Bioremediation is the process of reduction, elimination, alteration, and transformation of contaminants present in the natural environment like soil, sediments, air, and water through the application of microorganisms, fungi, green plants, or their enzymes.
It is a waste management technique that uses naturally occurring biological organisms to break down hazardous substances into less toxic or non-toxic forms.
Bioremediation includes a series of redox reactions for the production of energy within microbial cells for cell maintenance and reproduction.
Bioremediation is a global, regional and local application for removing pollutants from the environment restoring the contaminated sites.
Factors Affecting Bioremediation
The monitoring of soil physically and chemically is a time-consuming process, in order to measure the pollution of soil after contamination in a shorter period of time microbial and biochemical properties of the soil are to be determined.
The bioremediation process depends upon the different factors for the removal of the contaminants, some of them are:
1. Concentration of the contaminant
The concentration of the contaminants directly affects microbial activity. Lower the concentration of the contaminants there will be decreasing rate of degrading enzymes produced by bacteria in the soil. Toxic effects are observed in presence of higher concentrations of contaminants. The decomposition rate of catabolic enzymes can be increased by the synergistic interactions between different components of the contaminants.
2. Nutrient availability
Carbon, nitrogen, phosphorus, potassium, and calcium are the basic requirement for the growth of microorganisms, the concentration of the nutrient availability directly affects the degradation of the contaminants. The excessive presence of nitrogen, potassium, and phosphorus shows a negative impact on the degradation of hydrocarbons. The rate of bioremediation can also be determined by knowing the accessibility of organic matters towards microorganisms; which is known as bioavailability.
3. Surfactants; enhancers of bioavailability
Mostly, Chemical and food-grade surfactants are used to increase the hydrophobic organic contaminants. Triton X 100, Tween 80, and SDS are the petroleum-derived chemical surfactants and T-MAZ 28, T-MAZ 10 and T-MAZ 60 are food-graded surfactants used in bioremediation. Besides these surfactants produced by microbes are also used for the reduction of environmental contaminants.
4. Characteristics of the contaminated soil
The bioremediation process is significantly affected by the different parameters of the contaminated soil such as pH, texture, permeability, water holding capacity, temperature, and oxygen availability.
Optimum pH is required for the bioremediation process which ranges from 6-8. Neutral pH is suitable for the degradation of petroleum hydrocarbons whereas some fungi and acidophilic microbes degrade contaminants in an acidic environment.
The degradation of the contaminants is also affected by temperature especially in the case of hydrocarbons under both in situ and ex-situ conditions. It has been found that a higher temperature of 30°C-40°c increases the bioremediation in the soil as well as in the marine environment.
c. Oxygen availability
Oxygen is a very important factor to determine the extent and rate of biodegradation of contaminants. Aerobic biodegradation is much faster than anaerobic biodegradation. For the aerobic respiratory breakdown of organic contaminants, oxygen availability plays a significant role. In the majority of cases, the addition of hydrogen peroxide is used to introduce oxygen. Hydrogen peroxide is about seven times more soluble in water than oxygen.
Types of Bioremediation
On the basis of removal and transportation of waste for treatment, bioremediation is of two different types.
- In Situ Bioremediation
- Ex Situ Bioremediation
In Situ Bioremediation
In situ remediation is the in-site treatment of contaminants using biological agents. It is a cleanup approach between microbes and the contaminants directly for biotransformation.
There are two major types of in situ bioremediation.
- Intrinsic Bioremediation
- Intrinsic bioremediation or natural attenuation is a passive process of treatment of polluted sites without any human intervention through naturally occurring microbial population.
- It is the natural remediation of environmental pollutants into non-toxic or less toxic forms using the inherent capacity of microorganisms without taking any engineering steps to enhance the process.
- The major application of intrinsic bioremediation is for control of contaminants at the waste sites
- This types of remediation requires monitoring of the role of native microorganisms in eliminating contaminants via tests performed at field sites or on site-derived samples of soil, sediment, or water.
- Different environmental factors favor intrinsic bioremediation such as pH, concentration, temperature and nutrient availability.
- It is a cost effective method.
- Limitation of this process is the longer time to achieve the target level of pollutant concentration.
- Engineered Bioremediation
- Engineered bioremediation or accelerated bioremediation is the advanced process of application of engineered systems to stimulate microbial activity for remediation of environmental pollutants.
- When intrinsic bioremediation isn’t feasible, accelerated bioremediation is proceeded where, either substrate or nutrients are added to the environment to degrade the toxicity of contaminants making the microorganism grow more rapidly.
- This process accelerates the biodegradation process through optimization of physical and chemical conditions vitalizing the growth of microorganisms.
- Usually the microorganism is indigenous, but occasionally microorganism that are very efficient at degrading a certain contaminant are additionally added.
- Engineered bioremediation may be chosen over intrinsic bioremediation because of time and liability.
- Limitation of this process is ineffective reactivity with metal contaminants that are mixed with organic compounds.
In Situ Bioremediation Techniques
- Bio augmentation is the process of addition of culture microbial population which have the ability to degrade specific soil and groundwater contaminants.
- A technique of bioremediation in which strains of natural or genetically engineered bacteria with unique metabolic profiles are added to the contaminated site in order to supplement indigenous microflora and speed up biodegradation.
- Common application involves bio augmentation for chlorinated contaminants, petroleum hydrocarbon etc.
- Microorganisms are isolated either from contaminated sites, historical sites or genetically modified to support remediation process of contaminated sites.
- Microbes cannot use pollutants as the only source of energy thus they need to be accessed with supplied nutrients.
- Bio-stimulation is the process of environmental modification via addition of limiting nutrients and electron acceptors like phosphorous, nitrogen, oxygen or carbon in order to stimulate the existing microbial population which are involved in bioremediation.
- It is most common remediation approach against petroleum pollutants in soil.
- Bioslurping, also called as multi-phase extraction is the process of in situ aerobic bioremediation of contaminated soils using bioventing and vacuum enhanced free-product recovery that extracts light, no aqueous-phase liquids (LNAPLs) from the capillary fringe.
- Bioslurping is limited to 25 feet below ground surface as contaminants cannot be lifted more than 25 feet by this method and effective for removing free product that is floating on the water table.
- Biosparging the injection of a gas and gas-phase nutrients pressure into the saturated zone applying pressure to promote aerobic biodegradation.
- It is the most recommended approach for aerobic degredation of sites affected with lighter to heavier petroleum contaminants such as oils, diesel, gasoline, jetfuels etc.
- Lighter ones are removed easily but heavier ones due to minimum level of microbial bioavaibility requires longer process of treatment.
- In it the cost can be reduce by reducing the diameter of injection point.
- The most common in situ treatment and involves supplying air and nutrients through wells to contaminated soil to stimulate the indigenous microorganism.
- Bioventing is applied for remediation of petroleum hydrocarbons contaminants in soil through air supply to an unsaturated soil zone using a combination of pumps and blowers for continuous injection of low volumes of air.
- It can be categorized as either aerobic, anaerobic or co-metabolic depending on the amendments used.
- The slow removal of air and maintaining 5% oxygen in subsurface is generally practice for bioventing.
- Phytoremediation is the use of plant and its products for the decontamination or stabilization of contaminants and metals from soil.
- There are certain varieties of plants which have the ability to vacuum heavy metals the soil via root and concentrate them in the stems, shoots, and leaves.
- These plants possess genes that regulate the amount of metals taken up from the soil by roots and deposited at other locations within the plant.
- Depending on the underlying processes, applicability and types of contaminant, phytoremediation can be broadly categorized as:
- Percolation is the process of downward movement of water through soil under the influence of gravitational force.
- It depends on hydraulic activity and water content of the soil.
- If the hydraulic activity of soil is greater than the precipitation rate to near saturation, the movement of water will be downward through soil which leads to nutrient loss from soil.
Pump and Treat
- It is the remediation process treatment through extraction and restoration of contaminated groundwater with the help pf dissolved chemicals, metals and solvents.
- The extraction is done by pumping the water out in which extraction rate is the minimum rate suitable to prevent spreading of contaminated area.
- Restoration is done to remove the contaminants in which pumping rate is sufficiently larger than the required rate for removing contaminants so that clean water will flush at an accelerated rate.
Ex-situ bioremediation or off-site bioremediation is the removal/excavation of contaminants and pollutants by subsequent transportation of contaminants from one site to another. Similar to in situ techniques, remediation occurs with the role of microorganisms. These techniques are based on the type of contaminants, site of pollution, degree of pollution, and cost of treatment.
Techniques in ex situ bioremediation are:
- Land Farming
- Biofiltration is the biological treatment process of biodegradable waste which relies on biodegrading microbial populations.
- It is the process of purification of contaminated air evolved from volatile organic compounds.
- The treatment of contaminants is done using various materials like bio-filters, bio-trickling filters, bio-scrubbers, conventional bio-filters etc.
- Biofilter is treatment bed consisting of compost, soil or peat media inside which the pollutants come into contact with microorganisms and eventually get biodegraded.
- Biopile is type of remediation process that involves enhancement techniques via above-ground piling of excavated polluted soil, nutrient amendment, and sometimes aeration to increase the microbial population and their activity.
- This technique involves aeration, irrigation, nutrient and leachate collection systems, and a treatment bed.
- Biopile can reduce and limit volatilization of low molecular weight (LMW) pollutants, also help in effective remediation of extreme polluted environments
- It is a cost effective approach which ensures effective biodegradation.
- Bioreactor is an engineered system involving series of biological reactions in which pollutants are fed into the bioreactor vessel for their degradation that facilitate the growth of biological mass.
- There 5 different operating mechanisms in bio reactor which includes: batch, fed-batch, sequencing batch, continuous and multistage.
- Bioreactors maintains suitable controlled environment for the optimum growth conditions that leads to the proliferation of microbial populations.
- Pollutants are fed either in the dry or slurry form
- Composting is the process of degradation and decaying or organic waste under favorable controlled conditions with the action waste degrading microorganisms.
- Composting is a self-heating, substrate-dense and solid phase treatment process.
- Microbial population metabolize the organic waste and degrade it to volume by 50% reduction forming the end product called compost or humus.
- Compost is a nutrient rich soil which is very useful in application to the crops and plants for their effective growth.
- The steps involved in the process include sorting and separating, size reduction, and digestion of the refuse.
- One of the simplest bioremediation process of excavation of polluted soil transported to above the ground surface allowing aerobic biodegradation of pollutant by autochthonous microorganisms.
- The autochthonous microorganisms are stimulated by tilling process which involves nutrients amendments (nitrogen, phosphorous etc.), aeration process and irrigation.
- If the in-site treatment is the process can also fall under in-situ remediation.
- It is also a cost effective approach which requires minimal environment and energy for treatment of large volume of polluted soils.
Advantages of Bioremediation
- Complete remediation of harmful contaminants presents in the environment instead to transferring contaminants from one site to another.
- Cost effective method with minimal requirements of complex tools and equipment
- Environment friendly approach with use of microorganism instead of harmful chemicals
- In majority cases, can be carried out on site reducing transportation cost
- Minimum site destruction and disruption
- Lower liability level
- Low energy consumption
- Being a very effective reliable and easy approach, is therefore accepted publicly and by the regulatory authorities.
Disadvantages of Bioremediation
- Only limited to biodegradable waste and contaminants
- Requires extensive monitoring
- Being a biological process, specificity is a major drawback in terms factors like type of environmental growth conditions, types of microorganisms, type of nutrient requirements and type of contaminants.
- Possibility of production unknown and potentially toxic byproducts
- Comparatively a time consuming process
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