated zone in-situ bioremediation is the lack of adequate mass transport of the electron acceptor (usually oxygen). In this regard, the physical setting of the site is critical. Overall permeability, and the scale and degree of heterogeneity are the factors governing the advective and diffusional transport rates in the subsurface. If transport rates are too low, saturated zone in-situ bioremediation is not a viable option. It is worthwhile to repeat, most instances in which in-situ bioremediation has failed to meet expectations is due to inadequate mass transport.
Given adequate mass transport properties, site specific microbiological conditions can also impact the process. Unfortunately, the presence of indigenous microbes and efficient mass transport may still prove insufficient for effective bioremediation. Following are brief descriptions of common microbiologically specific reasons for the poor performance of in-situ bioremediation systems:
- There is uncertainty with regards to the effect of hydrocarbon availability on the effectiveness of biodegradation. Can bacteria degrade hydrocarbons adsorbed to surfaces or degrade hydrocarbons with low levels of solubility? Or must the hydrocarbon be solubilized before it can be biodegraded? Contradictory laboratory and field evidence has been published for both scenarios. The answer is likely consortia specific and dependent upon the ability of the organisms to synthesize appropriate biosurfactants. This ability may be absent in some instances.
- Although petroleum hydrocarbons are amenable to aerobic biodegradation, for it to occur the indigenous bacteria must have the appropriate genetic information. This genetic information is precise. The presence of a specific hydrocarbon will stimulate the synthesis of an oxygenase enzyme that is expressly configured to react with that stimulating hydrocarbon. For remediation, the question is whether the indigenous microbes posses the genetic information required for appropriate enzyme production and will the contaminant stimulate the production of those enzymes? In most instances the answer is yes, but not in all cases.
- General microbial stimulation has the potential to produce a large amount of biomass that may not take part in the biodegradation process and actually cause harm through biofouling and plugging of injection wells, galleries or surrounding formations. There is potential to lose critical subsurface mass transport capabilities. Bacterial Transport
- There are practical limits to the degree of clean-up obtainable using bioremediation. Hydrocarbons at the low ppm level may not be capable of supporting significant levels of microbial activity even under stimulation. Sites with relatively high levels of hydrocarbon impact may actually be better candidates for bioremediation than those lightly impacted at levels slightly above regulatory action levels.
Now to address an additional issue that is specific to xenobiotic industrial hydrocarbons. Xenobiotic compounds are by definition recalcitrant to direct aerobic microbial attack. However, over the last ten years a biodegradation process termed co-oxidation (or cometabolism) has been successfully demonstrated by researchers.
For example, the aerobic degradation of trichloroethylene (TCE) has been accomplished using mono oxygenase and dioxygenase enzymes produced through the use of petroleum hydrocarbons as a metabolizable substrate (food source) and stimulus for enzyme production. This general process is termed co-oxidation and the hydrocarbon substrate used as a food source is the cometabolite.
Many different hydrocarbon substrates have been observed to stimulate the generation of co- oxidation enzymes. The currently known cometabolic substrates fall into two broad classes:
- Analog substrates which are hydrocarbons that have a geometry similar to the targeted xenobiotic compound.
- Methanotrophic (which is different than methanogenic) microbial systems have proven particularly effective at generating xenobiotic active enzymes.
- Other microbial mediated reactions.
Remember, these microbial systems were unknown only a few years ago and this work represents a significant advancement for the science of bioremediation.
Unfortunately, there is a serious problem with regards to field scale implementation of the technology. It lies in the preferential selectivity for action of the generated enzyme. Enzymes with co-oxidizing potential have a strong natural affinity for the hydrocarbon originally stimulated its generation. After all that is the compound the enzyme was genetically tailored to react with. An example serves to illustrate the point, over 300 moles of methane are required to biodegrade 1 mole of TCE via co-oxidation. The efficiency of the co-oxidation process is extremely poor. Under field conditions where mass transport is a critical success factor, a 300 fold decrease in the effectiveness of reactants in the contaminated zone is impractical.
At any given site impacted with petroleum hydrocarbons bioremediation may by a viable clean-up option. Accurate assessment of potential limiting factors should be addressed before committing to that option. While not impossible, at the current state of technical development aerobic in-situ bioremediation of xenobiotic should be regarded as experimental.
Lastly, the implementation of bioremediation is a dynamic process. Prediction is a risky venture and results are best determined empirically by monitoring of the process in the field. This includes monitoring the hydrological, chemical and biological conditions over the life of a project. Understand the meaning of that data in the context of potential limitations and then act upon it. To do otherwise may significantly decrease the potential for a successful conclusion, a clean site.
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