The Microbiology of In Situ Bioremediation

by Eric Mellor, Paul Landin, Christine O'Donovan, Dave Connor

Spring 1996

Table of Contents

Introduction
Bacteria
Aerobic vs. Anaerobic Bioremediation
Enyzmes
Bioremediation Processes
Research on the Degredation of Metals
Conclusion

Introduction

In situ bioremediation is the application of biological treatment to the cleanup of hazardous chemicals in the soil and surface or subsurface waters. In the many forms of bioremediation, microorganisms are utilized and managed through the control of environmental factors to reduce environmental pollution. Normally bioremediation treats organic contaminants. Recently, progress has been made in the development of microbes degrading heavy metals. Most bioremediation processes utilize indigenous microorganisms, although some rely on the introduction of bacterial or fungal strains. Currently studies are being performed on genetically engineered microbes as well as the possibilities of algae for use in the field of bioremediation.

Bacterial digestion is the process of bacteria-consuming organic matter. The bacteria feed on the contamination, deriving nutrition for growth and reproduction. Undergoing complex chemical reactions, the waste is metabolized into the final metabolic waste products, water and carbon dioxide. This provides the bacteria with the energythey need to live. The end result of this natural process is that wastes are used up or converted into a less harmful form.

Bacteria

A bacteria is a single cell life form. Each individual cell is a separate, unique organism. Bacteria often grow into colonies but each cell remains an independent life. Bacteria reproduce by a process called cell division. A mature bacteria reproduces by dividing into two 'daughter cells', each cell identical to the other and the parent bacteria. Under ideal conditions, bacteria can reproduce rapidly, producing a new generation every 20 to 30 minutes. Thousands of dlfferent species of bacteria exist everywhere in our world, and most of them carry on bacterial digestion in some way. However, some of them are found only in a specific environment, require specialized types of food, or have very unique niches.

Following the reproduction process, the number of individual bacteria doubles with each generation. The population explodes as the number of microorganisms increases Logarithmically. At some point, the food source will be depleted, or some other change in the environment will cause the population to decrease. These changes could be pH, temperature, or oxygen content of the environment.

For waste digestion, we can identify several beneficial charateristics that bacteria should have. They must:

Consume (digest) organic waste.
Digest waste quickly and completely, without causing odors or noxious gas.
Not cause disease in man or animals (non-pathogenic).
Grow and reproduce readily in the environmental conditions of organic waste.

Aerobic vs Anaerobic Bioremediation

These bacteria can be further separated into aerobic types, whlch require oxygen to live, and anaerobic, which can live without oxygen. (Aerobic bioremediation usually is preferred because it degrades pollutants 10 to 100 times faster than anaerobic bioremediation.) Facultative types can thrive under either aerobic and anerobic conditions. Certain bacteria belonging to the Bacillus and Pseudomonas (show to the left) species have these desirable characteristics. They consume organic waste thousands of times faster than the types of bacteria that are naturally present in the waste. They grow and reproduce easily, are non-pathogenic, and do not produce foul odors or gas. These bacteria are cultured on a liquid or dry agar. These cultured bacteria are then freeze dried leaving them in a state of suspension. They remain alive and will function normally as soon as they are rehydrated and put into an acceptable environment. This environment should induce rapid growth and reproduction of these bacteria and must have:

A water medium containing food (organic waste) for them to eat.
Dissolved oxygen in sufficient quantities for the aerobic types that require it. (It provides an electron acceptor.)
The proper pH (not too acidic nor too alkaline; between 6 and 9 on the pH scale).
Moderate temperatures, between 50 and 100 degrees F.

Enzymes

Enzymes are necessary for the proper functioning of the bacteria. An enzyme is a chemical catalyst that breaks up long, complex waste molecules into smaller ones. The smaller particles can be digested directly by the bacteria. Essential nutrients are added to supply the vitamins and minerals required for the growth and activity of the bacteria. These vitamins and minerals might not be present at the contamination site, and a lack of any one of them will inhibit the growth or reproduction of the microbes. They must be added to the site to assure the fastest, most efficient waste digestion.

Bioremediation Processes

Mechanisms of bioremediation include bioaugmentation in which microbes and nutrients are added to the contaminated site or biostimulation in which nutrients and enzymes are added to supplement the intrinsic microbes. In the injection method, bacteria and nutrients are injected directly into the contaminated aquifer, or nutrients and enzymes, often referred to as 'fertilizer', that stimulate the activity of the bacteria are added. In soil remediation, usually nutrients and enzymes are added to stimulate the natural soil bacteria, though sometimes both nutrients and bacteria are added. When the treatment is stopped, the bacteria die. This technique works best on petroleum contamination.

Bacteria can degrade the following compounds with relative ease:

Petroleum or hydrocarbon products: gasoline, diesel, fuel oil.
Hazardous crude oil compounds: benzene, toluene, xylene, naphthalene.
Some polynuclear aromatics
Some pesticides: malathion
Coal compounds: phenols and cyanide in coal tars and coke waste.
Some industrial solvents: acetone.
Miscellaneous: ethers; simple alcohols such as methanol, and other ground water contaminants including: methylethylketone; ethylene glycol

Some chemicals are only partially degradable, or sometimes wastes that are so mixed and variable that they degrade at different rates and may leave some toxic chemicals behind. These include:

TCE (trichloroethylene)
PCE (perchloroethylene): it degrades to TCE when no oxygen is present
Pentachlorophenol and other ingredients in coal tar and wood preservatives
PCBs and dioxin
Arsenic, chromium and selenium
Currently experiments are being performed on the bioremediation of certain metals. Heavy metals are not biodegradable, but bacteria can concentrate them into forms that make them more easily disposable. These include: uranium, mercury, cadmium, sulfur, and DDT.

Research on the Degradation of Metals

In addition to the research being conducted on the different micobes degrading various metals, research is being performedon algae as well as genetically engineered microbe cultures. Among the algae, blue green algae also known as cyanobacteria (two examples shown to the left), appear to be the most promising. Despite the public outcry against the release of genetically engineered organisms, there are some advantages to these cultures. Many sites have more than one pollutant and genetically engineered microbes are more efficient and do not produce toxic intermediate products. Pseudomonas is often used in genetic engineering because certain species have degrative pathways coded for by plasmids (Dart and Stretton 1977). Plasmids are extra chromosomal DNA that are not associated with the nucleus of the cell. By altering the plasmids or adding to them, biodegradation may be accelerated or altered.

Conclusion

In conclusion, in situ bioremediation is the application of biological treatment to the clean up of contaminants in soil, groundwater, and surface waters. During the process, microorganisms, usually bacteria and fungi, feedon the contaminants. They derive nutrition and energy for growth and reproduction. The wastes are used up or converted into a less harmful form, such as water and carbon dioxide. Mechanisms of bioremediation include bioaugmentation, in which microbes and nutrients are added to the contaminated site; and biostimulation in which nutrients and enzymes, referred to as 'fertilizer,' are added to stimulate intrinsic microbes. Bioremediation has proven successful on petroleum and hydrocarbon contamination. Currently research is being performed on the use of microbes to degrade metals. The use of algae and genetically engineered cultures is also being researched.

Fungi are italicized

Works Consulted:

Dart and Stretton. Microbial Aspects of Pollution Control. New York: Elsevier Scientific Publishing Company, 1977.

Norris, Robert D. Fourth Quarter 1996 Practical Applications of Bioremediation Technology. Remediation Management

Rosenberg, E. Microorganisms to Combat Pollution. Boston: Kluwer Academic Publishers, 1993.

Sutherson, Suthan S. Remediation Engineering; Design Concepts. New York: CRC Lewis Publishers, 1997.

WWW Resources used:

Eo-zyme Environmental. Bioremediation: Minimizing Waste Naturally. Previously available at: http://www.cleanup.com/eozyme.article.html/

United States Geological Suvey. Bioremediaton: Nature's Way to a Cleaner Environment. Available at: http://h2o.usgs.gov/public/wid/html/bioremed.html

Washington State University and University of Washington. 1994. In situ Bioremediation of Groundwater. Previously available at: http://www.pnl.gov/WEBTECH/voc/biorem.html

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Copyright © 1998 Daniel Gallagher
Last Modified: June 7, 1998