After reading this article you will learn about:- 1. Meaning of Bioremediation 2. Classification of Bioremediation Technology 3. Features 4. Methods 5. Limitations.
Contents
Meaning of Bioremediation:
Bioremediation is a treatment technology that uses biodegradation of organic contaminants through stimulation of indigenous microbial populations by providing certain amendments, such as adding oxygen, limiting nutrients, or adding exotic microbial species.
It uses naturally occurring or externally-applied microorganisms to degrade and transform hazardous organic constituents into compound of reduced toxicity and/or availability.
Specific technologies fall into two broad categories:
(1) Ex situ technologies (e.g. slurry phase, land treatment, solid phase, composting), and
(2) In situ technologies.
Activity include addition of amendments such as nutrients or oxygen while passive remediation utilizes natural attenuation to adequately characterize, model and monitor the site to record natural attenuation and protection of potential receptors.
Classification of Bioremediation Technology:
Different kinds of bioremediation technologies are currently being used for soil treatment and many more innovative approaches involving bio-augmentation are being developed.
Considering the similarity in their cross-media transfer potential, a few examples of bioremediation technologies and processes are listed below:
1. Natural Attenuation
2. Biodegradation
3. Aerobic/Anaerobic biodegradation
4. Composting
5. Bio-piles
6. Bioreactors
7. Land Treatment
8. Dehalogenation
9. Bio-scrubbers
10. Binding of Metals
11. Methanotrophic Process (in Situ)
12. Fungal Inoculation Process
13. Plant Root Uptake (Phytoremediation)
14. Slurry Phase bioremediation
15. Bioventing
16. Solid Phase Bioremediation
17. Bio Wall for Plume Decontamination (In Situ)
18. Bioremediation of Metals (Changing the Valence)
The scope of bioremediation treatment is not limited to the above listed technologies. Any treatment technology that has similar key features, as described below, should be considered a bioremediation technology. A typical schematic solid phase bioremediation system is shown in Fig. 23.1.
Key Features of Bioremediation:
1. Most bioremediation treatment technologies destroy the contaminants in the soil matrix.
2. These treatment technologies are generally designed to reduce toxicity either by destruction or by transforming toxic organic compounds into less toxic compounds.
3. Indigenous micro-organisms, including bacteria and fungi, are most commonly used. In some cases, wastes may be inoculated with specific bacteria or fungi known to biodegrade the contaminants in question. Higher plants may also be used to enhance biodegradation and stabilise the soil.
4. The addition of nutrients or electron acceptors (such as hydrogen peroxide or ozone) to enhance growth and reproduction of indigenous organisms may be required.
5. Field application of bioremediation may involve:
(i) Excavation
(ii) Soil handling
(iii) Storage of contaminated soil piles
(iv) Mixing of contaminated soils
(v) Aeration of contaminated soils
(vi) Injection of fluid
(vii) Extraction of fluid
(viii) Introduction of nutrients and substrates
Bioremediation Technology Description:
Bioremediation involves the use of micro-organisms to chemically degrade organic contaminants. Aerobic processes use organisms that require oxygen to be able to degrade contaminants. In some cases, additional nutrients such as nitrogen and phosphorous are also needed to encourage the growth of biodegrading organisms.
A biomass of organisms—which may include entrained constituents of the waste, partially degraded constituents and intermediate biodegradation products—is formed during the treatment process. Identically, anaerobic microorganism also helps in bioremediation.
Although bioremediation is applied in many different ways, the description of typical solid phase bioremediation, composting, bioventing, and traditional in situ biodegradation is provided here, besides the description of a few common bioremediation technologies.
Solid Phase Bioremediation:
The solid phase bioremediation treatment can be conducted and lined land treatment units or in composting piles. A lined land treatment unit consists of a prepared bed reactor with a leachate collection system and irrigation and nutrient delivery systems. The unit may also contain air emission control equipment. The soil is placed on land lined with an impervious layer, such as soil, clay, or a synthetic liner.
Bioventing:
Bioventing uses relatively low-flow soil aeration technique to enhance the biodegradation of soils contaminated with organic contaminants. Although bioventing is predominantly used to treat unsaturated soils, applications involving the remediation of saturated soils and groundwater (augmented by air sparging) are becoming more common.
Generally, a vacuum extraction, an air injection, or a combination of both systems is employed. An air pump, one or more air injections or vacuum extraction probes, and emissions monitors at the ground surface level are commonly used.
Land-Farming:
Ex situ processes also include land farming, which involves spreading contaminated soils over a large area on which cropping or plantation can be made.
Bioreactors:
Bioremediation may also be conducted in a bioreactor, in which the contaminated soil or sludge is slurried with water in a mixing tank or a lagoon. Bioremediation systems require that the contaminated soil or sludge be sufficiently and homogeneously mixed to ensure optimum contact with the seed organisms.
Bioreactors function in a manner that is similar to sewage treatment plants. There are many ways in which a bioreactor can be designed; but most are a modification of one of two systems. In the first system, which is often referred to as a trickling filter or fixed media system.
The second common bioreactor design uses a sealed vessel to mix the contaminants, amendments and micro-organisms. Recent research has expanded the capabilities of this technology, which along with its generally lower cost, has led to bioremediation becoming an increasingly attractive clean-up technology.
Bioremediation Application:
Bioremediation is the biological clean up alternatives. During past few decades this process opened up a new option in pollution abatement and cleans up of various contaminated environment. Usually variety of micro-organisms viz., bacteria and fungi were known to be the major players in the whole technology. In contrary the term phytoremediation involves the use of higher plants in clean process so that environment is harmless.
The objective of bioremediation is to exploit naturally occurring bio-degradative process to clean up contaminated sites. Firstly large number of microorganisms were characterised for use in bioremediation of diverse toxicants viz., metals, radio nuclides, phenolic compounds, pesticides and polyaromatic compounds. The process of microbial biodegradation may be of aerobic or anaerobic categories.
Bio-Treatment of Metal and Radionuclide:
There are many metal tolerant microbes which are capable of accumulating and transforming toxic metals and thus helps in detoxification processes. A number of processes are involved in metal removal by different tolerant microorganisms.
These includes:
1. Precipitation of heavy metals and radionuclides by production of extra cellular materials which interact with metal cations forming insoluble precipitate;
2. Biotransformation of metals and radio nuclides either by oxidation, reduction or alkylation reactions;
3. Intercellular accumulation or extra cellular accumulation
The major mechanisms for bacterial metals precipitation is through the formation of hydrogen sulphide and the immobilisation of the metal cations as metal sulphides. Aerobic bacteria like Citrobactar sp produces metal sediment as phosphate salt through phosphatase reactions, where hydrogen phosphate is formed from organic phosphates, such hydrogen phosphate (HPO4-) subsequently precipitates metals and radionuclides (such as lead, cadmium and uranium). The sulphur reducing bacteria viz., species of Desulfovibrio and Desulfotomaculum produce metal sediment in anaerobic environment.
In contrary several microorganisms transforms metals and radionuclides by oxidation, reduction or alkalization reactions. Ferrous (Fe2+) and manganous (Mn2+) compounds can be deposited through oxidation reactions catalysed by species of bacteria, fungi, algae and protozoa. For example, Leptothrix is very common ferromanganese oxidizing bacteria produces he (OH)3 and MnO2 within a surface bound exopolymer.
Similarly Thiobaallus ferroxidans and Leptospirillum ferroxidans can solubilize metal from minerals allowing the extraction and recovery of metals such as Cu, Cd, Gold and Uranium from low grade ores. All these are oxidative reactions.
On the other hand several microbes help in reduction of metal likes mercury, iron, manganese, selenium, arsenic and thus reduces the toxicity of metal ions. Identically tin, selenium and lead can be volatilized by bacteria through the production of alkylated metals. The major bacteria like species of Pseudomonas and Corynebacteria and fungi like Alterneria alternate form these reactions in presence of methylating agents.
Bioaccumulation of metals by microbes is quite well known. Microbes often accumulate metals in intercellular region by active transport or extracellular surface binding. Filamentous fungi like Aspergillus niger and Penicillium species are quite well known for their bio-adsorption.
A variety of biopolymers like polysaccharides, protein and polyphenols has proformed metal binding properties. Metal binding proteins such as metallothioneins (cystine rich small peptides) and phytochelation appears to be commonly produced by microbes. In addition in certain categories of microbes, metal chelating agents like siderophores are known. The siderophores are catechol or hydroxamate derivatives.
Biodegradation of Aromatics:
Several microbes are now well recognised as aromatic degrading organism. Sometime they act individually or act together called consortium. A wide variety of bacteria and fungi can carry out aromatic transformation, both partial and complete, under a variety of environmental conditions.
The bacteria Pseudomonas putida or fungi like Phanerochaete chrysosporium are well known for aromatic compound biotransformation reactions. Under aerobic conditions the most common initial transformation is a hydroxylation that involves the incorporation of molecular oxygen. The enzymes involved in these initial transformations are either monooxygenases or dioxygenases (Fig. 23.2, Fig. 23.3 and Fig. 23.4).
Methods of Bioremediation:
There are two broad classes of bioremediation:
1. In-situ bioremediation—On site treatment for detoxification
2. Ex-situ bioremediation—Of site treatment toxic materials
3. Sometimes bioremediation takes place by natural ways and means called Intrinsic bioremediation or natural attenuation. There are many instances where bioremediation technology received better appreciation and viable technology. But there are numbers environmental conditions that influence the bioremediation processes.
These include the oxygen availability and nutrient availability for microbial actions in on site treatment areas. Thus bioventing (a technique used to add oxygen directly to a contaminated site) through external aeration pipeline or air spraying through forceful injection at contaminated site.
The primary nutrient like sources of C, N, P needs to be added in contaminated site for rapid microbial biodegradation process as needed. Surfactant addition has been proposed as a technique for increasing the bioavailability and hence biodegradation of contaminants. The details of various bioremediation techniques are given in Fig. 23.5.
If appropriate biodegrading microorganisms are not present in soil or if microbial populations have been reduced because of contaminant toxicity, specific microorganisms can be added as “introduced organisms” to enhance the existing populations.
This process is known as bio-augmentation. Scientists are now capable of creating ‘superbugs’—organisms that can degrade pollutants at extremely rapid rates. Such organisms can be developed through successive adaptations under laboratory condition or can be genetically engineered.
Limitations of Bioremediation:
The problems of onsite bioremediation by microbes are often seen for two major reasons:
First, the introduced microbe often cannot establish a niche in the environment. In fact, these introduced organisms often do not survive in a new environment beyond a few weeks.
Second, there are difficulties in delivering the introduced organisms to the site of contamination, because microorganisms like contaminants, can be strongly absorbed by solid surfaces.
An overall scenario in application bioremediation is given below in Table 23.1.