Table of Contents
- What is bioremediation?
- What is biodegradation?
- Bioremediation process
- Bioremediation techniques
- Ex-situ bioremediation
- In-situ bioremediation
- Bioremediation techniques
- Types of bioremediation
- Example of Bioremediation
- Advantages of Bioremediation
- Disadvantages of bioremediation
What is bioremediation?
Bioremediation in ecology refers to a type of biotechnology that involves the use of living organisms to remove pollutants, contaminants, and toxins from water, soil, and other environments. Such living organisms used include plants, microbes, bacteria, and fungi and fall under the study of ecology.
This biotechnology is mainly used to clean up oil spills, soil contaminated with acidic mining drainage, crime scenes, underground pipe leaks, or contaminated groundwater. Bioremediation is done by altering environmental conditions to enhance the growth of microbes to degrade the target pollutants. These target pollutants are metabolized by enzymes present in these microbes.
Bioremediation has been seen as a solution for emerging contaminant problems. A number of microorganisms are involved in the bioremediation process including fungi, aerobic, and anaerobic bacteria.
Bioremediation involves the removal, degradation, immobilization, or detoxification of diverse chemical wastes and physical hazardous materials from the environment through the action of microorganisms. The main concept is degrading and converting these contaminants and pollutants to their less toxic forms.
Furthermore, the process of bioremediation involves oxidation-reduction reactions. In an oxidation-reduction reaction, there is oxidation between the electron donor and the electron acceptor. Oxidation is the transfer of electrons away from a compound. The electron donor is the compound that donates electrons in the reaction and is said to be oxidized.
Whereas, the electron acceptor is the compound that receives electrons and is said to be reduced. This oxidation-reduction reaction normally supplies the energy that microbes use for reproduction and growth. In bioremediation, electron acceptors are oxygen, sulfate, iron, and nitrate. The organic contaminant usually serves as the electron donor.
Hence, in the bioremediation process, it could involve the addition of an electron acceptor like oxygen to initiate the oxidation of a reduced pollutant like hydrocarbons. Likewise, it can also involve the addition of an electron donor like an organic substrate to reduce oxidized pollutants like explosives, oxidized metals, chlorinated solvents, nitrates, propellants, and perchlorate. Human activities such as industrialization and agricultural processes create unwanted byproducts that bioremediation can be used to reduce.
Bioremediation however is less expensive and more sustainable compared to other alternatives for remediation. Some other remediation techniques are vitrification, air stripping, bioleaching, soil washing, rhizofiltration, and thermal desorption. The bioremediation end goal is the reduction and removal of harmful compounds to improve water quality and soil.
However, these harmful compounds can be reduced and removed with bioremediation techniques that could be in-situ or ex-situ. These techniques are grouped based on location. In-situ bioremediation techniques will treat contaminated sites in a non-destructive manner and are very cost-effective. Whereas, ex-situ bioremediation techniques will warrant the evacuation of the polluted site which invokes and increases costs.
However, in both in-situ and ex-situ bioremediation, nutrients, vitamins, pH buffers, and minerals may be added to optimize conditions for the microbes.
Currently, several methods and strategies are employed for the bioremediation process. One of the techniques involves the addition of specialized microbial cultures to further promote biodegradation. This type of bioremediation is called biostimulation. Other types of bioremediation include bioaugmentation, phytoremediation, bioventing, landfarming, biopiles, windrows, bioattenuation, and biosparging.
Bioremediation definition in biology
Bioremediation can be defined in biology as a process whereby biological organisms are used to degrade or remove an environmental pollutant through their metabolic process. These biological organisms include plants as in phytoremediation and microbes such as bacteria, fungi, algae, and bacteria.
On earth, microbes grow in the widest range of habitats. They can grow in water, soil, animals, plants, and even in freezing ice environments. However, the large population of microorganisms and their appetite for a large range of chemicals makes them the perfect agent for bioremediation.
Bioremediation is treated as a waste management technique in biology. This technique involves the use of living organisms to remove or neutralize pollutants from a contaminated site. It is seen as a treatment technique, where microorganisms are used to break down harmful substances into less toxic or non-toxic substances.
Bioremediation, however, is different from other treatment processes and waste management techniques because it uses no toxic chemicals. There are other remediation processes that polluted or contaminated solid or water is purified by chemical treatment, burial in landfill, and incineration. Also, there are other waste management techniques like nuclear waste management, solid waste management, etc.
Bioremediation in microbiology
In microbiology, bioremediation is the use of microorganisms to break down contaminants and pollutants that pose as health and environmental hazard. In this process, natural existing microbial communities are stimulated to break down a pollutant or contaminant by providing these microbes with nutrients and other needs.
Bioremediation involves the removal, degradation, immobilization, or detoxification of diverse chemical wastes and physical hazardous materials from the environment through the action of microorganisms. The main concept is degrading and converting these contaminants and pollutants to their less toxic forms.
However, the process of removing a pollutant is mainly dependent on the nature of the pollutant. The pollutant or contaminant could range from heavy metals, greenhouse gasses, hydrocarbons, nuclear waste, xenobiotic compounds, pesticides, organic halogens, agrochemicals, chlorinated compounds, dyes plastics to sludge.
- Pseudomonas putida
- Pseudomonas stutzeri
- Achromobacter spp
- Bacillus megaterium
- Pseudomonas fluorescens
- Achromobacter xylosoxidans
- Pseudomonas pseudoalcaligenes
- Cupriavidus necator
- Bacillus licheniformis
What is the goal of bioremediation?
The bioremediation end goal is the reduction and removal of harmful compounds to improve water quality and soil. The concept goal is stimulating microorganisms with nutrients and other chemicals for them to be able to destroy the contaminants. Today, the bioremediation systems in operation rely on microbes that are native to the contaminated sites. The microbes are encouraged to work by supplying the optimum levels of nutrients and other chemicals needed for their metabolism. However, bioremediation systems are limited today by the capabilities of the native microbes.
As a result, scientists and researchers are currently investigating ways to introduce nonnative microbes and genetically engineered microbes to polluted sites. Genetically modified microbes can be suited to degrade the pollutant of concern at particular sites. However, bioaugmentation, a type of bioremediation could increase the range of possibilities for future bioremediation systems.
Also understanding how these microbes degrade or destroy pollutants is important to understanding bioremediation. Hence, the type of microbial process that will be utilized in the clean-up will determine what nutritional supplements the bioremediation system has to supply. Moreso, the byproducts of microbial processes can indicate the success of bioremediation.
What is biodegradation?
Biodegradation is the breaking down of organic material by microbes like fungi and bacteria. The process of biodegradation involves 3 stages of biodeterioration, biofragmentation, and assimilation.
Biodeterioration is explained at times as a surface-level degradation. During biodeterioration, the physical, mechanical, and chemical properties of the material are altered. This usually occurs when the material is exposed to environmental abiotic factors. Thus, by weakening the structure of the material, it allows further degradation.
Some of these environmental abiotic factors that influence this alteration are compression (mechanical), light, temperature, and chemicals in the environment. Biodeterioration is the first stage of biodegradation although in some cases it can happen alongside biofragmentation.
Actions like the breakdown of stone facades of buildings, esthetic changes on man-made products by the growth of microbes, and corrosion of metals by microorganisms are examples of biodeterioration.
Biofragmentation of a substance is the process where the bonds within the substance are broken down into oligomers and monomers. The process involved in the fragmentation of a substance varies based on the presence of oxygen. When oxygen is present in the system, the fragmentation of a substance by microbes is aerobic digestion. However, when oxygen is absent, the fragmentation process by microbes is anaerobic digestion.
Both reactions release carbon dioxide, water, new biomass, and some type of residue. Though usually, anaerobic reactions release methane while aerobic reactions do not. However, aerobic digestion happens faster than anaerobic digestion even though anaerobic digestion reduces the volume and mass of organic material better. This is why in addition to this attribute and the production of methane, anaerobic digestion technology is used widely as a source of biomass energy and for waste management systems.
After biofragmentation, the resulting products are then integrated into microbial cells. This is the process of assimilation. Some of these products are easily transported within the cell by membrane carriers. Whereas, some still need to undergo biotransformation reactions. This will enable it to yield products that can then be transported inside the cell. Once these products are inside the cell, they enter catabolic pathways. Either leading to the production of adenosine triphosphate (ATP) or elements of the cells.
Furthermore, almost all chemical compounds and materials can be biodegraded. The significant difference, however, is that the rate of biodegradation of some material would take days, some weeks, years, or even centuries. Several factors like water, light, temperature, and oxygen determine the rate at which the degradation of organic compounds occurs.
How bioremediation work is basically by stimulating the growth of microorganisms. This is because microbes use the contaminant as a source of food and energy. These microorganisms convert the pollutants into small amounts of water and harmless gasses such as carbon dioxide.
For the process of bioremediation, the right temperature, food, and nutrients are needed. Without these elements, the cleanup of the pollutants may be prolonged. However, unfavorable conditions for bioremediation can be enhanced by the addition of amendments to the environment. Such amendments could be vegetable oil, molasses, or simple air added to optimize conditions for the microorganism to flourish. As these microbes flourish, it accelerates the completion of the bioremediation process.
Microbes like fungi and bacteria are the primary agents in the process of bioremediation. Bacteria, however, are the most crucial microorganism in the process. This is because they degrade the waste compounds into nutrients and organic matter. Bacteria can easily digest contaminants like chlorinated pesticides or clean oil spills easily. But microbes cannot destroy heavy metals like cadmium and lead. So, even though bioremediation may be an efficient waste management process, it destroying contaminants is not 100%.
Currently, bioremediation is commercially used to clean up a limited range of pollutants. It is used mostly to clean up hydrocarbons found in gasoline. Microbes have the ability to biodegrade almost all organic and many inorganic pollutants. Microorganisms use pollutants for their reproduction and growth. These organic pollutants provide electrons that microbes can extract to gain energy.
Also, from organic pollutants, a source of carbon is derived which is the main building block of new cell constituents from microorganisms. Microorganisms break down contaminants because in the process of degradation they gain energy that aids their growth and reproduction. This energy is gotten by microbes from the pollutants by breaking chemical bonds. They transfer electrons from the pollutants to an electron acceptor like oxygen. This chemical reaction is called an oxidation-reduction reaction.
In an oxidation-reduction reaction, there is oxidation between the electron donor and the electron acceptor. Oxidation is the transfer of electrons away from a compound. The electron donor is the compound that donates electrons in the reaction and is said to be oxidized. Whereas, the electron acceptor is the compound that receives electrons and is said to be reduced. This oxidation-reduction reaction normally supplies the energy that microbes use for reproduction and growth. In bioremediation, electron acceptors are oxygen, sulfate, iron, and nitrate while the organic contaminant usually serves as the electron donor.
Then, microbes use the energy together with some electrons and carbon gotten from the pollutants to generate more cells. However, the electron donor and electron acceptor are crucial for cell growth. They are referred to as primary substrates. Hence, in the bioremediation process, it could involve the addition of an electron acceptor like oxygen to initiate the oxidation of a reduced pollutant like hydrocarbons. Likewise, it can also involve the addition of an electron donor like an organic substrate to reduce oxidized pollutants like explosives, oxidized metals, chlorinated solvents, nitrates, propellants, and perchlorate.
Several microbes use molecular oxygen as the electron acceptor. However, the process of microbes degrading organic compounds with the help of oxygen is called aerobic respiration. Thus, in aerobic respiration, microorganisms use oxygen to oxidize part of the carbon in the pollutant to carbon dioxide. Then, they use the rest of the carbon to generate a new cell mass.
Furthermore, the process of bioremediation can be done in-situ or ex-situ. In-situ bioremediation is done at the site whereas ex-situ bioremediation is done away from the site. However, ex-situ bioremediation may be needed if the climate is too cold to support microbial activities. It can also be necessary if the soil is too dense for nutrients to be evenly distributed.
Excavating and cleaning the soil above ground may even be required in ex-situ bioremediation. This, however, may incur significant costs to the bioremediation process. Anywhere, the bioremediation process may take several months to years to complete. Although this depends on variable conditions. Conditions like the size of the polluted area, temperature, the concentration of contaminants, soil density, and whether bioremediation will be in-situ or ex-situ.
Techniques in bioremediation can be done ex-situ and in-situ. There are selection standards to be considered before selecting any bioremediation technique. Such selection standards include the nature of the pollutant, environment type, depth and amount of pollution, cost, location, and environmental policies. Nutrient concentration, pH, temperature, oxygen concentrations, and other abiotic factors determine how successful the bioremediation processes will be.
These bioremediation techniques include excavating pollutants from polluted sites and transporting them for treatment to another site. Ex-situ bioremediation techniques are usually considered based on the pollutant type, depth of pollution, extent of pollution, geographical location of the polluted site, and cost of treatment. The performance standards also control the choice of ex-situ techniques.
Techniques in ex-situ bioremediation
Solid-phase treatment is an ex-situ technology. In this ex-situ technique, the contaminated soil is excavated and placed into piles. Pipes are distributed throughout the pile and microbial growth is then moved through pipes. However, air pulling through the pipes is crucial for microbial respiration and ventilation. A solid-phase system needs a huge amount of space. Also, the cleanups need more time to be completed as compared to slurry-phase treatment. However, solid-phase treatment processes include windrows, biopiles, windrows, composting, and land farming.
Slurry-phase treatment processes are relatively faster compared to other treatment processes. In this technique, contaminated soil is excavated and combined with water, oxygen, and nutrient in the bioreactor to form the optimum environmental condition for the microbes to degrade the pollutants in the soil. This process involves the separation of rubbles and stones from the polluted soil. The concentration of water added depends on the following:
- The rate of the biodegradation process
- Concentration of pollutants
- The soil’s physicochemical properties
After the slurry treatment process is completed, the soil is removed and dried up. The soil is dried using vacuum filters, pressure filters, and centrifuges. Then, the next procedure is soil disposition and advanced treatment of the resultant fluids. Furthermore, biodegradation is greater in a bioreactor system than in solid-phase systems. This is because the polluted environment is more manageable, regulatable, and predictable.
Types of ex-situ bioremediation
- Land farming
Advantages of ex-situ bioremediation
- Ex-situ bioremediation is suitable for a wide range of pollutants.
- The suitability of ex-situ bioremediation for a clean-up exercise is relatively simple to assess from site investigation data.
Disadvantages of ex-situ bioremediation
- Ex-situ is not suitable for chlorinated hydrocarbons or heavy metal contamination.
- Non-permeable soil will need additional processing.
- Through soil washing or physical extraction, the pollutants may be stripped from the soil before being placed in a bioreactor.
This bioremediation technique involves decontaminating polluted sites by using either indigenous or external microbes. In in-situ bioremediation, the bioremediation occurs at the site of contamination. Hence, it happens without the translocation of the polluted substance. Whereas, ex-situ bioremediation occurs at a location that is separate from the contaminated site involving the translocation of the contaminated substance.
Microorganisms are used to neutralize pollutants such as nitrates, hydrocarbons, heavy metals, chlorinated compounds, and other pollutants. This neutralization process is done through a variety of chemical mechanisms. The microbes used in this process of bioremediation can either be cultivated or implanted within the site. This is done by applying fertilizers and other nutrients. Common polluted sites that bioremediation is used for are polluted soil and groundwater or aquifers.
Also through bioremediation, improvements have been seen in aquatic ecosystems affected by oil spills. The Exxon Valdez oil spill (1989) and the Deepwater Horizon oil spill (2010) are the most notable cases of oil spill recovery. Moreso, in situ bioremediation, can further be categorized by the level of human involvement and the metabolism occurring as aerobic and anaerobic.
Techniques in in-situ bioremediation
In-situ techniques involve treating the polluted substances at the pollution site. Excavation is not required and does not disturb the soil construction. In-situ techniques are cost-effective compared to ex-situ techniques. Techniques such as biosparging, bioventing, and phytoremediation are in-situ bioremediation techniques that may be enhanced or engineered. Whereas, bioremediation techniques like natural attenuation or intrinsic bioremediation progress without any form of enhancement. Furthermore, all these in-situ bioremediation techniques have been used effectively to treat heavy metals, chlorinated solvents, dyes, and hydrocarbons polluted sites.
Types of in-situ bioremediation
- Intrinsic bioremediation
- Engineered bioremediation
Intrinsic bioremediation is also known as natural reduction or natural attenuation. This is an in-situ bioremediation technique that involves passive remediation of polluted sites without human intervention or any external force. This bioremediation process involves stimulating the native or naturally occurring microbial population. The intrinsic bioremediation process is based on microbial aerobic and anaerobic metabolism biodegrading polluting components. This technique is inexpensive compared to other in-situ bioremediation techniques because external forces are not needed.
Engineered in-situ bioremediation
This in-situ bioremediation techniques involve the addition of certain microbes to the contaminated site. The genetically engineered microbes that are used in the in-situ bioremediation speed up the degradation process. This is done by enhancing physicochemical conditions to promote the growth of microbes. Bioventing, bioslurping, biosparging, phytoremediation, permeable reactive barrier, bioaugmentation, and biostimulation are all examples of engineered in-situ bioremediation.
Advantages of in-situ bioremediation
- In-situ bioremediation treats both solid and dissolved contaminants.
- Techniques in in-situ bioremediation do not require excavation of the contaminated soil.
- In-situ bioremediation is cost-effective and there is minimal site disruption.
- In this type of bioremediation, it is possible to convert organic pollutants to harmless compounds like water, carbon dioxide, and ethane.
- The time duration in using engineered in-situ bioremediation to treat sub-surface pollution is usually faster than pump and treatment processes.
Disadvantages and Limitations of in-situ bioremediation
- Depending on the specific site, some pollutants may not be completely converted into harmless products.
- In-situ bioremediation usually needs the acclimatization of microbes, which may not develop for spills and adamant compounds.
- Some recalcitrant pollutants cannot be biodegradable and if conversion stops at an intermediate compound, the intermediate may be more toxic and mobile than the parent compound.
- When microbes are incorrectly applied, the injection wells may become blocked by profuse microbial growth. This growth is a result of the addition of nutrients, electron acceptors, and electron donors.
- Heavy metals and organic compounds and heavy metals concentration may inhibit the activities of native microbes.
Types of bioremediation
- Natural attenuation
- Bioreactor-based bioremediation
- Permeable reactive barrier (PRB)
- Land farming
Biostimulation is a type of bioremediation in which the bacteria is stimulated to initiate the process of biodegradation. The environment is modified in order to stimulate the existing bacteria that are able to carry out bioremediation. This bioremediation process can be done by adding several forms of nutrients and electron acceptors. Phosphorus, oxygen, carbon, or nitrogen are usually added. The contaminated substance is mixed first with special nutrients and other important components in the form of gas or liquid. This stimulates the growth of microorganisms. Hence, causing quick and efficient biodegradation of the pollutants by the microbes.
On the other hand, biostimulation for the remediation of halogenated pollutants in anaerobic environments involves the addition of organic substrates as electron donors. This will permit native microbes to use the halogenated pollutants as electron acceptors. Through injection wells, Anaerobic Bioremediation Technologies Additives are usually added to the subsurface. Although injection well technology for biostimulation processes is still upcoming. However, biostimulation can be enhanced by bioaugmentation. For the remediation of hydrocarbon and high production volume chemical spills, biostimulation is used. Also, it is used for the treatment of contaminant spills like pesticides and herbicides.
The main advantage of biostimulation is that the bioremediation process will be carried out by already existing indigenous microbes. These native microbes are well-suited to the subsurface environment. Hence, within the subsurface, they are evenly distributed spatially. Furthermore, the introduction of additives in a way that permits them to be easily available to subsurface microbes is based on the local geology of the subsurface. This, however, is the major disadvantage of biostimulation. Compacted impermeable subsurface lithology such as tight clays or other fine-grained materials makes it difficult to spread additives throughout the contaminated area. This is because fractures in the subsurface form advantageous pathways in the subsurface. Then, additives preferentially follow these pathways preventing even distribution of additives.
Recently, several products that permit bioremediation using biostimulation methods have been introduced. These products using biostimulation may harness native bacteria. The stimulation is done by creating a favorable environment for hydrocarbon-devouring microorganisms. Alternatively, foreign bacteria may be introduced into the site. However, there are suggestions that the introduction of foreign bacteria to an environment may lead to the mutation of the native organisms which will, in turn, affect the biome. To develop a successful biostimulation system, investigation to find out the subsurface features is very important. Subsurface characteristics include:
- Natural groundwater velocity during ambient conditions
- Hydraulic conductivity of the subsurface
- The lithology of the subsurface
Also, before full-scale design and implementation of the biostimulation system, a pilot-scale study of the potential system should be done. Some biostimulation agents can be used in disorganized surfaces like open water and sand. They can be used as far as these agents bond to hydrocarbons exclusively. Also, sinking in the water column bonding to oil where they later float to the surface of the water. Thus exposing the hydrocarbon to more oxygen and sunlight where the abundant activity of aerobic microbes will be promoted. However, some biostimulants possess such quality whereas others don’t.
Bioaugmentation or biological augmentation is a type of bioremediation that involves the addition of archaea or bacterial cultures needed to enhance the rate of degradation of a pollutant. Microbes that are native to polluted areas may be able to degrade the polluting compounds but perhaps slowly and inefficiently.
In this type of bioremediation, studying the native organisms present in the location is required to know if biostimulation is possible. The native bacteria in the location are discovered and studied to know if the native bacteria can metabolize the pollutant. If they can’t, more of the native bacteria will be introduced into the location to enhance the biodegradation of the pollutants. As long as the native microbes do not have the metabolic capability to perform the bioremediation process, exogenous organisms with such sophisticated pathways are introduced.
Bioaugmentation is quite different from biostimulation. In biostimulation, nutritional supplements are added for the native bacteria to increase their microbial metabolism. Whereas in bioaugmentation, more archaea or bacterial cultures are introduced to promote the degradation of the contaminant. There are certain sites such as in municipal wastewater where microbes are needed to extract the contaminants. In such cases, bioaugmentation is used. The only disadvantage of this bioaugmentation process is that in the process of extracting the specific contaminant, it almost becomes impossible to regulate the growth of the microbes.
In municipal wastewater treatment, bioaugmentation is commonly used to restart activated sludge bioreactors. Most of the cultures available contain microbial cultures that already contain all needed microbes such as Aspergillus sp., Streptomyces, B. licheniformis, B. stearothermophilus, Penicillium sp, Arthrobacter, B. thuringiensis, Flavobacterium, Saccharomyces, P. polymyxa, Pseudomonas, etc. Activated sludge systems are however based on microbes such as nematodes, bacteria, rotifers, protozoa, and fungi. These microbes are capable of breaking down biodegradable organic matter. The use of bioaugmentation profers many positive outcomes like improvement in the efficiency and speed of degradation and the reduction of toxic substances in the area.
Natural attenuation is also known as natural reduction or intrinsic bioremediation. This is a type of bioremediation that involves passive remediation of polluted sites without human intervention or any external force. This bioremediation process involves stimulating the native or naturally occurring microbial population. The natural attenuation process is based on microbial aerobic and anaerobic processes biodegrading polluting components. This bioremediation type is inexpensive compared to other in-situ bioremediation techniques because external forces are needed.
This process of bioremediation is most effective in water and soil. This is because these two biomes always have a high likelihood of being contaminated and polluted. The process of natural attenuation is mostly used in underground places such as underground petroleum tanks. It is difficult to detect a leakage in such places. Hence, contaminants and toxins through these leaks can find their way to enter and contaminate the petrol. Thereby, only microorganisms can clean the tanks and remove the toxins.
Bioventing is a type of bioremediation that involves controlled stimulation of airflow by providing oxygen to the unsaturated areas to increase native microbial activities for bioremediation. In this type of bioremediation, alterations are made by adding nutrients and moisture. This is to increase bioremediation that will achieve microbial conversion of pollutants to harmless substances. Among other in-situ bioremediation techniques, bioventing has gained popularity.
This type of bioremediation technique combines vacuum-enhanced pumping, soil vapor extraction, and bioventing. It is used to achieve soil and groundwater remediation. This is done by indirectly providing oxygen and the stimulation of contaminant biodegradation. This bioremediation technique is implemented to remediate soils that are polluted with semi-volatile and volatile organic compounds.
The process makes use of a slurp that spreads into the free product layer, pulling up liquids from this layer. The pumping machine transports LNAPLs by an upward movement to the surface. At the surface, LNAPLs become separated from air and water. LNAPL means light non-aqueous phase liquid examples are toluene, benzene, xylene, and other hydrocarbons. It is a groundwater contaminant that is not soluble in water and has a lower density than water.
However, this bioremediation technique is not suitable for low permeable soil remediation, it is a cost-effective operation. This is because as a result of less amount of groundwater, it minimizes treatment, storage, and disposal costs.
In this bioremediation type, the air is injected into the soil subsurface to enhance microbial activities that will stimulate pollutant removal from polluted sites. Biosparging is similar to bioventing. In bioventing, however, the air is injected into the saturated zone, which can help in an upward movement of volatile organic compounds to the unsaturated zone. This stimulates the biodegradation process.
Furthermore, the efficiency of biosparging is based on 2 main factors. These factors are pollutant biodegradability and soil permeability. Biosparging has been used in the treatment of aquifers that are contaminated with kerosene and diesel.
Phytoremediation is a type of bioremediation that involves depolluting the contaminated soils with plants. This process is based on the plant interactions in contaminated sites to diminish the toxic properties of pollutants. There are several mechanisms involved in phytoremediation such as phytostimulation, phytoextraction, phytodegradation, rhizofilteration, phytoaccumulation, phytostabilization, and phytovolatilization.
Contaminants such as radionuclides and heavy metals are commonly removed by extraction, transformation, and sequestration. Then, organic contaminants like chlorinated compounds and hydrocarbons are mostly removed by rhizoremediation, phytodegradation, phytovolatilization, and phytostabilization. Also, they can be removed with mineralization when some plants like willow and alfalfa are used.
In phytoremediation, plants are directly used to clean up contaminants in the soil. This type of bioremediation will help reduce environmental pollution without excavating the contaminant material and disposing of it elsewhere. Some important factors of a plant being a phytoremediator include:
- The plant root system- either fibrous or tap depending on the depth of pollutant
- Above-ground biomass
- Toxicity of pollutant to plant
- Existence of the plant and its adaptability to predominant environmental conditions
- Plant growth rate
- Site monitoring
- Time mandatory to achieve the preferred level of cleanliness
- The plant’s resistance to diseases and pests
The phytoremediation process of removing a pollutant or contaminant involves the uptake of the substance and translocation from roots to shoots. The translocation and accumulation depend on transpiration and partitioning. Hence, the process of phytoremediation is likely to change, depending on other factors like the nature of the plant and contaminant. Most plants growing in polluted sites are good phytoremediators.
However, the success of the phytoremediation process depends on improving the remediation potentials of the indigenous plants growing in polluted sites. This is done either by bioaugmentation with endogenous or exogenous plants. The major advantage of phytoremediation is that some precious metals can bioaccumulate in some plants and be recovered after remediation. This process is known as phytomining.
Some bioremediation processes involve above-ground piling of dug contaminated soil which is followed by aeration and nutrient enhancement to promote microbial metabolic activities. This kind of bioremediation is called biopile. It involves:
- Leachate collection
- Treatment bed systems
Biopile is cost-effective. In biopile, the pH, temperature, nutrients, and aeration are effectively controlled. This bioremediation process is used to treat volatile low molecular weight pollutants. Also, biopile is used to remediate extremely cold environments that are polluted. The flexibility of this technique permits remediation time to be shortened.
Also to increase microbial activities, a heating system can be integrated into biopile design to enhance biodegradation. Heated air is injected into biopile design to provide air and heat. This is for the enhancement of bioremediation. However, extreme heating of air can result in the drying of the soil undergoing bioremediation. This can reduce the metabolic activities of microbes and rather stimulate volatilization instead of biodegradation. Also, in biopile construct, bulking agents and other organic materials are added to improve the process of remediation. Examples of such bulking agents are straw sawdust, bark, and wood chips.
The bioreactor is actually a vessel in which organic materials, after series of biological reactions are converted to specific products. There are several operational modes of bioreactors. They include:
- Sequencing batch
- Continuous and multistage
The bioreactor is filled with contaminated samples and it supplies optimal growth conditions for bioremediation to occur. However, the bioreactor-based treatment of contaminated soil has some advantages over other types of bioremediation processes. This is because the bioreactor-based bioremediation process has good control over agitation and aeration, pH, temperature, as well as substrate and inoculum concentrations. Thereby, efficiently reducing the time it will take for bioremediation to be completed. The regulation and manipulation process of parameters in a bioreactor instigate biological reactions. Bioreactor system designs are flexible as they permit maximum biodegradation while minimizing abiotic losses.
Permeable reactive barrier (PRB)
This bioremediation technique is used as a physical method for remediating polluted groundwater. The biological mechanisms used in the PRB process are precipitation, degradation, and sorption of pollutants. Alternative terms used for the PRB technique are biological PRB, bio-enhanced PRB, and passive bio-reactive barrier. Generally, the permeable reactive barrier is an in-situ technique used for the bioremediation of groundwater polluted with heavy metals and chlorinated compounds.
Another type of bioremediation technique is the windrows. It involves the periodic rotation of the piled polluted soil to enhance bioremediation. The microbial degradation activities of native and transient hydrocarbonoclastic bacteria in polluted soil are increased. This is done by the periodic turning of polluted soil to increase aeration. Also, the addition of water, uniform distribution of nutrients, pollutants, and microbial activities increases the bioremediation rate.
Furthermore, windrow treatment shows a higher rate of hydrocarbon removal as compared to biopile treatment. Hence, windrow is quite effective for the remediation of hydrocarbon from the soil. Though the periodic turning associated with windrow treatment is not suitable in the bioremediation of polluted soil with toxic volatile compounds. Also, the release of methane, a greenhouse gas has been associated with the use of windrows treatment. This is as a result of the formation of an anaerobic zone inside piled polluted soil that reduces aeration.
One of the simplest bioremediation techniques is landfarming. It is inexpensive and requires less equipment for its operation. In the land farming process, the polluted soil is usually excavated and tilled on the site of treatment. However, the depth of the pollutant is important in landfarming. Land farming can be done ex-situ or in-situ. In the process, excavated polluted soils are carefully applied above the ground surface on fixed layer support. This allows autochthonous microbes to aerobically biodegrade pollutants. Generally, land farming is a type of bioremediation technique that is very simple to design and implement. This is because it requires low capital input. Additionally, it can be implemented to treat large volumes of polluted soil with little energy required and minimal environmental impact.
Example of Bioremediation
- Bioremediation in crime scene cleanup
- Cleaning contaminated soil through bioremediation
- Bioremediation in oil spill clean up
Bioremediation in crime scene cleanup
An example of bioremediation is seen in crime scene cleanup. Bioremediation in crime scene cleanup involves the cleaning up of blood and bodily fluids with enzymes cleaners. The blood and bodily fluids in the scene can pose health risks like hepatitis, Methicillin-resistant Staphylococcus aureus (MRSA), and HIV. Hence, forensic professionals use enzyme cleaners instead of using standard cleaning agents like bleach or ammonia. Cleaning the crime scene with enzymes gets rid of and destroys the structure of the dirt or substance. Why enzymes? Enzymes are naturally occurring compounds that can break down the cell structures of substances that soils and stains. The dirt or substance once attacked by enzymes, losses its grip on the surface. This makes the substance easier to be removed.
An example of a frequently used enzyme cleaner is the International Product Corporation’s (IPC) Zymit Pro enzyme cleaner. It is used frequently by forensic professionals to ensure the accuracy and cleanliness of their work. Also, Aftermath is a company that specializes in this area of bioremediation in crime scenes.
Cleaning contaminated soil through bioremediation
Another example of bioremediation is seen in the cleanup of contaminated soil. During bioremediation, microorganisms use the chemical contaminants in the soil as an energy source. Then, via oxidation-reduction reactions, the contaminant is metabolized into useable energy for microbes.
Bioremediation has been seen as a solution for emerging contaminant problems. A number of microorganisms are involved in the bioremediation process including fungi, aerobic, and anaerobic bacteria. Bioremediation involves the removal, degradation, immobilization, or detoxification of diverse chemical wastes and physical hazardous materials from the environment through the action of microorganisms. The main concept is degrading and converting these contaminants and pollutants to their less toxic forms.
Bioremediation in oil spill clean up
The Exxon Valdez oil spill (1989) and the Deepwater Horizon oil spill (2010) are the most notable cases of oil spill recovery. An example of bioremediation is seen in oil spill cleanup. In 2010, the Deepwater Horizon oil spill happened. About 3.19 million barrels of oil spilled off the Gulf of Mexico. Two bioremediation methods which were biostimulation and bioaugmentation were used to clean up the oil spill.
Also in 1989, the Exxon Valdez oil tanker spilled approximately 11 million gallons of oil. During this period, bioremediation was gaining popularity as a viable alternative for oil cleanups. However, the EPA and Exxon Mobil Corporation (XOM) began testing different compounds. Between 1989 and 1990, pounds of fertilizer were applied to the affected areas. Then, in the mid-1992, the oil spill cleanup was considered complete.
Advantages of Bioremediation
- Since bioremediation is solely dependent on natural processes, it minimizes damage to the ecosystem.
- It is sustainable and eco-friendly compared to other cleanup methods.
- Bioremediation produces relatively few harmful byproducts as the process involves converting contaminants and pollutants into water and carbon dioxide.
- The process of bioremediation is cheaper than most cleanup methods.
- Bioremediation takes little time as an adequate waste treatment process.
- The bioremediation procedures require very little effort.
- Bioremediation can be carried out on site of pollution and suppresses the transport amount of waste off-site.
- Through bioremediation, possible threats to human health and the environment are eradicated.
- In bioremediation, contaminants, and pollutants are destroyed rather than being transferred to different environments.
- It is non-intrusive and allows the continued use of the site.
Disadvantages of bioremediation
- Bioremediation may be an efficient waste management process, but it destroying contaminants is not 100%. Bacteria may easily digest contaminants like chlorinated pesticides or clean oil spills easily. But microbes cannot destroy heavy metals like cadmium and lead.
- Not all compounds can be degraded quickly and completely, hence bioremediation is only restricted to compounds that are biodegradable.
- Depending on the specific site, some pollutants may not be completely converted into harmless products and if the conversion of some recalcitrant pollutants stops at an intermediate compound, the intermediate may be more toxic and mobile than the initial compound.
- Most times in bioremediation implementation, research is needed in order to develop and engineer bioremediation technologies that are convenient for the polluted sites with complex mixtures of pollutants that are not dispersed evenly in the environment. This could take a longer time compared to other treatment processes.
- Biological processes in bioremediation are highly specific and for remediation to occur certain criteria must be met. It requires suitable environmental growth conditions for the presence of metabolically active microbial communities and nutrients, as well as contaminants, should be available.