Bioremediation: Types, Advantages, and Risks 

Pollution all around the world has increased exponentially due various human activities. Recently, New Delhi was claimed to be one of the highly polluted city in the world. So, the leading countries of the World now, is focusing on controlling the pollution in an environmentally friendly and most cost-effective approach. 

. Not only controlling the pollutants, bioremediation, helps in restoring the areas  impacted by different pollution.  

The term bioremediators or biodegraders are used for denoting organisms used in bioremediation. These organisms have the ability to transform harmful substances into less toxic forms through their metabolic processes.  

The bioremediators can either degrade the pollutants directly or by producing enzymes and other biochemical agents which aid in degrading the contaminants. There are different factors that determine the effectiveness of the bioremediation methods like environmental circumstances and biodegraders used. Based on the factors, bioremediation method are divided in various types. 

Types of Bioremediation

Bioremediaton has been able to remedify wide range of pollutants like petroleum hydrocarbons, pesticides, solvents, heavy metals, and many organic compounds. It is sustainable alternative to traditional remediation method like excavation and chemical treatment. It is also applicable in areas where available methods can be expensive and impractical. 

The effectiveness and efficacy of biormediation depends on many other factors like concentration and type of concentrations, availability of biodegraders, and different environmental conditions. So, based on these factors there are seven different types of bioremediation; in situ bioremediation, phytoremediation, mycoremediation, ex situ bioremediation, biosimulation, and bioaugumentation.   

In situ bioremediation

The biological treatment of contaminants in the contaminated area or surface without removing any polluted material is called in situ bioremediation (ISB). It typically occurs in the cleaning of groundwater. ISB applies the concept of microbiology, chemistry, hydrogeology, and engineering for planned and controlled microbial degradation of specific classes of organic matter. ISB manipulates the degree of oxidation or reduction, which helps degrade the chemicals with microbial-catalyzed biochemical reactions. Types of microorganisms, geological conditions, and contaminant types should be considered for accomplishing the degradation.  

For example, chlorinated solvents in groundwater plumes require adding an organic carbon source, electron acceptor, nutrients, and microbial cultures to stimulate the degradation. Anaerobic reductive dechlorination, oxidation, reductive dechlorination, and aerobic co-metabolism are the primary biological processes degrading the chlorinated solvent compounds.  

The delivery method of the various amendments to the target area is crucial when forming the ISB systems. The delivery mechanisms commonly used are:

• Good vertical recirculation.
• Horizontal good recirculation.
• Direct liquid amendment injection.
• Gas amendment injection.
• Filtration trench recirculation.
• Pass-through (reactive cell) designs. 

Ex situ bioremediation

Ex-situ bioremediation treats soil and water by exacting in a controlled environment outside the original location. Petroleum hydrocarbon mixtures, phenols, cresols, some pesticides, and polycyclic aromatic compounds are the source of carbon and energy for microorganisms. 

Microbes degrade the carbon source into carbon dioxide and water. Different types of ex-situ bioremediation commonly available are; biopiles, composting, land farming, and bioreactors. 

1. Composting is the simple and most common technique of ex-situ bioremediation technique. It is usually applicable in treating agricultural, municipal solid wastes, and sewage sludge. These organic wastes converts into valuable components like hummus. It is also applicable in the treatment of contaminated soil. Mesophilic and thermophilic bacteria degrade, transform, or stabilizes the organic waste compounds. 
2. Biopiles are also called bio-cells, bio-heaps, bio-mounds, and compost cells. It is heavily applicable for remediating different petrochemical contaminants in soils and sediments. This technique combines landfarming and composting, providing a favorable environment for indigenous aerobic and anaerobic microbes. It also controls the physical losses of the contaminants by leaching and volatilization. The piling technique consists of a treatment bed, an aeration system, an irrigation and nutrient system, and a leachate collection system. 
3. Landfarming, also called land treatment or application, is simple ex-situ bioremediation. Here the materials are excavated and deposited in a large flat impermeable surface for increasing interaction between polluted soil and the atmosphere. This interaction helps in improving the aerobic microbial activity in the ground. 
4. The bioreactor is a biochemical processing system helpful in removing pollutants from wastewater or pumped groundwater using microbes and treatment of polluted soils in liquid or solid slurry. The mixing of contaminated soil with water and other additives occurs inside the bioreactor system to maintain contact between microbes and pollutants in optimum environmental conditions for degradation. 

Bioaugmentation

Adding selected strains/mixed cultures of organisms to the contaminated areas increases the ability of a pre-existing microbial community to degrade the pollutants. The two main strategies applied in bioaugmentation are augmentation by enrichment indigenous microbes and augmentation by enrichment of non-indigenous microbes.  

The most common method of applying bioaugmentation is the addition of microorganisms directly to the affected site so that these can transform harmful compounds into less dangerous material. The addition may attach to the carrier to form biofilm or exist alone and use contaminants as the primary carbon and energy sources. Compared to pure bacterial culture, mixed flora is preferrable for bio enhancement and bioaugmentation due to its higher ability to degrade and adapt according to the situation. 

Biosimulation

Factors like nutrients, temperature, pH, oxygen, moisture, soil properties, oxygen, and contaminants can limit hydrocarbon bioremediation. Biostimulation consists of modifying the environment to stimulate existing bacteria with bioremediation ability. The stimulation occurs by addition of various limiting nutrients and electron acceptors like phosphorus, oxygen, carbon, or nitrogen. These nutrients may be low, which constrains microbial activity. 

The addition of compounds in the treatment areas can improve pollutant degradation by optimizing the conditions like pH, temperature control, aeration, and the addition of nutrients. This technique is appropriate for the removal of petroleum pollutants from the soil. The advantage of this technique is the use of already present native microorganisms, which are already acclamatized to the subsurface environment and are distributed spatially within the subsurface. 

Mycoremediation

Using fungi or their derivatives to remediate environmental pollutants is called mycoremediation. It is a comparatively eco-friendly and cost-effective method. 

Fungi are an ideal choice because their growth is robust, they have vast hyphal networks, they can produce versatile extracellular ligninolytic enzymes, and they resist heavy metals. Additionally, the fungus has a high surface area to volume ratio and adaptability to fluctuation pH, temperature, and metal-binding proteins. 

This technique applies in the in-situ bioremediation of pollutants like dyes, pharmaceutical drugs, and herbicides. It can also occur in bioreactors with controlled physiochemical conditions to promote microbial growth. 

Phytoremediation

Using plants and associated soil microbes to reduce the concentrations and toxic effects of contaminants from the environment is called phytoremediation. It is a cost-effective environmental restoration technology. 

The method is an alternative to engineering procedures that are usually more destructive. The plants uptake pollutants through their roots and either store them in their tissues or break them down by natural metabolic processes. Phytoextraction, photodegradation, phytostabilization, and hemofiltration are some of the methods used for phytoremediation. 

However, this technique is limited to the root zone of plants and with limited application where the concentrations of contaminants are toxic to plants. 

Factors Affecting Efficacy and Effectiveness 

Some factors influence the efficacy and effectiveness of the methods used for bioremediation, which are as follows: 

1. Environmental conditions: Environmental conditions play a significant role in the growth of microbes and the proper degradation of contaminants. The environmental conditions include oxygen concentration, pH, temperature, moisture content, and nutrient level. Considering these environmental factors are necessary for implementing the correct method of bioremediation. 
2. Contaminant concentration and type: The higher the contaminant concentration, the more difficult to remediate the pollutants because the elevated concentration limits microbial activity. Additional steps may be required to enhance the activity in increased concentrations. Likewise, the type of contaminant also influences the selection of suitable bioremediation methods. Contaminants are either easily degradable or recalcitrant or hard to degrade. Microbes and plants can quickly degrade simple pollutants. The recalcitrant pollutants require specific enzymes or metabolic pathways for degradation. 
3. Bioavailability of contaminants: If the pollutants are easily accessible and susceptible to degradation, i.e., more bioavailable, it is easier to interact with bio remediate. But if the contaminants are tightly bound, i.e., less bioavailable, the accessibility of mediators may decrease. So, further treatments like adding surfactants or chelating agents must be done to increase the bioavailability of the pollutants. 
4. Diversity and population of microbes: Types and the number of microbes present in the contaminated areas also play essential roles when selecting bioremediation techniques. The indigenous people of microbes can have the ability to degrade the contaminants, but sometimes there might be need for addition of diverse strains.
5. Toxicity and inhibitory substances: Some contaminants, after degradation, can produce or are themselves toxic or inhibit microbes’ growth. The limitation of microbes’ growth reduces the effectiveness of bioremediation. A thorough study of the type of contaminant and their toxicity is critical in developing strategies to detoxify these compounds for successful bioremediation.   
6. Timeframe: Some bioremediation techniques require more time to complete remediation, like biosimulation techniques. In contrast, some methods require less time for completion but involve logistic challenges like landfarming bioremediation. So, the timeframe is essential for increasing effectiveness and efficacy. 
7. Site characteristics: The geographical features, hydrogeological conditions, soil type, and other aspects of the site for bioremediation is other factors that play a crucial role in the efficacy and effectiveness of bioremediation. Studying the site characteristics beforehand will aid in selecting the correct technique for bioremediation.  

Advantages of Bioremediation

The advantages of bioremediation for addressing environmental contaminations are as follows:

1. Cost-effective: There are other methods of remediating pollutants of environment except bioremediation like excavation and incineration which are expensive. Bioremediation is the most cost-effective option available for controlling and removing pollution from the environment. The equipment required, energy, and labor requirements is less expensive especially in the cases the site is inaccessible and extensive.  
2. Environmental sustainability: Bioremediation is an environmentally friendly technique because it relies on natural processes and organisms to degrade or transform pollutants. It protects from harsh chemicals and damaging excavation methods. These properties minimize further harm to the environment and ecosystems.  
3. Restoration of affected area: The degradation by another method besides bioremediation may cause disruption and damage to the affected area. Bioremediation believes in the degradation of pollutants in natural processes, which restores soil fertility, water quality, and overall ecosystem health. This method not only helps in the restoration of the affected area but also the growth of the area.
4. Long-term and versatile effectiveness: Bioremediation applies to contaminants like petroleum hydrocarbons, pesticides, heavy metals, solvents, and organic compounds. Different organisms and techniques are applicable for specific pollutants, which allows versatility in degrading different types of contamination. Likewise, this method targets the source of contamination, which promotes the degradation of persistent pollutants. This method can also adapt and respond to changing conditions, assisting in ongoing or complex contamination scenarios.    

Risks of Bioremediation

Although bioremediation is highly beneficial and widely used for environmental cleaning, it has some risks, which are as follows:

1. Bioremediation may lead to the production of more toxic compounds than already existing components. These toxic compounds can also be motile and transfer from a contaminated area to another unaffected area. For example, trichloroethane (TCA) degradation produces vinyl chloride, which is more toxic and carcinogenic than TCA.
2. Polycyclic aromatic compounds and some chlorinated compounds are not easily degradable by microbes. 

So, there is requirement of proper research in bioremediation selection to remove environmental pollutants successfully.

References and further reading

• Cristorean, Carmen & Micle, Valer & Sur, Ioana. (2016). A critical analysis of ex-situ bioremediation technologies of hydrocarbon polluted soils. ECOTERRA – Journal of Environmental Research and Protection. 
• Godleads Omokhagbor Adams, Prekeyi Tawari Fufeyin, Samson Eruke Okoro, and Igelenyah Ehinomen, “Bioremediation, Biostimulation and Bioaugmention: A Review.” International Journal of Environmental Bioremediation & Biodegradation, vol. 3, no. 1 (2015): 28-39. doi: 10.12691/ijebb-3-1-5.
• Wexler, P. (2014) ‘Bioremediation’, in Encyclopedia of toxicology. third. Amsterdam: Academic Press, pp. 435–489. 
• Ren, H., & Zhang, X. (2020). Biological technologies for cHRPs and risk control. In High-risk pollutants in wastewater (pp. 209–236). Amsterdam, Netherlands: Elsevier.
• Akhtar, N., & Mannan, M. A. (2020). Mycoremediation: Expunging environmental pollutants. Biotechnology Reports, 26. doi:10.1016/j.btre.2020.e00452  
• Greipsson, S. (2011) Phytoremediation. Nature Education Knowledge 3(10):7 
• Kapahi, M., & Sachdeva, S. (2019). Bioremediation Options for Heavy Metal Pollution. Journal of health & pollution, 9(24), 191203. https://doi.org/10.5696/2156-9614-9.24.191203