Introduction:
- It is a process which employs naturally occurring biological systems such as microorganisms like bacteria and fungi or plants to destroy or degrade or reduce concentrations of contaminants from the polluted sites such as sediments, soils, water or air.
- Microbes and plants have a natural capability to attenuate or reduce the mass, detoxify, volume, concentration of pollutants without human interventions under controlled conditions.
- Cometabolism is a natural process where microorganisms synthesize enzymes that utilize pollutants as a nutrient and potentially destroy it.
- The efficiency of bioremediation is determined by a number of factors, including microorganisms, availability of nutrients, selection and characterisation of site, environment, and the types and state of contaminants.
- This allows a natural process of treatment that aims to lessen toxicity by destroying or converting harmful organic molecules.
Type of contaminants:
- Biodegradable products such as petroleum products, gas, diesel, fuel oil.
- Uranium, mercury, and DDT are examples of non-biodegradable/recalcitrant chemicals.
- Crude oil chemicals include benzene, toluene, xylene, and naphthalene.
- Some pesticides (malathion) and industrial solvents.
- TCE (Trichloroethane) a persistent hazard to groundwater, while PCE (Perchloroethane) a dry-cleaning solvent.
- Arsenic, chromium, and selenium.
- Not degradable / Recalcitrant such as uranium, mercury, DDT(Dichlorodiphenyltrichloroethane).
- Coal compounds include phenols, cyanide in coal tars, and coke waste.
Microorganisms in Bioremediation:
Aerobic bacteria
Many of these bacteria grow in the presence of oxygen that use the contaminant as the sole source of carbon and energy.
It degrades pesticides and hydrocarbons, both alkanes and polyaromatic compounds.
Examples: Pseudomonas, Alcaligenes, Sphingomonas, Rhodococcus, and Mycobacterium
Methanotrophs/ Methylotrophs
Bacteria that convert methane into carbon and energy.
These bacteria produced enzymes such as methane monooxygenase that has a broad substrate specificity and is active against a wide spectrum of compounds including chlorinated aliphatic like trichloroethylene and 1,2-dichloroethane.
Examples: Bacillus methanicus, Pseudomonas methanica, Methanomonas methanooxidans,
Methylococcus capsulatus.
Anaerobic bacteria
These bacteria are not used as frequently as aerobic bacteria and grow in absence of oxygen.
Can often be applied to bioremediation of polychlorinated biphenyls (PCBs) in river sediments, dichlorination of the solvent trichloroethylene (TCE), and chloroform.
Examples: Escherichia coli, Staphylococcus genus, Clostridium genus
Fungi
Capable of degrading a wide diverse range of persistent or toxic environmental pollutants.
Bioremediation Strategies:
In situ Bioremediation
In situ bioremediation is when the contaminant is cleaned up onsite exactly where it occurred where there is no need to excavate or remove soils or water in order to accomplish remediation.
Types
Intrinsic Bioremediation
- This is a process that rely on the natural microbial activity at the site of contamination.
- No any external supplies are executed where existing microbial populations the pollutants in a natural manner.
- It is the most commonly used type of bioremediation because it is the cheapest, most efficient. which can be more sustainable and environmentally friendly where there is no any involvement of human intervention.
Engineered Bioremediation
This is the second approached of in-situ bioremediation that involves supplies of microorganisms, oxygen, and nutrients to the site of contamination to stimulate naturally occurring bacteria to degrade contaminants.
Generally, it is employed when the conditions of the site are not suitable that accelerates the degradation process by enhancing the various physicochemical conditions to encourage the growth of microorganisms.
Types of engineered bioremediation
Bioventing
This kind of strategy is linked with the supply of air and nutrients to contaminated soil in order to stimulate and enhance the aerobic degradation of pollutants by the indigenous or naturally existing bacteria and fungus.
Biosparging
It involves the injection of oxygen or other gas to increase oxygen concentrations into contaminated zones and availability of oxygen to the microbial community in order to enhance the rate of biological degradation of contaminants. It uses sparging wells to pump pressurized air or gas.
Bioaugmentation
- It is one of the mechanisms of biodegradation that involved addition of engineered microbes to increase the degradative efficiency of a wide range of environmental pollutant
- These types of approach involved collection of microbes from the contaminated site, separately cultured, genetically modified or engineered as per the requisite, and returned to the site.
- Genetically modified microbes have demonstrated the potential for bioremediation of soil, groundwater, and activated sludge, displaying increased degrading capabilities of a broad spectrum of chemical and physical contaminants.
Ex-situ Bioremediation
It is a biological procedure that involves the removal of pollutants from the site and subsequently transporting them to another site for treatment based on the cost of treatment, depth of pollution, type of pollutant, degree of pollution, geographical location and geology of the polluted site.
Some types of ex-situ bioremediation include:
Composting
It is a technique that involves mixing contaminated soil with organic compounds such as agricultural wastes or manure.
The presence of these organic materials supports the growth of a microbial population and elevated temperature characteristic of composting produce useful compost as a by-product.
Land farming
This is a basic procedure that involves excavating contaminated soil, spreading it over a prepared bed, and tilling it on a regular basis until contaminants are degraded. The practice is limited to treating the top 10–35 cm of soil.
Biopiling
This is a combination of land farming and composting. Essentially, designed cells are built like aerated compost piles. Typically employed to address surface pollution with petroleum hydrocarbons, they are a refined version of land farming that controls physical losses of contaminants by leaching and volatilization. Biopiles create an ideal environment for indigenous aerobic and anaerobic bacteria.
Bioreactors
Large, sealed containers that serve as a petri dish for the development of specialized organisms capable of breaking down the pollutant.
Contaminated material is placed in a controlled reactor vessel where environmental conditions (pH, temperature, oxygen levels, and nutrition supply) are tuned to promote microbial decomposition.
Advantage of Bioremediation:
- Eco-friendly, sustainable, simple, Cost-efficient, and less intensive
- Contaminants are eliminated, not just shipped to another environmental medium.
- Easy to implement and Effective for a wide range of organic contaminants,
- No requirements of sophisticated equipment.