Types of bioremediation are microbial bioremediation, mycoremediation and phytoremediation. Methods of bioremediation are in situ and ex situ bioremediation
This article discusses the types and methods of bioremediation, according to the following outline;
Types of Bioremediation
1). Microbial Bioremediation as one of the Types of Bioremediation
Microbial bioremediation is the use of microorganisms and microbial enzymes to breakdown and eliminate environmental contaminants .
The mechanism behind microbial bioremediation is biodegradation.
This mechanism ensures that contaminants are removed through a series of biochemical reaction involving microbial enzymes, and resulting in the conversion and mineralization of contaminants.
Microbial bioremediation is the most common type of bioremediation and is often used as the basis of definition of the overall concept. It is also versatile and can be used in various scenarios.
Some factors may affect the rate, efficiency and effectiveness of microbial bioremediation. These are the same factors that affect biodegradation, and they include temperature, pH, salinity, nutrients, enzymes and microbes.
Bacteria are a very common tool used in microbial bioremediation. Examples of bacteria which are relevant in this area include Pseudomonas, Phanerochaete and Deinococcus.
In microbial bioremediation, the contaminant serves as a source of nutrients to the microbes, which breakdown the contaminant to access these nutrients.
It is therefore necessary for any given contaminant to be supplied with the right microorganisms that are capable of breaking it down.
Mycoremediation is derived from ancient Greek words; μύκης (mukēs), which means fungus; and remedium, which means to restore balance.
Mycoremediation is the use of fungi to breakdown and eliminate environmental contaminants.
We may consider mycoremediation to be an aspect of microbial bioremediation, since fungi are also microorganisms. However, it is treated as a distinct type of bioremediation because the field of microbial bioremediation is fairly dominated by bacteria.
In mycoremediation, the natural metabolic capabilities of fungi are used to transform toxic materials to less-harmful end-products.
Mycoremediation is important because it provides an eco-friendly, effective, economical, and agriculturally beneficial strategy to treat contaminants.
The agricultural benefit of micoremediation stems from the fact that fungi themselves can act as a source of nutrients to the soil when they undergo biodegradation, as well as through nutrient mineralization and fixation.
At the same time, some fungi can be eaten as food, by other organisms. These include yeasts and mushrooms .
Contaminants which can be eliminated through mycoremediation include hydrocarbons, heavy metals, and pesticides .
3). Phytoremediation as one of the Types of Bioremediation
Phytoremediation is a type of bioremediation which uses plants to remove contaminants in soil, water and air.
This type of bioremediation is mostly used for elemental contaminants  that are relatively easy for plants to extract and absorb. It is also most applicable and effective for contaminated soil and water.
Aside extracting and absorbing contaminants, phytoremediation could involve transforming or destroying contaminants.
The mechanism behind phytoremediation is very similar to that behind microbial bioremediation. In phytoremediation, the contaminant acts as a source of nutrients to the plants, which extract the needed components from the contaminant, subsequently absorbing them into its stem and leaves.
When elements are extracted from the contaminant, it breaks down and degrades in the process; transforming into other, less-harmful materials.
Various plants can be used for phytoremediation, including trees, aquatic plants, grasses and shrubs . Also, phytoremediation has been successfully used in the removal of various pollutants; such as pesticides , hydrocarbons , and heavy metals .
Methods of Bioremediation
Methods of bioremediation are biopiling, bioreaction, windrowing, landfarming, bioslurping, bioventing, permeable reactive barrier, biosparging, bioaugmentation, intrinsic bioremediation, and biostimulation.
These methods of bioremediation can be broadly grouped into two categories; in-situ and ex-situ bioremediation.
-In Situ Bioremediation
In situ bioremediation is the biological elimination of contaminants using tools and techniques on the site of contamination.
Some methods or techniques of in situ bioremediation include bioaugmentation, biostimulation, intrinsic bioremediation, bioventing, biosparging, permeable reactive barrier usage, and bioslurping.
1). Bioaugmentation as one of the Methods of Bioremediation
Bioaugmentation is the introduction of additional microbe cultures like archaea and bacteria to the contaminated medium, so as to enhance the rate and effectiveness of bioremediation.
In simpler terms, bioaugmentation is the addition of more microbial cultures to the contaminated site to improve contaminant degradation .
The use of bioaugmentation is necessary where the size of the microbe population on the site of contamination (or in the contaminated medium) is too small to breakdown the contaminant. It is also necessary where the required species of microbes to breakdown a particular contaminant, are not present.
Bioaugmentation is possible where there is a pre-grown, cultured microbial community already available to be added to the contaminated medium. The method is used for different cases of bioremediation such as wastewater and polluted soil.
Biostimulation is the addition of materials to facilitate the survival, growth and metabolism of microorganisms, mostly for the purpose of biodegradation.
As the term implies, the goal of biostimulation is to stimulate microbial growth and activity . In order to achieve this, nutrients are usually supplied to the site, to nourish the microbes, thereby resulting in their increased growth, reproduction, and metabolism.
The need for biostimulation arises when the required microbes are present on the site, however, not in sufficient numbers or with sufficient activity-levels to carry out biodegradation effectively.
Materials which could be used as source of nutrients in biostimulation include nitrogen, carbon, and phosphorus . These nutrients can be provided from fertilizer, cellulose; straw, or compost, among other materials.
Both biostimulation and bioaugmentation can both be applied in ex situ bioremediation, when the prevalent conditions demand the use of these methods. However, they are most commonly used in in-situ bioremediation projects.
3). Intrinsic Bioremediation as one of the Methods of Bioremediation
Intrinsic bioremediation is a rather spontaneous method of bioremediation, which relies on the innate metabolic capabilities of microbes in an environment, to degrade contaminants.
It involves minimal (little or no) human interference, and depends on native microorganisms that are already occurring on the site .
Based on the description above, it is arguable that intrinsic bioremediation is not a conventional method, since it is a mostly-natural process and does not necessarily involve human intervention.
However, it is classified as a method of bioremediation, because it still utilizes biological processes and organisms, to eliminate contaminants.
Intrinsic bioremediation is used mostly in cases where the contaminated site is not easy to access, such as where the contaminant occurs underground in subsurface soil and groundwater.
Bioventing is a method of bioremediation whereby oxygen is supplied to contaminated subsurface soil in a regulated manner, so as to enhance the activities of microbes and facilitate biodegradation.
The bioventing method is used in cases where the contaminant is located in the vadose zone of the soil profile, which is usually deficient in oxygen.
When a controlled stream of air is supplied to this soil layer, it increases the amount of available oxygen; thereby stimulating an increase in the growth and activities of microbes.
Bioventing is often carried out in combination with biostimulation, so that both nutrients and oxygen are provided to the microbial community.
5). Biosparging as one of the Methods of Bioremediation
Biosparging is the method of bioremediation whereby air is injected into the saturated zone of the subsurface soil profile, in order to increase the activities of microorganisms.
There are great similarities between biosparging and bioventing, however, the difference between the two methods lies in the fact that bioventing involves air-injection into the vadose zone, while biosparging involves injection of air into the saturated zone.
The saturated zone is characterized by low organic activity, mainly because it is saturated with water, and has less oxygen than the unsaturated zone of the soil profile.
In biosparging, the air which is injected, forces the volatile materials (like volatile organic compounds- VOCs) in the saturated zone, to migrate into the unsaturated zone of the soil profile, thereby increasing the prospects of biodegradation.
The mechanism described above is best for volatile, low-density contaminants. Biosparging is usually applied in the bioremediation of contaminated aquifers, where the contaminant is a low-density material like hydrocarbons.
5). Permeable Reactive Barrier Usage
Permeable reactive barrier (PRB) is a passive method of bioremediation which uses a permeable medium to trap pollutants, which are degraded and eliminated through a series of reactions.
The mechanism behind the use of permeable reactive barriers is similar to that which is used for reverse osmosis filtration. Also, the method is useful for bioremediation projects involving a liquid, like wastewater treatment and groundwater remediation.
The permeable reactive barrier is placed in the flow pathway of the polluted medium. As the medium (liquid) flows through the permeable barrier, the contaminants react with the barrier, and are degraded and eliminated as a result.
Although the mechanism of this method is physical, the processes by which the pollutants are removed are mostly biological.
The most common use of PRB is in groundwater remediation, where the contaminant may be heavy metals, halogens, or other reactive materials. Also, the effectiveness of this method depends on various factors, like site location, severity of contamination, depth of contaminant spread, and geologic conditions.
6). Bioslurping as one of the Methods of Bioremediation
Bioslurping is a method of bioremediation which involves the use of bioventing, soil-vapor extraction and vacuum-enhanced technology, to recover free-product contaminants from a medium.
The use of bioslurping is possible in cases where the contaminant can be recovered without complex biochemical reactions.
While the use of bioventing (which is incorporated into this method) helps to increase microbial activity, the mechanism of bioslurping is designed to extract capillary, light non-aqueous phase liquids (LNAPLs) from contaminated soil in the saturated and unsaturated zones .
The vacuum-enhanced technology pulls light non-aqueous phase liquid contaminants to the surface, where they are separated from water and air. Effectiveness of the method generally increases with increase in soil permeability. Hydrocarbon is a common pollutant that is eliminated using this method.
Advantages of bioslurping include its cost-effectiveness and low-demand for waste disposal. However, it is applicable to only a specified type of environment and leads to soil dewatering, which can subsequently inhibit microbial activity.
-Ex Situ Bioremediation
Ex situ bioremediation is the biological degradation and elimination of contaminants from an environment by the physical removal and subsequent treatment of the contaminated medium.
In order to remove the contaminated material, excavation (in the case of soil) or pumping (in the case of water, is usually employed . This material is then transported to a designated site for bio-remedial treatment.
Ex situ bioremediation is required in a number of scenarios. These include cases where the contamination is severe in concentration and area of coverage; as well as in cases of sensitive or unfavorable environmental characteristics.
Other scenarios requiring ex situ bioremediation include cases where the required microbes are not available on the site, or compatible with the site of contamination.
Ex situ bioremediation has been applied successfully for different contaminants. including hydrocarbons and heavy metals.
Methods of ex situ bioremediation include biopiling, bioreaction, windrowing, and landfarming.
7). Biopiling as one of the Methods of Bioremediation
Biopiling (or ‘biopile’) is a bioremediation method whereby contaminated material is accumulated (that is; piled) above-ground, within contact with a nutrient-amendment and air-distribution system.
The presence of the air-distribution and nutrient amendment systems implies that biopiling is a combined bioremediation technique, as it includes the functions of bioventing and biostimulation (by aeration).
In this method, contaminated soil is excavated from the site and accumulated over an aeration system, while having access to nutrient supply. This accumulated material nay be called a biopile.
The aim of this mechanism is to facilitate an increased rate of microbial activity through the supply of oxygen and nutrients, thereby influencing higher biodegradation rates.
There are some conditions under which biopiling is applicable and suitable.
One of these is where the depth of contamination is shallow. Biopiling is also applicable for low molecular weight contaminants. Another applicable condition is where the temperature of the contaminated site is too low to support microbial activity.
The process of bioremediation by the biopilng method, is flexible. It can be made to proceed at a faster rate by altering some variables like pH, oxygen/nutrient supply, and temperature.
Biopiling has been used successfully for the treatment of soils contaminated by volatile organic compounds . Nutrient supplementation can be achieved by addition of biomass to the contaminated material, like wood chips and straw.
Bioreaction is a method of bioremediation, involving the use of a bioreactor to breakdown and convert biodegradable materials through a series of biochemical reactions.
A bioreactor is a vessel which is capable of converting biodegradable raw materials to inorganic products, through biochemical reactions .
Bioreactors can be operated in different ways, including sequence-batch, batch, fed-batch, multistage and continuous modes of operation.
In order for bioremediation to be effective and efficient; it is often necessary to use the bioreactor under controlled conditions of pH, temperature, and temperature.
Like biopiling, bioreaction is a flexible method of bioremediation, as various variables of the process can be adjusted to alter the rate of biological transformation.
9). Windrowing as one of the Methods of Bioremediation
Windrowing is a method of bioremediation, whereby the contaminated material is accumulated in a pile, which is rotated periodically to improve aeration and enhance microbial activity.
Windrowing is especially applicable to ex situ bioremediation projects involving polluted soil.
The method has been used successfully for soils contaminated by hydrocarbons . It is comparable to biopiling, in terms of the objective of air circulation and microbial enhancement.
However, windrowing is believed to be associated with significant greenhouse emissions , because the periodic rotation may lead to the growth of anaerobic microbes and the occurrence of extensive anaerobic biodegradation in parts which have minimal exposure.
Landfarming is a method of bioremediation which involves the excavation, tilling, and nutrient amendment of soil, to stimulate microbial activity.
The method may be used for in situ or ex situ bioremediation, although it is more commonly used for ex situ projects. When used in situ, there is usually no excavation of soil; rather, the soil is tilled directly on the site.
In landfarming, the ultimate goal is to enhance the rate of aerobic biodegradation of the contaminant, by stimulating the growth and activity of autochthonous microbes .
The method is beneficial in terms of its simplicity, flexibility and effectiveness.
Phases of Ex Situ Bioremediation
Based on the state of the contaminated material, there are two phases of bioremediation; which are the solid phase and the slurry phase.
In the solid phase, the excavated material is accumulated or piled.
This pile is then treated using any of various techniques including aeration, landfarming, bioreaction and windrowing.
The aim of these applications is to enhance microbial activity and biodegradation, both of which help to breakdown the contaminant(s).
Slurry phase bioremediation involves the addition of water to the contaminated material. The purpose of this is to increase moisture and humidity, which is necessary for most biochemical and microbial processes.
In addition to water, slurry phase treatment often involves the addition of nutrients and oxygen to the contaminated material.
The use of slurry phase treatment depends on factors like the concentration of pollutant, the size of soil aggregates, the original moisture content, and the degree of aeration.
Types of bioremediation are;
- Microbial Bioremediation
The methods of bioremediation are;
- Intrinsic Bioremediation
- Permeable Reactive Barrier Usage
These may also be called the techniques of bioremediation.
Bioremediation methods can be grouped broadly into ex situ and in situ methods.
In situ bioremediation involves contaminant treatment on the site, while ex situ bioremediation involves treatment off the site.
Examples of ex situ bioremediation are landfarming, bioreaction, biopiling and windrowing.
Examples of in situ bioremediation are bioventing, biosparging, bioaugmentation, biostimulation, permeable reactive barrier usage, intrinsic bioremediation and bioslurping.
Ex situ bioremediation is generally more expensive than in situ bioremediation. However, it is also generally more effective.
Some factors that determine the type, method or technique of bioremediation that is suitable for any given scenario include;
- Severity of contamination
- Depth and areal span of contamination
- Degree of aeration
- Moisture content
- Nutrient availability
- Number and species of microorganisms present
- Environmental and geologic conditions (soil type, temperature, pH, salinity, saturation)
1). Adams, G.; Tawari-Fufeyin, P.; Okoro, S. E.; Ehinomen, I. (2015). “Bioremediation, Biostimulation and Bioaugmention: A Review.” Available at: https://doi.org/10.12691/ijebb-3-1-5. (Accessed 25 March 2022).
2). Azu, C.; Chikere, C.; Okpokwasili, G. (2016). “Bioremediation techniques-classification based on site of application: principles, advantages, limitations and prospects.” World Journal of Microbiology and Biotechnology 32(11):180. Available at: https://doi.org/10.1007/s11274-016-2137-x. (Accessed 25 March 2022).
3). Bailey, C. (2017). “Fungal Fermented Foods.” Available at: https://www.mycopia.com/blog/2017/07/10/fungal-fermented-foods. (Accessed 25 March 2022).
4). Beltran-Siñani, M.; Gil, A. (2021). “Accounting Greenhouse Gas Emissions from Municipal Solid Waste Treatment by Composting: A Case of Study Bolivia.” Eng. 2021, 2(3), 267-277. Available at: https://doi.org/10.3390/eng2030017. (Accessed 25 March 2022).
5). Chirakkara, R. A.; Reddy, K. R. (2015). “Plant Species Identification for Phytoremediation of Mixed Contaminated Soils.” Journal of Hazardous, Toxic, and Radioactive Waste. Available at: https://doi.org/10.1061/(ASCE)HZ.2153-5515.0000282. (Accessed 25 March 2022).
6). Coulon, F.; Al-Awadi, M.; Cowie, W.; Mardlin, D.; James, S.; Cunningham, C.; Risdon, G.; Arthur, P.; Semple, K. T.; Paton, G. L. (2010). “When is a soil remediated? Comparison of biopiled and windrowed soils contaminated with bunker-fuel in a full-scale trial.” Environmental Pollution 158(10):3032-40. Available at: https://doi.org/10.1016/j.envpol.2010.06.001. (Accessed 25 March 2022).
7). Germaine, K. J.; Byrne, J.; Liu, X.; Keohane, J.; Culhane. J.; Lally, R. D.; Kiwanuka, S. Ryan, D.; Dowling, D. N. (2015). “Ecopiling: a combined phytoremediation and passive biopiling system for remediating hydrocarbon impacted soils at field scale.” Front. Plant Sci., 05 January 2015. Available at: https://doi.org/10.3389/fpls.2014.00756. (Accessed 25 March 2022).
8). Hall, J.; Soole, K.; Bentham, R. H. (2011). “Hydrocarbon Phytoremediation in the Family Fabacea —A Review.” International Journal of Phytoremediation 13(4):317-32. Available at: https://doi.org/10.1080/15226514.2010.495143. (Accessed 25 March 2022).
9). Höckenreiner, M.; Neugebauer, H.; Lakshmanan, E. (2013). “Ex situ bioremediation method for the treatment of groundwater contaminated with PAHs.” International journal of Environmental Science and Technology 12(1). Available at: https://doi.org/10.1007/s13762-013-0427-5. (Accessed 25 March 2022).
10). Jaiswal, S.; Shukla, P. (2020). “Alternative Strategies for Microbial Remediation of Pollutants via Synthetic Biology.” Front. Microbiol., 19 May 2020. Available at: https://doi.org/10.3389/fmicb.2020.00808. (Accessed 25 March 2022).
11). Kanissery, R. G; and Sims, G. K. (2011). “Biostimulation for the Enhanced Degradation of Herbicides in Soil”. Applied and Environmental Soil Science, vol. 2011, Article ID 843450. Available at: https://doi.org/10.1155/2011/843450. (Accessed 25 March 2022).
12). Rhodes, C. J. (2014). “Mycoremediation (bioremediation with fungi) –growing mushrooms to clean the earth.” Chemical Speciation & Bioavailability, 26:3, 196-198. Available at: https://doi.org/10.3184/095422914X14047407349335. (Accessed 25 March 2022).
13). Sharma, I. (2020). “Bioremediation Techniques for Polluted Environment: Concept, Advantages, Limitations, and Prospects”, in M. A. Murillo-Tovar, H. Saldarriaga-Noreña, A. Saeid (eds.), Trace Metals in the Environment – New Approaches and Recent Advances, IntechOpen, London. Available at: https://doi.org/10.5772/intechopen.90453. (Accessed 25 March 2022).
14). Suman, J.; Uhlik, O.; Viktorova, J.; Macek, T. (2018). “Phytoextraction of Heavy Metals: A Promising Tool for Clean-Up of Polluted Environment?” Front. Plant Sci., 16 October 2018 | https://doi.org/10.3389/fpls.2018.01476. (Accessed 25 March 2022).
15). Tarla, D. N.; Erickson; Hettiarachchi, G. M.; Amadi; Galkaduwa; Davis; Nurzhanova, A. A.; Pidlisnyuk (2020). “Phytoremediation and Bioremediation of Pesticide-Contaminated Soil.” Applied Sciences 10(4):1217. Available at: https://doi.org/10.3390/app10041217. (Accessed 25 March 2022).
16). Tyagi M.; da Fonseca M. M.; de Carvalho C. C. (2011). “Bioaugmentation and biostimulation strategies to improve the effectiveness of bioremediation processes.” Biodegradation. 2011 Apr;22(2):231-41. Available at: https://doi.org/10.1007/s10532-010-9394-4. (Accessed 25 March 2022).
17). Yan, A.; Wang, Y.; Tan, S. N.; Yusof, M. L. M.; Ghosh, S.; Chen, Z. (2020). “Phytoremediation: A Promising Approach for Revegetation of Heavy Metal-Polluted Land.” Front. Plant Sci., 30 April 2020. Available at: https://doi.org/10.3389/fpls.2020.00359. (Accessed 25 March 2022).
18). Yen, H.; Chang, N. B.; Lin, T. (2003). “Bioslurping Model for Assessing Light Hydrocarbon Recovery in Contaminated Unconfined Aquifer. II: Optimization Analysis.” Practice Periodical of Hazardous Toxic and Radioactive Waste Management 7(2). Available at: https://doi.org/10.1061/(ASCE)1090-025X(2003)7:2(114). (Accessed 25 March 2022).