In Situ Bioremediation
In situ bioremediation involves the treatment of the conamination on site. In the case of soil contamination, in situ bioremediation involves the addition of mineral nutrients. These nutrients increase the degradation ability of the microorganisms that are already present in the soil.
Sometimes new microorganisms are added to the contaminated area. Microorganisms can sometimes be genetically engineered to degrade specific contaminants. An example of a microorganism that has been genetically engineered is Pseudomonas fluorescens HK44. These genetically engineered microorganisms can be designed for the conditions at the site.
Which approach is taken depends upon the relationship between the type of contamination and the type(s) of microorganisms already present at the contamination site. For example, if the microorganisms already present are appropriate to break down the type of contamination, cleanup crews may only need to "feed" these microorganisms by the addition of fertilizers, nutrients, oxygen, phosphorus, etc.
There are two frequently used methods of supplying oxygen to the microorganisms.
Bioventing - This consists of blowing air from the atmosphere into the contaminated soil. First, injection wells must be dug into the contaminated soil. How many wells, how close together they go, how deep they are dug all depends on the factors affecting the rate of degradation (type of contamination, type of soil, nutrient levels, concentration of conatminants, etc). Once all of the injection wells are dug, an air blower is used to control the supply of air that is given to the microorganisms. These injection wells can also be used to add nitrogen and phosphorus, maximizing the rate of degradation.
Hydrogen Peroxide Injection - In cases in which the contamination has already reached the groundwater, bioventing will not be very successful. Instead, hydrogen peroxide is used. It functions much the same as bioventing, using the hydrogen peroxide instead of air blowers to deliver oxygen to the microorganisms. If the soil is shallow (the groundwater is fairly close to the surface) the hydrogen peroxide can be administered through sprinkler systems. If the groundwater is fairly deep beneath the surface, injection wells are used.
Ex Situ Bioremediation
Ex situ bioremediation involves the physical extraction of the contaminated media to another location for treatment. If the contaminants are just in the soil, the contaminated soil is excavated and transported for treatment. If the contamination has reached the groundwater, it must be pumped and any contaminated soil must also be removed.
One major thing that this removal of the contaminants does right away is stop the spread of the contamination. Provided that the cleanup crews to a good job in the excavation process, there should ideally be no remaining contaminants, but it is also usually alright to have a minimal amount of contaminants rermaining. If minimal contaminants do remain in the soil, they can likely be broken down by the naturally occuring microorganisms already present.
There are two main types of ex situ bioremediation. They are refered to as phases:
Solid Phase - Solid phase treatment consists of placing the excavated materials into an above ground enclosure. Inside this enclosure, the contaminated soil is spread onto a treatment bed. This treatment bed usually has some kind of built-in aeration system. Using this system, cleanup crews are able to control the nutrients, moisture, heat, oxygen and pH. This allows them to maximize the efficiency of the bioremediation. The soil can also be tilled like farmland, helping to provide oxygen and enable additional aerobic biodegradiation of the contamination. Solid phase treatment is especially effective if the contaminants are fuel hydrocarbons. However, it does require a lot of space and sometimes it cannot be used for that very reason.
There are three solid phase bioremediation techniques. They are: landfarming, biopiling, and composting.
Slurry Phase - the contaminated soil is excavated and removed from the site as completely as possible. The contaminants are then put into a large tank which is known as a bioreactor. Cleanup crews use this bioreactor to mix the contaminants and the microorganisms. This mixing process keeps the microorganisms in constant contact with the contaminants. Water, oxygen, and nutrients are added. Since the cleanup crews have complete control of the conditions in the bioreactor, they can adjust things until they acheive the optimal conditions for the degradation of the contaminants. Since the degradation can be kept at or very close to optimal conditions, it does not take very long to break down the contaminants.
In fact, slurry phase bioremediation is much faster than many other bioremediation techniques. It is very useful in cases in which the contaminants need to be broken down very quickly. Another advantage to slurry phase bioremediation is the fact that it can be a permanant solution to the problem.
However, it is not always a permanant solution. It's success is highly dependant upon the chemical properties of the soil and contaminations. Slurry phase bioremediation definately has some diadvantages. For example, the rate of treatment is limited by the size of the bioreactor. That is, if a small bioreactor is being used, the rate of degradation will be very slow. Also, additional treatment of the wastewater is required. After the additional treatment, the waste water must then somehow be disposed of. These things add quite a bit of cost. They are part of the reason that slurry phase bioremediation has a high operating cost as well as a fairly high capital cost.