Article Index

4.2. Bioremediation Technologies

Bioremediation technologies can be grouped into five techniques: in situ, ex situ, bioreactor, natural attenuation, and phytoremediation. These techniques do primarily one of two things: they either remove the contaminants from the substratum (decontamination or cleanup techniques) or reduce the risk posed by the contaminants by reducing exposure (stabilization techniques).


4.2.1. In situ Bioremediation Techniques

Bioremediation technologies that are used “in place” without removal of the contaminated matrix from air, water and soil. The advantages and disadvantages of these techniques are given in Table 4.1. Commonly used in situ treatments are:

Bioventing

Bioventing involves supplying air and nutrients through wells to contaminated soil to stimulate the local bacteria. If the contamination is deep under the surface, this technique can be used for environmental cleaning. Bioventing supply low air flow rates and provides only the amount of oxygen necessary for the biodegradation while minimizing volatilization and release of contaminants to the atmosphere.

Biodegradation

Biodegradation involves supplying oxygen and nutrients by circulating aqueous solutions through contaminated soils to stimulate indigenously occuring multiple organisms to degrade organic contaminants. Generally, this technique can be used for contaminated soil and groundwater.

Biosparging

This method is one of the in situ remediation technologies that use local microorganisms to biodegrade organic ingredient in the saturated site. In order to increase the biological activity of the naturally occuring microorganisms, air and nutrients are injected into the saturated zone. Biosparging is used to clean groundwater and soil contaminated with petroleum constituents. The ease and low cost of installing smalldiameter air injection points allows considerable flexibility in the design and construction of the system.

Bioaugmentation

This method involves the addition of microorganisms naturally occuring or exogenous to the contaminated sites. This method is especially useful for contaminated soil with inorganic compounds.

Biostimulation

Biostimulation is a type of natural remediation that can improve pollutant degradation by optimizing conditions such as aeration, addition of nutrients, pH and temperature control. This method can be considered as an appropriate remediation technique for petroleum pollutant’s removal in soil and requires the evaluation of both the intrinsic degradation capacities of the autochthonous microflora and the environmental parameters involved in the kinetics of the in situ process.

Table 4.1. Advantages and disadvantages of in situ techniques

Advantages Disadvantages

•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 the soil is 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)

•The seasonal variation of microbial activity exists

•The sites are directly exposed to environmental factors like temperature, oxygen supply etc.

•Problematic application of treatment additives like nutrients, surfactants, oxygen etc.

•It is a very tedious and time consuming process.


4.2.2. Ex Situ Bioremediation Techniques

Ex situ technologies are remediation options where the affected soil (excavation) and water (pumping) are removed from their original location and cleaned on-site or off-site. The most important ex situ bioremediation treatment processes include landfarming, biopiles, and composting. The advantages and disadvantages of these techniques are given in Table 4.2.

Land farming

These technique applications are the easiest and most effective method for petroleum contaminated soil. The main aim is to situmulate natural biodegradative organisms and to facilitate their aerobic degradation of pollutants. Land farming received much attention as a disposal alternative technique because it has the potential to reduce monitoring and maintenance costs, as well as clean-up liabilities. However, the practice is limited to the treatment of superficial 10–35 cm of soil.

Composting

Composting method is the process of degrading organic wastes by microorganisms under thermorphilic condition (40-65°C). This method has been applied to soils and biosolids contaminated with petroleum hydrocarbons, solvents, chlorophenols, pesticides, herbicides, PAHs, and nitro-aromatic explosives.

Biopiles

Biopile is a bioremediation technology in which land farming and composting are used as a hybrid system. This technique, which is a refined version of land farming that tend to control physical losses of the contaminants by leaching and volatilization is used for treatment of surface contamination with petroleum hydrocarbons. Biopiles have been considered as a feasible, cost effective and less destructive remediation technique for petroleum contaminated soils.

Table 4.2. Advantages and disadvantages of ex situ techniques

Advantages Disadvantages

  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.

•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.


4.2.3. Bioreactors

It is a large fermentation chamber for growing organisms (bacteria or yeast) that are used in the biotechnological production of substances such as pharmaceuticals. It is also used for the conversion of harmful waste to less harmful substances. This technique is used to remediate contaminated soil and water with fuel hydrocarbons and organics. Bioreactor has two phases (dry and slurry). The slurry phase bioremediation is a relatively more rapid process compared to the dry phase. In a slurry phase bioreactor, contaminated soil is combined with water and other additives in a large chamber. In contrast to slurry phase, dry phase bioreactor does not include extra water. The advantages and disadvantages of this technique are given in Table 4.3.

Table 4.3. Advantages and disadvantages of bioreactor technique.

Advantages Disadvantages

•  Relatively rapid treatment

•  Reduced pellet formation

•  Increased slurry homogenization

•  Increased bioavailability

•  Soil-water separation can become a problem.

•  There is a need for wastewater treatment after the soil is dewatered.


4.2.4. Natural Attenuation

The natural attenuation is a process to reduce the mass, toxicity, mobility, volume, or concentration of contaminants (organic and inorganic) in soil, groundwater and surface water. These processes are grouped as physical, chemical and biological. Physical phenomena are advection, dispersion, dilution, diffusion, volatilization and sorption/desorption. Chemical processes are ion exchange, complexation and abiotic transformation. Biological processes are aerobic and anaerobic biodegradation, plant and animal uptake. Natural attenuation is a cost-effective remediation technology. The advantages and disadvantages of this technique are given in Table 4.4.

In order to use natural attenuation as a cleaning process, sites must meet one or more of the following criteria:

•It must be located in an area with little risk to human health or to the environment.

•The contaminated soil or groundwater must be located an adequate distance from potential receptors.

•There must be evidence that natural attenuation is actually occurring at the site.

•High permeability speeds contaminant spread, low permeability slows the breakdown. Ideally, natural attenuation works best in soils whose permeability ranks somewhere between high and low.

 Table 4.4. Advantages and disadvantages of natural attenuation technique

Advantages Disadvantages

•  In situ destruction - no wastes generated and no cross-media transfer

•  Little risk to human health or to the environment

•  The most toxic and mobile contaminants usually biodegrade most quickly and reliably

•  Non-intrusive

•  Cost-effective

•  Easily combined with other remedies

•  No down time due to equipment failures

•  Time frames may be as long as remediation by groundwater extraction and treatment  

•  Long-term monitoring

•  Aquifer heterogeneity complicates site characterization (not unique to natural attenuation)

•  Intermediates of biodegradation may be more toxic than the original contaminants

•  Occasionally more expensive than other remedies, especially pump and treat (due to monitoring requirements)


4.2.5. Phytoremediation

Phytoremediation is a bioremediation technique that uses various types of plants and associated microorganisms to remove, transfer, stabily, and destroy contaminants in the soil, sludge, sediments, wastewater, groundwater and air. Plants are able to indicate, exclude, accumulate, and hyperaccumulate or metabolise toxic inorganic or organic substances. Thereby they contribute significantly to the fate of chemicals, and they can be used to remove unwanted compounds from the biosphere. Phytoremediation offers an environmentally friendly, cost effective, and carbon neutral approach for the cleanup of toxic pollutants from the environment. The advantages and disadvantages of this technique are given in Table 4.5.

Phytoremediation mechanisms involve phyto-extraction, phyto-stabilization, phyto-volatilization, phyto-degradation, phyto-accumulation, rhizofiltration, rhizosphere biodegradation, and hydraulic control. These mechanisms are shown in Figure 4.1.

Phyto-extraction

Phyto-extraction uses plants or algae to remove contaminants from soils, sediments or water. In this mechanism, plants remove heavy metals or radionuclides from soil through their root system and accumulate them in the root or transport them up into shoot or leaves. Plant may continue to remove contaminants until it is harvested. After harvest, if the contaminated soil does not reach significant cleaning up level, the growth/harvest cycle must usually be repeated through several crops to achieve a significant cleanup.

The main advantage of phyto-extraction is being environmental friendly. It does not cause any harm to soil quality. Another benefit of phyto-extraction is that it is less expensive than any other clean up process. As this process is controlled by plant, it takes more time than any traditional soil clean up process.

Phyto-stabilization

In this mechanism, the plants immobilize the contaminated chemical compounds from soil and water. Contaminants are taken up by roots and precipitated in the rhizosphere. This prevents the contaminant getting into food chain.

Phyto-volatilization

In this mechanism, water containing organic contaminants is taken up by plants, and the contaminants are released into the air through by their leaves. The contaminant may become modified along the way, as the water travels along the plant's vascular system from the roots to the leaves, whereby the contaminants evaporate or volatilize into the air surrounding the plant.

Phyto-degradation

In this mechanism, contaminants (organic) are metabolized and destroyed within plant tissues. These smaller pollutant molecules may then be used as metabolites by the plant as it grows, thus becoming incorporated into the plant tissues.

Phyto-accumulation

In this mechanism, contaminants along with other nutrients and water are taken up by plant roots. This contaminant mass is not destroyed but accumulate in the plant shoots and leaves.

Rhizofiltration

This mechanism uses both terrestrial and aquatic plants to absorb, concentrate and precipitate metal from contaminanted water (surface or groundwater).

Rhizosphere biodegradation (Rhizodegration)

In this mechanism, plants relief common substances through their roots, supplying nutrients to microorganisms in the soil. So the microorganisms enhance biological degradation. For instance, plant root exudates carbohydrate sources (sugars, alcohols, and organic acids) for the soil microflora. These compounds enhance microbial growth and activity, and also they act as chemotactic signals for microbes.

Hydraulic control

In this mechanism, contaminated ground water is remediated by trees.

LO1 1

 Figure 4.1. Phytoremediation mechanisms

 Table 4.5. Advantages and disadvantages of phytoremediation technique

Advantages     Disadvantages

•  Phytoremediation is less expensive than the old "pump and treat" method for the treatment of contaminated water.

•  Phytoremediation is also much less expensive than digging out the contaminated site.

•  Up to 95% of TCE present in water could be removed by simply planting trees and letting them grow.

•  Phytoremediation takes no maintenance once instituted.

•  Since phytoremediation uses plants, it is aesthetically pleasing.

•  After plants are introduced, wildlife is able to flourish at the once uninhabitable site.

•  Solar energy is used to drive the cleansing activity.

•  Phytoremediation is limited to sites with lower contaminant concentrations.

•  Phytoremediation is restricted to sites with contamination as deep as the roots of the plants being used.

•  The food chain could be adversely affected by the degradation of chemicals.

•  The air could be contaminated by the burning of leaves or limbs of plants containing dangerous chemicals.

 backnext

Intranet