CONTAMINATED LAND BIOREMEDIATION INTRODUCTION Contaminated land is another

CONTAMINATED LAND & BIOREMEDIATION

INTRODUCTION • Contaminated land is another example of a widely appreciated, yet often poorly understood, environmental problem. • The importance of land remediation in cleaning up the residual effects of previous human activities on a site lies in two spheres. • Firstly, throughout the world, environmental legislation is becoming increasingly stringent and the tightening up of the entire regulatory framework has led to both a real drive for compliance and a much greater awareness of liability issues within industry. • Secondly, as the pressure grows to redevelop old, unused or derelict so called ‘brown-field’ sites, rather than develop previously untouched ‘green-field’, the need to remove any legacy of previous occupation is clear. • A number of technologies are available to achieve such a cleanup, of which bioremediation, in its many individual forms, is only one.

• The legal definition of a contaminated land according to the Section 57 of the UK Environment Act 1995 is as follows: “any land which appears. . . to be in a condition that. . . significant harm is being caused or there is a significant possibility of significant harm. . (or). . . pollution of controlled waters”. • In this, harm is expressly defined as to human health and environment.

REMEDIATION METHODS The currently available processes for soil remediation can be divided into five generalized categories: • biological; • chemical; • physical; • solidification/ vitrification; • thermal.

1. BIOLOGIACL REMEDIATION • Biological methods involve the transformation or mineralization of contaminants to less toxic, more mobile, or more toxic but less mobile, forms. • The main advantages of these methods are their ability to destroy a wide range of organic compounds, their potential benefit to soil structure and fertility and their generally nontoxic, ‘green’ image.

2. CHEMICAL REMEDIATION • Toxic compounds are destroyed, fixed or neutralized by chemical reaction. • The principal advantages are that under this approach, the destruction of biologically recalcitrant chemicals is possible and toxic substances can be chemically converted to either more or less biologically available ones, whichever is required.

3. PHYSICAL REMEDIATION • This involves the physical removal of contaminated materials, often by concentration and excavation, for further treatment or disposal.

4. SOLIDIFICATION/ VITRIFICATION • Solidification is the encapsulation of contaminants within a monolithic solid of high structural integrity, with or without associated chemical fixation, when it is then termed ‘stabilization’. • Vitrification uses high temperatures to fuse contaminated materials. • One major advantage is that toxic elements and/or compounds which cannot be destroyed, are rendered unavailable to the environment.

5. THERMAL REMEDIATION • Contaminants are destroyed by a heat treatment using incineration, gasification, pyrolysis or volatization processes. • Clearly, the principal advantage of this approach is that the contaminants are most effectively destroyed.

BIOREMEDIATIO N

BIOREMEDIATION • Bioremediation is a soft bioengineering technique to clean up contaminated lands and soils using microbes, plants and earthworms. OR • It is also a technique to stabilize the eroded lands and prevent soil erosion. OR • Bioremediation is the use of microorganisms to destroy or immobilize waste materials.

• Microbes are adapted to thrive in ‘adverse conditions’ of high acidity, alkalinity, toxicity and high temperature. • Under favorable conditions of growth, microbes can biodegrade and biotransform the complex hazardous organic chemicals into simpler and harmless ones. • Environmentalists are viewing microbes such as yeast, bacteria, algae, diatoms and actinomycetes as an ‘eco-friendly factories’ for metal remediation. • Bioremediation is a key area of ‘white’ biotechnology, because the elimination of a wide range of pollutants from water and soils is an absolute requirement for sustainable development.

• Industrial and domestic wastes are produced in the three physical states, i. e. as solids, liquids and gases. • Solid wastes pollute the soil with which they come in contact. • Liquid wastes flow horizontally over the ground or percolate vertically into the layers below thus polluting the soil layers and ground water with which they come in contact. • Gaseous and vaporous wastes spread horizontally and vertically in the atmosphere, get washed out by rain water and ultimately reach the ground polluting the soil and water. • Irrespective of their physical state i. e. whether solid, liquid or gas the waste can be classified as: biodegradable and non-biodegradable. • The biodegradability of a waste can be rapid, moderate or slow.

• Soil is the loose material of earth primarily composed of mineral fraction (Inorganic matter as Si. O 2, Al, Fe and less quantities of Ca, Mg, K, Mn, Na, N, P and S) and organic fraction (plant and animal debris, microbes and humus). It has moisture and space between particles where gases accumulate. • Groundwater gets accumulated in porous beds over an impervious layer and this porous bed containing water is called an aquifer. • Both soil and water form an important base for bacteria (microscopic plants) to flourish. • However these microbes need be provided with nutrients as N, P & K which may be deficient in the waste as well as in soil as the soil is of varying degree of fertility (may be highly fertile or totally barren). • Of all the processes of waste treatment biological treatment is cheaper than both physical and chemical processes provided the waste is biodegradable, more soluble in water and is non-toxic to microbes.

• The technology to treat vaporous wastes biologically is termed as Biofiltration. • The time taken and the efficiency of the treatment depends on the type of soil, temperature, p. H and the microbial environment prevailing for the decomposition of the waste. • Depending on the type of bacteria that are responsible for the degradation i. e. in the presence of free oxygen or oxygen in combined state, bioremediation is classified as ‘aerobic’ or ‘anaerobic’.

SOURCES OF CONTAMINATION • • Septic tank effluents, discharges from waste stabilization ponds, gasoline leaks from underground storage tanks bursting of mobile chemical containers, accidental spills of toxins, agricultural discharges rich in pesticides, oils and cleaning solvents from garage ----- responsible for soil and water contamination.

• Every industry contributes industrial wastes which add to the pollution of soil and ground water. • Pesticides when reach drinking water, cause nervous disorders in addition to cancers of different origin. • Continuous exposure to diesel and petrol vapors is carcinogenic. • Paints and varnish wastes (from sanitary landfills) contribute to disorder of nervous system and heavy metal poisoning besides being carcinogenous.

FACTORS OF BIOREMEDIATION FACTORS CONDITIONS REQUIRED Microorganism Aerobic or Anaerobic Biological processes Catabolism and Anabolism Environmental factors Nutrients Temperature, p. H , Oxygen content, Electron acceptor/donor Carbon , Nitrogen , Oxygen etc Soil moisture 25 -28% of water holding capacity Type of soil Low clay or silt content

BIOREMEDIATION STRATEGIES • Following are the two strategies of bioremediation: 1. In-situ bioremediation 2. Ex-situ bioremediation

IN SITU vs. EX SITU techniques • A common way in which all forms of remediation are often characterized is as in situ or ex situ approaches. • IN SITU: unmoved, unaltered, unchanged - remaining in an original state; "persisting unaltered through time“ • EX SITU: Off-site; most commonly used in the context of ex-situ remediation of contaminated soil, which means removing the soil and replacing it with clean soil.

IN SITU • Generally speaking, in situ methods are suited to instances where the contamination is widespread throughout, and often at some depth within, a site, and of low to medium concentration. • Additionally, since they are relatively slow to act, they are of most use when the available time for treatment is not restricted.

EX SITU • The main characteristic of ex situ methods is that the soil is removed from where it originally lay, for treatment. Strictly speaking this description applies whether the material is taken to another venue for clean-up, or simply to another part of the same site. • The main benefits are that the conditions are more readily optimized, process control is easier to maintain and monitoring is more accurate and simpler to achieve. • In addition, the introduction of specialist organisms, on those occasions when they may be required, is easier and/or safer and generally these approaches tend to be faster than corresponding in situ techniques.

1. In-situ Bioremediation • Following techniques are studied under the strategy: • Biosparging • Bioventing • Bioaugmentation • Biopiling

Biosparging: • Biosparging involves the injection of air under pressure below the water table to increase groundwater oxygen concentrations and enhance the rate of biological degradation of contaminants by naturally occurring bacteria. • Biosparging increases the mixing in the saturated zone and thereby increases the contact between soil and groundwater. • The ease and low cost of installing small diameter air injection points allows considerable flexibility in the design and construction of the system.

Bioventing: • Bioventing is a promising new technology that stimulates the natural in-situ biodegradation of any aerobically degradable compounds within the soil by providing oxygen to existing soil microorganisms. • Bioventing uses low air-flow rates to provide only enough oxygen to sustain microbial activity. • Oxygen is most commonly supplied through direct air injection into residual contamination in soil by means of wells. • Adsorbed fuel residuals are biodegraded, and volatile compounds also are biodegraded as vapors move slowly through biologically active soil.

Bioaugmentation: • Bioaugmentation is the introduction of a group of natural microbial strains or a genetically engineered variant to treat contaminated soil or water. • It is commonly used in municipal wastewater treatment to restart activated sludge bioreactors. • Most cultures available contain a research based consortium of microbial cultures, containing all necessary microorganisms at sites where soil and groundwater are contaminated with chlorinated ethenes, such as tetrachloroethylene and trichloroethylene, bioaugmentation is used to ensure that the in situ microorganisms can completely degrade these contaminants to ethylene and chloride, which are non-toxic. • monitoring of this system is difficult.

Biopiling: • Biopile treatment is a full-scale technology in which excavated soils are mixed with soil amendments, placed on a treatment area, and bioremediated using forced aeration. • The contaminants are reduced to carbon dioxide and water. • The basic biopile system includes a treatment bed, an aeration system, an irrigation/nutrient system and a leachate collection system. • Moisture, heat, nutrients, oxygen, and p. H are controlled to enhance biodegradation. • The irrigation/nutrient system is buried under the soil to pass air and nutrients either by vacuum or positive pressure. Soil piles can be up to 20 feet high and may be covered with plastic to control runoff, evaporation and volatilization and to promote solar heating. • If volatile organic compounds (VOCs) in the soil volatilize into the air stream, the air leaving the soil may be treated to remove or destroy the VOCs before they are discharged into the atmosphere. • Treatment time is typically 3 to 6 months.

2. Ex-situ Bioremediation • Ex situ treatment involves excavation of the contaminated soil, transporting it to the place of disposal, mixing it with bulking agents as manure and moisture and treating by inoculating with the necessary microbes along with nutrients required for their proliferation . • Composting (solid phase) • slurry phased Bioremediation (Bioreactor)

Composting: • Composting is a process by which organic wastes are degraded by microorganisms, typically at elevated temperatures. • Typical compost temperatures are in the range of 55° to 65°C. The increased temperatures result from heat produced by microorganisms during the degradation of the organic material in the waste.

Bioreactors: • used for ex situ treatment of contaminated soil and water. • A slurry bioreactor may be defined as a containment vessel and apparatus used to create a three-phase (solid, liquid, and gas) mixing condition to increase the bioremediation rate of soil bound and water-soluble pollutants as a water slurry of the contaminated soil and biomass(usually indigenous microorganisms) capable of degrading target contaminants. • The rate and extent of biodegradation are greater in a bioreactor system than in situ or in solid-phase systems because the contained environment is more manageable and hence more controllable and predictable. • there are some disadvantages. The contaminated soil requires pretreatment (e. g. , excavation) or alternatively the contaminant can be stripped from the soil via soil washing or physical extraction (e. g. , vacuum extraction) before being placed in a bioreactor.

ADVANTAGES OF BIOREMEDIATION • • • It is a natural process, it takes a little time, as an acceptable waste treatment process for contaminated material such as soil. Microbes able to degrade the contaminant increase in numbers when the contaminant is present; when the contaminant is degraded, the biodegradative population declines. The residues for the treatment are usually harmless product sort. Bioremediation also requires a very less effort and can often be carried out on site, often without causing a major disruption of normal activities. This also eliminates the need to transport quantities of waste off site and the potential threats to human health and the environment that can arise during transportation. Bioremediation is also a cost effective process as it lost less than the other conventional methods that are used for clean-up of hazardous waste. It also helps in complete destruction of the pollutants, many of the hazardous compounds can be transformed to harmless products, this feature also eliminates the chance of future liability associated with treatment and disposal of contaminated material. It does not use any dangerous chemicals. The nutrients added to make microbes grow are fertilizers commonly used on lawns and gardens. Because bioremediation changes the harmful chemicals into water and harmless gases, the harmful chemicals are completely destroyed.

DISADVANTAGES OF BIOREMEDIATION • Bioremediation is limited to those compounds that are biodegradable. Not all compounds are susceptible to rapid and complete degradation. • There are some concerns that the products of biodegradation may be more persistent or toxic than the parent compound. Biological processes are often highly specific. Important site factors required for success include the presence of metabolically capable microbial populations, suitable environmental growth conditions, and appropriate levels of nutrients and contaminants. • It is difficult to extrapolate from bench and pilot-scale studies to full-scale field operations. • Research is needed to develop and engineer bioremediation technologies that are appropriate for sites with complex mixtures of contaminants that are not evenly dispersed in the environment. Contaminants may be present as solids, liquids, and gases. • Bioremediation often takes longer than other treatment options, such as excavation and removal of soil or incineration.

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