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Bioremediation

How Bioremediation Works

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The Basics
 
     Bioremediation is used to clean up contaminated sites that are environmentally hazardous.  The first thing that you need to understand is that there are microorganisms of some type everywhere.  Although not all microorganisms are able to break down contaminants, many are able to break them down at least partially.  Therefore, when a contamination occurs, the microorganisms present on site are usually able to break down the contamination on some level, however small.  It is rare to find a site where the microorganisms are able to degrade the contamination completely without assistance from some form of bioremediation.  The microorganims are used to break the contaminants down into harmless carbon dioxide (CO2) and water (H2O).  These products can then be used by plants and animals to carry out their daily functions.
 

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This illustration shows oil being broken down into carbon dioxide and water by microorganisms

     Essentially, the goal of bioremediation is to overcome the factors that are limiting the natural degradation of the contaminants.  There are many ways to accomplish this goal.  One way is simply to add nutrients and oxygen to enhance the capabilities of any microorganisms already present at the contamination site.  This method is usually attempted to some degree at every site because it is the simplest approach.  Another way to accomplish the goal is through a process known as bioaugmentation.  Bioaugmentation is when the cleanup crews apply specifically engineered microorganisms that are specially designed to break down the contaminants.

     Each case in which bioremediation is used is unique.  When trying to determine which type of bioremediation to use, we must consider several factors:

  • Type(s) of contamination
  • Extent of contamination
  • Toxicity of contaminants
  • Location and characteristics of the site
  • economic situation
  • microbes source of carbon and energy
  • electron acceptor availability.
  • temperature and other weather conditions
  • nutrients available to the microbes - this includes types of nutrients and amount of nutrients
  • pH of contaminated site - must be between 5 and 10 work in general. Optimal conditions are pH 6-8.
  • moisture content (for soil contamination)

 

     With this many variables, there cannot be a standard bioremediation prodedure in place.  Each case must be analyzed individually.  Taking those factors into account, cleanup crews fromulate a treatment plan that is tailored to the particular situation.  For example, gasoline has over 100 different substances in it. This is because it is a mixture of organic compounds.  The problem with this large number of substances is that no one type of microorganism can break them all down.  Therefore, multiple types of microorganisms will have to be used.  

     Bioremediation will not work at every site. Bioremediation cannot be used in cases where the contaminated site has:

  • High Metal Concentrations -  Metals that cannot be broken down include arsenic, cadmium, mercury, copper, chromium and zinc.  Mercury is the most toxic of heavy metals.
  • Highly Chlorinated Organic Molecules  -  These are organic molecules that have multiple chlorine atoms attached to them.
  • Inorganic salts  -  This includes substances such as sodium thiosulfate, sulfur dioxide, sodium metabisulfite, trimethylamine hydrochoride, and ferric chloride.

     These types of contaminated site cannot be cleaned up by means of bioremediation because the contaminants are so toxic that they kill the microorganisms.

     Upon arrival at the contamination site, cleanup crews perform a wide variety of tests.  They will test for the factors listed above and that will help them determine whether or not bioremediation can be used.  If it can be used, these tests will also decide which type can be used.  In certain cases, the cleanup crew will know before the tests which type they will have to use if the rest of the conditions are fine.  For example, in situ bioremeditation is the only option if the contamination is under a road or building. This example will make much more sense as you learn about the different types.

Soil Sample Collection
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Soil samples are collected from different depths

Lab Testing of the Soil
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The soil sample is taken to a lab where it is tested