Bioremediation techniques are destruction techniques to stimulate the growth of microorganisms, using the contaminants as a food and energy source.
- These techniques have been successfully used to remediate soils/sludges & groundwater contaminated by petroleum hydrocarbons, solvents, pesticides, wood preservatives, and other organic chemicals.
- Oxygen, water & nutrients are added, and the temperature and pH are controlled.
- The rate at which microorganisms degrade the contaminants is influenced by: the specific contaminants present, their concentrations, the oxygen supply, moisture, temperature, pH, nutrient supply, bio-augmentation, and co-metabolism.
- Micro-organisms can be adapted to degrade specific contaminants or enhance the process.
In situ bioremediation
- In situ — it involves treatment of the contaminated material at the site.
- Bioventing: supply of air and nutrients through wells to contaminated soil to stimulate the growth of indigenous bacteria. It is used for simple hydrocarbons and can be used where the contamination is deep under the surface.
- Biosparging: Injection of air under pressure below the water table to increase groundwater oxygen concentrations and enhance the rate of biological degradation of contaminants by naturally occurring bacteria.
- Bio augmentation: Microorganisms are imported to a contaminated site to enhance degradation process.
- Bioleaching: It is the extraction of metals from their ores through the use of living organisms. This is much cleaner than the traditional heap leaching using cyanide.
- No need to excavate & transport soils - typically less expensive
- Can treat a large volume of soil at once.
- Causes less contaminants to be released than ex situ techniques
- Creates less dust
- Most effective if permeable sandy soil (uncompacted)
- Least effective in clays/highly layered subsurface environments - oxygen cannot be evenly distributed throughout the treatment area.
- May be slower to reach cleanup goal (if less easily degradable contaminant, requires years).
- May be more difficult to manage (than ex situ techniques).
- Bioreactors: These are large vessels where the contaminated material can be monitored and conditions for bioremediation can be controlled. Biological organisms typically have conditions where they operate best.
In bioreactors we can control the mixing rate, temperature, pH, and nutrient levels to suit the organisms breaking down our contaminant.
- Land farming: It involves spreading contaminated soil into a lined bed (to prevent leaching) and periodically applying nutrients and mixing the soil to boost biological activity.
- Bio piling: It places the contaminated soil into piles that are well aerated and nutrients are added to speed up bioremediation. In all cases, the contaminant levels are monitored to verify that bioremediation is taking place and steps are taken to ensure that contaminated material stays out of contact with the environment.
Advantages & Disadvantages of ex situ bioremediation:
Ex situ techniques can be faster, easier to control, and used to treat a wider range of contaminants and soil types than in situ techniques.
There is more certainty about the uniformity of treatment because of the ability to homogenize, screen, and continuously mix the soil.
However, they require excavation of soils, leading to increased costs and engineering for equipment,
More risk of material handling/worker exposure conditions.
Usually requires treatment of the contaminated soil before and, sometimes, after the actual bioremediation step.
Ex situ techniques include: slurry & solid phase bioremediation:
Solid-phase soil treatment processes include land farming, soil bio piles, and composting.
Slurry-phase soil treatment processes include the slurry phase bio-reactor.