With oil spills occurring worldwide, much media and practical attention has been given in recent years to the rapidly maturing field of hydrocarbon bioremediation, particularly with application to marine spills.
Hydrocarbon contamination of soil and groundwater, although less visible, is even more widespread and has provided the background for the numerous studies presented in this book, in addition to those devoted to shoreline spills. Chapters address a wide variety of theory and practice and cover important subjects such as biofiltration, natural attenuation, surfactants, and the use of in situ bioventing compared to soil venting.
This unique book represents the collective global experience of practitioners and researchers in North America, Europe, Africa, and Asia. It describes experiences in tying laboratory studies to field applications. Nowhere else can anyone involved in hydrocarbon bioremediation find more up-to-date, relevant information on field experience using the various techniques and combinations of techniques in remediating hydrocarbons by biological means. Paul Guyer, P. Introductory technical guidance for civil, environmental, and geotechnical engineers interested in vapor extraction and bioventing of contaminated soil.
Here is what is discussed: 1. Author : W. Author : DavidJ. Desc4ibes the ex shu technique of thermal desorption of soil contaminants-a low-cost aftemative to incineration for the removal of organics.
Author : M. This book provides information essential to students taking courses in biotechnology as part of environmental sciences, environmental management, or environmental biology programs. It is also suitable for those studying water, waste management, and pollution abatement. Topics include biodiversity, renewable energy, bioremediation technology, recombinant DNA technology, genetic engineering, solid waste management, composting, vermicomposting, biofertilizer, chemical pesticides, biological control of pests, and genetically modified organisms.
The book also discusses bioethics and risk assessment, intellectual property rights, environmental cleanup technologies, and environmental nanotechnology. This book presents a comprehensive collection of various in situ and ex-situ soil remediation regimes that employ natural or genetically modified microbes, plants, and animals for the biodegradation of toxic compounds or hazardous waste into simpler non-toxic products.
These techniques are demonstrated to be functionally effective in connection with physical, chemical, and biological strategies. Soil and water contamination through heavy metals, hydrocarbons and radioactive wastes is of global concern, as these factors have cumulative effects on the environment and human health through food-chain contamination.
The book discusses the utilization of algae, plants, plant-associated bacteria, fungi endophytic or rhizospheric and certain lower animals for the sustainable bioremediation of organic and inorganic pollutants. In addition, it explores a number of more recent techniques like biochar and biofilms for carbon sequestration, soil conditioning and remediation, and water remediation. It highlights a number of recent advances in nanobioremediation, an emerging technology based on biosynthetic nanoparticles.
Lastly, it presents illustrative case studies and highlights the successful treatment of polluted soils by means of these strategies. The introduction of contaminants, due to rapid urbanization and anthropogenic activities into the environment, causes distress to the physio-chemical systems including living organisms, which possibly is threatening the dynamics of nature as well as the soil biology by producing certain xenobiotics.
Hence, there is an immediate global demand for the diminution of such contaminants and xenobiotics that can otherwise adversely affect the living organisms. Some toxic xenobiotics include synthetic organochlorides such as PAHs and some fractions of crude oil and coal.
Over time, microbial remediation processes have been accelerated to produce better, more eco-friendly, and more biodegradable solutions for complete dissemination of these xenobiotic compounds.
The advancements in microbiology and biotechnology led to the launch of microbial biotechnology as a separate area of research and contributed dramatically to the development of areas like agriculture, environment, biopharmaceutics, fermented foods, and more. The Handbook of Research on Microbial Remediation and Microbial Biotechnology for Sustainable Soil provides a detailed comprehensive account for microbial treatment technologies, bioremediation strategies, biotechnology, and the important microbial species involved in remediation.
The chapters focus on recent developments in microbial biotechnology in the areas of agriculture and environment and the physiology, biochemistry, and the mechanisms of remediation along with a future outlook.
This book is ideal for scientists, biologists, academicians, students, and researchers in the fields of life sciences, microbiology, environmental science, environmental engineering, biotechnology, agriculture, and health sciences. An introduction to the principles and practices of soil and groundwater remediation Soil and Groundwater Remediation offers a comprehensive and up-to-date review of the principles, practices, and concepts of sustainability of soil and groundwater remediation.
In addition, it was reported that biopile could be used to treat large volume of polluted soil in a limited space. Biopile setup can easily be scaled up to a pilot system to achieve similar performance obtained during laboratory studies Chemlal et al. Important to the efficiency of biopile is sieving and aeration of contaminated soil prior to processing Delille et al.
Although biopile systems conserve space compared to other field ex situ bioremediation techniques, including land farming, robust engineering, cost of maintenance and operation, lack of power supply especially at remote sites, which would enable uniform distribution of air in contaminated piled soil via air pump are some of the limitations of biopiles.
More so, excessive heating of air can lead to drying of soil undergoing bioremediation, which will result in inhibition of microbial activities, and promote volatilization rather than biodegradation Sanscartier et al.
The periodic turning of polluted soil, together with addition of water bring about increase in aeration, uniform distribution of pollutants, nutrients and microbial degradative activities, thus speeding up the rate of bioremediation, which can be accomplished through assimilation, biotransformation and mineralization Barr Windrow treatment when compared to biopile treatment, showed higher rate of hydrocarbon removal; however, the higher efficiency of the windrow towards hydrocarbon removal was as a result of the soil type, which was reported to be more friable Coulon et al.
Nevertheless, due to periodic turning associated with windrow treatment, it may not be the best option to adopt in remediating soil polluted with toxic volatiles. The use of windrow treatment has been implicated in CH 4 greenhouse gas release due to development of anaerobic zone within piled polluted soil, which usually occurs following reduced aeration Hobson et al.
Bioreactor, as the name implies, is a vessel in which raw materials are converted to specific product s following series of biological reactions. There are different operating modes of bioreactor, which include: batch, fed-batch, sequencing batch, continuous and multistage.
The choice of operating mode depends mostly on market economy and capital expenditure. Conditions in a bioreactor support natural process of cells by mimicking and maintaining their natural environment to provide optimum growth conditions.
Polluted samples can be fed into a bioreactor either as dry matter or slurry; in either case, the use of bioreactor in treating polluted soil has several advantages compared to other ex situ bioremediation techniques.
Excellent control of bioprocess parameters temperature, pH, agitation and aeration rates, substrate and inoculum concentrations is one of the major advantages of bioreactor-based bioremediation. The ability to control and manipulate process parameters in a bioreactor implies that biological reactions within can be enhanced to effectively reduce bioremediation time. Importantly, controlled bioaugmentation, nutrient addition, increased pollutant bioavailability, and mass transfer contact between pollutant and microbes , which are among the limiting factors of bioremediation process can effectively be established in a bioreactor thus making bioreactor-based bioremediation more efficient.
Further, it can be used to treat soil or water polluted with volatile organic compounds VOCs including benzene, toluene, ethylbenzene and xylenes BTEX. The applications of different bioreactors for bioremediation process have resulted in removal of wide range of pollutants Table 1. The flexible nature of bioreactor designs allows maximum biological degradation while minimizing abiotic losses Mohan et al.
Short or long-term operation of a bioreactor containing crude oil-polluted soil slurry allows tracking of changes in microbial population dynamics thus enabling easy characterization of core bacterial communities involved in bioremediation processes Chikere et al. Furthermore, it allows the use of different substances as biostimulant or bioaugmenting agent including sewage sludge. In addition, bioreactor being an enclosed system, genetically modified microorganism GEM can be used for bioaugmentation after which the organism GEM can be destroyed before treated soils are returned to field for landfilling.
This containment of GEM in a bioreactor followed by destruction will help ensure that no foreign gene escapes into an environment after bioremediation. With bioreactor, the role of biosurfactant was found to be insignificant due to efficient mixing associated with bioreactor operations Mustafa et al. Despite that bioreactor-based bioremediation has proven to be efficient as a result of different operating parameters, which can easily be controlled, establishing best operating condition by relating all parameters using one-factor-at-a-time OFAT approach would likely require numerous experiments, which is time-consuming.
This particular challenge can be overcome by using design of experiment DoE tone, which provides information on optimal range of parameters using a set of independent variables controllable and uncontrollable factors over a specified region level Mohan et al.
Notwithstanding, understanding microbiological processes is of great importance when optimizing bioremediation processes Piskonen et al. Moreover, bioreactor-based bioremediation is not a popular full-scale practice due to some reasons. Firstly, due to bioreactor being ex situ technique, the volume of polluted soil or other substances to be treated may be too large, requiring more manpower, capital and safety measures for transporting pollutant to treatment site, therefore, making this particular technique cost ineffective Philp and Atlas Lastly, pollutants are likely to respond differently to different bioreactors; the availability of the most suitable design is of paramount importance.
Above all, cost of a bioreactor suitable for a laboratory or pilot-scale bioremediation makes this technique to be capitally intensive. Land farming is amongst the simplest bioremediation techniques owing to its low cost and less equipment requirement for operation. In most cases, it is regarded as ex situ bioremediation, while in some cases, it is regarded as in situ bioremediation technique.
This debate is due to the site of treatment. Pollutant depth plays an important role as to whether land farming can be carried out ex situ or in situ. When excavated polluted soil is treated on-site, it can be regarded as in situ; otherwise, it is ex situ as it has more in common with other ex situ bioremediation techniques. Generally, excavated polluted soils are carefully applied on a fixed layer support above the ground surface to allow aerobic biodegradation of pollutant by autochthonous microorganisms Philp and Atlas ; Paudyn et al.
Tillage, which brings about aeration, addition of nutrients nitrogen, phosphorus and potassium and irrigation are the major operations, which stimulate activities of autochthonous microorganisms to enhance bioremediation during land farming. Nevertheless, it was reported that tillage and irrigation without nutrient addition in a soil with appropriate biological activity increased heterotrophic and diesel-degrading bacterial counts thus enhancing the rate of bioremediation; dehydrogenase activity was also observed to be a good indicator of biostimulation treatment and could be used as a biological parameter in land farming technology Silva-Castro et al.
Similarly, in a field trial, Paudyn et al. Land farming is usually used for remediation of hydrocarbon-polluted sites including polyaromatic hydrocarbons Silva-Castro et al. Land farming system complies with government regulations, and can be used in any climate and location Besaltatpour et al. The construction of a suitable land farming design with an impermeable liner minimizes leaching of pollutant into neighbouring areas during bioremediation operation da Silva et al. Over all, land farming bioremediation technique is very simple to design and implement, requires low capital input and can be used to treat large volume of polluted soil with minimal environmental impact and energy requirement Maila and Colete Although the simplest bioremediation technique, land farming like other ex situ bioremediation techniques has some limitations, which include: large operating space, reduction in microbial activities due to unfavourable environmental conditions, additional cost due to excavation, and reduced efficacy in inorganic pollutant removal Khan et al.
Moreover, it is not suitable for treating soil polluted with toxic volatiles due to its design and mechanism of pollutant removal volatilization , especially in hot tropical climate regions.
These limitations and several others make land farming-based bioremediation time consuming and less efficient compared to other ex situ bioremediation techniques. One of the major advantages of ex situ bioremediation techniques is that they do not require extensive preliminary assessment of polluted site prior to remediation; this makes the preliminary stage short, less laborious and less expensive.
Due to excavation processes associated with ex situ bioremediation, pollutant inhomogeneity as a result of depth, non-uniform concentration and distribution, can easily be curbed by effectively optimizing some process parameters temperature, pH, mixing of any ex situ technique to enhance bioremediation process. These techniques allow modifications of biological, chemical and physico-chemical conditions and parameters necessary for effective and efficient bioremediation. Importantly, the great influence of soil porosity, which governs transport processes during remediation, can be reduced when polluted soils are excavated.
Ex situ bioremediation techniques are unlikely to be used in some sites such as under buildings, inner city and working sites Philp and Atlas On the other hand, the excavation features of ex situ bioremediation tend to disrupt soil structure; as a result, polluted and surrounding sites alike experience more disturbances.
Moderate to extensive engineering required for any ex situ bioremediation techniques implies that more workforce and capital are required to construct any of the technique. In most cases, these techniques require large space for operation. Generally, ex situ bioremediation techniques tend to be faster, easier to control and can be used to treat wide range of pollutants Prokop et al.
These techniques involve treating polluted substances at the site of pollution. It does not require any excavation; therefore, it is accompanied by little or no disturbance to soil structure. Ideally, these techniques ought to be less expensive compared to ex situ bioremediation techniques, due to no extra cost required for excavation processes; nonetheless, cost of design and on-site installation of some sophisticated equipment to improve microbial activities during bioremediation is of major concern.
Some in situ bioremediation techniques might be enhanced bioventing, biosparging and phytoremediation , while others might proceed without any form of enhancement intrinsic bioremediation or natural attenuation. In situ bioremediation techniques have been successfully used to treat chlorinated solvents, dyes, heavy metals, and hydrocarbons polluted sites Folch et al.
Notably, the status of electron acceptor, moisture content, nutrient availability, pH and temperature are amongst the important environmental conditions that need to be suitable for a successful in situ bioremediation to be achieved Philp and Atlas Unlike ex situ bioremediation techniques, soil porosity strongly influences the application of in situ bioremediation to any polluted site. This technique involve controlled stimulation of airflow by delivering oxygen to unsaturated vadose zone in order to increase bioremediation, by increasing activities of indigenous microbes.
In bioventing, amendments are made by adding nutrients and moisture to enhance bioremediation with the ultimate goal being to achieve microbial transformation of pollutants to a harmless state Philp and Atlas A study by Sui and Li modelled the effect of air injection rate on volatilization, biodegradation and biotransformation of toluene-contaminated site by bioventing. It was observed that at two different air injection rates However, at the earlier stage of the study day , it was observed that high air injection rate resulted in enhanced toluene removal by volatilization compared to low air injection rate.
In other words, high airflow rate does not bring about increase in biodegradation rate nor make pollutant biotransformation more effective. This is due to early saturation of air by high or low air injection rate in the subsurface for oxygen demand during biodegradation.
Nonetheless, low air injection rate resulted in a significant increase in biodegradation. It thus demonstrates that in bioventing, air injection rate is among the basic parameters for pollutant dispersal, redistribution and surface loss. Similarly, Frutos et al. Interestingly, Rayner et al.
It becomes apparent that though airflow rates and air intervals are amongst the basic parameters of bioventing, the success of bioventing-based bioremediation relies on the number of air injection points, which helps to achieve uniform distribution of air. Despite the fact that bioventing design is to encourage aeration in unsaturated zone, it can be used for anaerobic bioremediation process especially in treating vadose zone polluted with chlorinated compounds, which are recalcitrant under aerobic conditions.
In this latter process, in lieu of air or pure oxygen, mixture of nitrogen together with low concentrations of carbon dioxide and hydrogen can also be injected to bring about reduction of chlorinated vapour, with hydrogen acting as electron donor Mihopoulos et al.
In a soil with low-permeability, injection of pure oxygen might lead to higher oxygen concentration compared to air injection. Furthermore, ozonation might be useful for partial oxidation of recalcitrant compounds in order to accelerate biodegradation Philp and Atlas Although both techniques use identical hardware, the configuration, philosophical design and operation differ significantly Diele et al. SVE may be regarded as physical method of remediation due to its mechanism of pollutant removal, however, the mechanism involved in pollutant removal for both techniques are not mutually exclusive.
During on-site field trials, achieving similar results obtained during laboratory studies is not always attainable due to other environmental factors and different characteristics of the unsaturated zone to which air is injected; as a result, with bioventing, treatment time may be prolonged.
Apparently, high airflow rate leads to transfer of volatile organic compounds to the soil vapour phase, which requires off-gas treatment of the resulting gases prior to release into the atmosphere Burgess et al.
This technique combines vacuum-enhanced pumping, soil vapour extraction and bioventing to achieve soil and groundwater remediation by indirect provision of oxygen and stimulation of contaminant biodegradation Gidarakos and Aivalioti The technique is designed for free products recovery such as light non-aqueous phase liquids LNAPLs , thus remediating capillary, unsaturated and saturated zones.
It can also be used to remediate soils contaminated with volatile and semi-volatile organic compounds. The pumping mechanism brings about upward movement of LNAPLs to the surface, where it becomes separated from water and air.
Following complete free products removal, the system can easily be made to operate as a conventional bioventing system to complete remediation process Kim et al. In this technique, excessive soil moisture limits air permeability and decreases oxygen transfer rate, in turn reducing microbial activities. Although the technique is not suitable for remediating soil with low permeability, it saves cost due to less amount of groundwater resulting from the operation thus minimizes storage, treatment and disposal costs Philp and Atlas Establishing a vacuum on a deep high permeable site and fluctuating water table, which could create saturated soil lenses that are difficult to aerate are amongst the major concerns of this particular in situ technique.
This technique is very similar to bioventing in that air is injected into soil subsurface to stimulate microbial activities in order to promote pollutant removal from polluted sites. However, unlike bioventing, air is injected at the saturated zone, which can cause upward movement of volatile organic compounds to the unsaturated zone to promote biodegradation. The effectiveness of biosparging depends on two major factors namely: soil permeability, which determines pollutant bioavailability to microorganisms, and pollutant biodegradability Philp and Atlas As with bioventing and soil vapour extraction SVE , biosparing is similar in operation with a closely related technique known as in situ air sparging IAS , which relies on high airflow rates to achieve pollutant volatilization, whereas biosparging promotes biodegradation.
Similarly, both mechanisms of pollutant removal are not mutually exclusive for both techniques. Biosparging has been widely used in treating aquifers contaminated with petroleum products, especially diesel and kerosene.
Kao et al. The major limitation however, is predicting the direction of airflow. This technique relies on the use of plant interactions physical, biochemical, biological, chemical and microbiological in polluted sites to mitigate the toxic effects of pollutants. Depending on pollutant type elemental or organic , there are several mechanisms accumulation or extraction, degradation, filtration, stabilization and volatilization involved in phytoremediation.
Elemental pollutants toxic heavy metals and radionuclides are mostly removed by extraction, transformation and sequesteration. On the other hand, organic pollutants hydrocarbons and chlorinated compounds are predominantly removed by degradation, rhizoremediation, stabilization and volatilization, with mineralization being possible when some plants such as willow and alfalfa are used Meagher ; Kuiper et al.
Some important factors to consider when choosing a plant as a phytoremediator include: root system, which may be fibrous or tap depending on the depth of pollutant, above ground biomass, which should not be available for animal consumption, toxicity of pollutant to plant, plant survival and its adaptability to prevailing environmental conditions, plant growth rate, site monitoring and above all, time required to achieve the desired level of cleanliness.
In addition, the plant should be resistant to diseases and pests Lee It has been reported Miguel et al. Further, translocation and accumulation depend on transpiration, and partitioning between xylem sap and adjacent tissues, respectively.
Nonetheless, the process is likely to differ, depending on other factors such as nature of contaminant and plant type. It is plausible that most plants growing in any polluted site are good phytoremediators. Therefore, the success of any phytoremediation approach primarily depends on optimizing the remediation potentials of native plants growing in polluted sites either by bioaugmentation with endogenous or exogenous plant rhizobacteria, or by biostimulation.
It was reported that the use of plant growth-promoting rhizobacteria PGPR might play an important role in phytoremediation, as PGPR tends to enhance biomass production and tolerance of plants to heavy metals and other unfavourable soil edaphic conditions Yancheshmeh et al.
In addition, Grobelak et al. Similarly, during phytoremediation of metal-contaminated estuaries with Spartina maritima , bioaugmentation with endogenous rhizobacteria resulted in increased plant subsurface biomass, metal accumulation and enhanced metal removal Mesa et al.
Addition of biosurfactant produced by Serratia marcescens to gasoline-contaminated soil to which Ludwigia octovalvis were planted, resulted in On the contrary, Maqbool et al. This was ascribed to the long fibrous root of the plant, which aided in proliferation of rhizobacteria and increased interaction with the contaminant, resulting in unfavourable competition in the rhizosphere of inoculated plant. Different plant species have been reported to have innate ability to remove organic and elemental pollutants from polluted sites Table 2.
Brachiaria mutica and Zea mays have also been reported as potential phytoremediators of heavy metal-contaminated sites Ijaz et al. Other plants with phytoremediation potentials have been extensively described Kuiper et al. One of the major advantages of using plants to remediate polluted site is that some precious metals can bioaccumulate in some plants and recovered after remediation, a process known as phytomining.
A study by Wu et al. Other advantages of phytoremediation include: low cost, environmentally friendly, large-scale operation, low installation and maintenance cost, conservation of soil structure, prevention of erosion and leaching of metal Van Aken ; Ali et al.
Moreover, following phytoremediation, there might be improved soil fertility due to input of organic matter Mench et al. However, longer remediation time, pollutant concentration, toxicity and bioavailability to plant, depth of plant roots and plant slow growth rate are likely to limit the application of phytoremediation Kuiper et al.
In some cases, harvesting of plant for biomass management following remediation might incur additional cost Wang et al. Besides, there is a possibility that accumulated toxic contaminants may be transferred along food chain.
Plants by their nature are autotrophic unable to use organic compounds as sources of carbon and energy , therefore lack catabolic enzymes needed to fully mineralize organic pollutants to carbon dioxide and water; this presents another pitfall for phytoremediation Lee Recombinant deoxyribonucleic acid DNA technology has been used to regulate the expression of some plant specific genes in order to increase metabolism and tolerance to heavy metals Dowling and Doty Composting of contaminated soil before planting resulted in enhanced TPH degradation, which in turn favoured rhizodegradation by Suaeda glauca Wang et al.
Recently, Thijs et al. Four major strategies plant selection in function of microbiome, root exudate interference, disturbance, and feeding of the supply lines were identified as the strategies to adopt to ensure that in polluted sites, opportunistic and pathogenic microbial populations are kept in check, to enable improved phytoremediation processes by degradative and PGP microbes.
Further, it was suggested that plant-microbiome interaction might not always be optimal for phytoremediation; therefore, human interventions are required to optimize such interaction for enhance contaminant removal. This technique is mostly perceived as a physical method for remediating contaminated groundwater, due to its design and mechanism of pollutant removal.
Nevertheless, researchers Thiruvenkatachari et al. Although alternative terms such as biological PRB, passive bioreactive barrier, bio-enhanced PRB have been proposed to accommodate the bioremediation or biotechnology aspect of the technique, the role of microorganisms have been reported to be mostly enhancement rather than an independent biotechnology Philp and Atlas In this section, PRB will be used to describe all variants of this technique including the permeable reactive barrier itself unless otherwise stated.
In general, PRB is an in situ technique used for remediating groundwater polluted with different types of pollutants including heavy metals and chlorinated compounds Table 3. As polluted water flows through the barrier under its natural gradient, pollutants become trapped and undergo series of reactions resulting in clean water in the flow through Thiruvenkatachari et al.
Ideally, the barriers are usually reactive enough to trap pollutants, permeable to allow the flow of water but not pollutants, passive with little energy input, inexpensive, readily available and accessible De Pourcq et al. The effectiveness of this technique depends mostly on the type of media used, which is influenced by pollutant type, biogeochemical and hydrogeological conditions, environmental and health influence, mechanical stability, and cost Obiri-Nyarko et al.
Similarly, Mena et al. Apparently, these combined techniques allowed polluted soil to maintain appropriate environmental conditions pH, temperature, nutrients needed for microbial growth, and resulted in surfactant biomass distribution across such polluted soil.
During performance evaluation of PBR for remediation of dissolved chlorinated solvents in groundwater, formation of carbonate precipitate in the iron zone was found not to be the major limitation to the observed performance; rather, accurate measurement of groundwater velocity through a PRB was implicated Vogan et al. Although maintaining barrier reactivity is vital for performance of PRB technique, preserving the barrier permeability is crucial for PRB success and can be achieved by maintaining appropriate particle size distribution Mumford et al.
Decrease in long-term performance due to reduction in reactivity of the barrier, zero-valent iron ZVI , loss of porosity and inability to apply the technique to site contaminated with some chlorinated hydrocarbons and recalcitrant compounds are amongst the major operational challenges associated with PRB technique.
Nevertheless, it was reported that polyhydroxybutyrate PHB , a biodegradable polymer, has a slow-release nutrient carbon capability, which promoted biological activity when used as a barrier, resulting in enhanced removal of chlorinated compounds Baric et al. Variations in climatic conditions, which can cause difficult hydrogeological site characterization, together with design flaws can result in reduced efficiency of PRB Henderson and Demond Therefore, cost-effective advanced site characterization methods and improved PRB designs will in turn increase the effectiveness of the technique Gibert et al.
Furthermore, the use of iron sulphide FeS barrier would help overcome some of the challenges loss of permeability under certain geological conditions associated with the use of ZVI Henderson and Demond In addition, model significant uncertainties are likely to affect the extrapolation of PBR performance based on laboratory-scale column experiments; these uncertainties can be reduced by independent experiments and field observation geared towards better understanding of surface deactivation mechanism in iron PRBs Carniato et al.
Other designs, reactive media, advantages, limitations and contaminants removed by PRB technique have been extensively described Thiruvenkatachari et al. Intrinsic bioremediation also known as natural attenuation is an in situ bioremediation technique, which involves passive remediation of polluted sites, without any external force human intervention. The process relies on both microbial aerobic and anaerobic processes to biodegrade polluting substances including those that are recalcitrant.
The absence of external force implies that the technique is less expensive compared to other in situ techniques. Nevertheless, the process must be monitored in order to establish that bioremediation is ongoing and sustainable, hence the term, monitored natural attenuation MNA.
Further, MNA is often used to represent a more holistic approach to intrinsic bioremediation. According to the United States National Research Council US NRC , there are three criteria that must be met in intrinsic bioremediation and these include: demonstration of contaminants loss from contaminated sites, demonstration based on laboratory analyses that microorganisms isolated from contaminated sites have the innate potentials to biodegrade or transform contaminants present at contaminated site from which they were isolated and evidence of realization of biodegradation potentials in the field Philp and Atlas It was further reported that during monitoring of intrinsic bioremediation of chronically polluted marine coastal environment, the most polluted sediments tended to have higher total bacterial diversity, abundance and culturable hydrocarbon degraders and contributed to natural attenuation of such site; therefore, suggesting that bacterial communities could be used as sensitive indicators of contamination in marine sediment Catania et al.
With respect to chlorinated compounds, Adetutu et al. MNA is widely gaining acceptance in most European countries with exception of very few, due to cold climate condition that is likely to exert negative effect on biodegradation process Declercq et al. Furthermore, biodegradation has been implicated as the main mechanism of pollutant removal during intrinsic bioremediation MNA. One of the major limitations of intrinsic bioremediation is that it might take a longer time to achieve the target level of pollutant concentration, given that no external force is incorporated to expedite the remediation process.
It thus follows that prior to application of intrinsic bioremediation, risk assessment needs to be carried out to ensure that remediation time is less than the time stipulated for pollutant to reach exposure point relative to the closest human and animal populations.
It is clear from the foregoing that bioremediation techniques are diverse and have proven effective in restoring sites polluted with different types of pollutants.
Microorganisms play crucial role in bioremediation; therefore, their diversity, abundance and community structure in polluted environments provide insight into the fate of any bioremediation technique provided other environmental factors, which can impede microbial activities are maintained at the optimal range.
Nutrient limitation, low population or absence of microbes with degradative capabilities, and pollutant bioavailability are among the major pitfalls, which may hinder the success of bioremediation. Since bioremediation depends on microbial process, there are two major approaches to speed up microbial activities in polluted sites, namely: biostimulation and bioaugmentation.
Biostimulation involves the addition of nutrients or substrates to a polluted sample in order to stimulate the activities of autochthonous microbes. As microorganisms are ubiquitous, it is apparent that pollutant degraders are naturally present in polluted sites, their numbers and metabolic activities may increase or decrease in response to pollutant concentration; hence, the use of agro-industrial wastes with appropriate nutrient composition especially nitrogen, phosphorus and potassium, will help solve the challenge of nutrient limitation in most polluted sites.
Nonetheless, it was reported that excessive addition of stimulant resulted in suppressed microbial metabolic activity and diversity Wang et al. On the other hand, bioaugmentation is a critical approach aimed at introducing or increasing microbial population with degradative capabilities. Microbial consortium has been reported to degrade pollutants more efficiently than pure isolates Silva-Castro et al. This is due to metabolic diversities of individual isolates, which might originate from their isolation source, adaptation process, or as a result of pollutant composition, and will bring about synergistic effects, which may lead to complete and rapid degradation of pollutants when such isolates are mixed together Bhattacharya et al.
Microorganisms are well known for their ability to breakdown a huge range of organic compounds and absorb inorganic substances. Bioremediation has been successfully used to clean up pollutants including crude oil, petrochemicals, pharmaceuticals, pesticides, sewage and chlorinated solvents used in cleaning supplies. They are hazardous because of potential toxicity, carcinogenicity and mutagenicity. The cost of bioremediation is less than other clean-up methods.
Key words: Bioremediation, Microorganisms, Pollutant. Introduction Due to the industrial revolution hazardous compounds are released into the environment. Bioremediation refers to the process of using microorganism to remove the environmental pollutant i.
The removal of organic waste by microbes for environmental cleanup is the essence of bioremediation. Biotechnological treatment is carried out at lower temperature and pressure which requires less energy than the conventional physico-chemicaltreatment technology like incineration, adsorption or extraction. Conventional techniques like air sparging, air stripping, adsorption and vapour phase extraction are used for the removal of benzene Atagana But these methods are expensive and cannot lead complete decomposition of contaminants.
Bioremediation to be an economicaland environmentally sound approach. Other terms are used for bioremediation are biotreatment, bioreclamation and biorestoration. Xenobiotics term is also involved in biodegradation or bioremediation.
Xenobiotic means unnatural, foreign and synthetic chemicals such as pesticides, herbicides, refrigerant and other organic compounds Jain Microbial degradation is an effective and economical means of disposing of toxic chemicals, specifically environmental pollutants.
Types of Bioremediation The most important aspects of environmental biotechnology is the effective management of toxic and hazardous pollutant.
The environmental clean-up by bioremediation can be achieved in two ways:in situ and ex situ bioremediation. In situ bioremediation: In situ Bioremediation is direct approach of biological treatment to clean up hazardous compounds pollutant present at the site. Addition of optimum quantity ofessential nutrients nitrogen, phosphorus etc. When microorganisms are exposed to pollutant,they develop metabolic ability to degrade them.
This method is effective for cleanup of oil spillages, beaches etc Perelo There are two types ofin situbioremediation: Intrinsic and engineered bioremediation.
Intrinsic in situ bioremediation:occurring microbes to degrade pollutants without taking any engineering steps to enhance the process.
Engineered in situbioremediation:Innate capability of microbes is generally slow and limited. By using suitable physico-chemical means good nutrient and oxygen supply, addition of electron acceptors, optimum temperature, the bioremediation process can be engineered for more efficient degradation of pollutants.
There are two techniques of engineered or enhancedbioremediationbiostimulation and bioaugmentation. Biostimulation:Biostimulationinvolves the modification of the environment to stimulate existing bacteria capable of bioremediation.
This can be done by addition of various forms of rate limiting nutrients and electron acceptor, such as phosphorus, nitrogen,oxygen or carbon.
Supplementation with co-substrates e. And addition of surfactants to disperse the hydrophobic compounds in water. In all these situations, the result is that there occurs biostimulation by effective bioremediation of polluted soil or waste land Godleads There are four techniques of biostimulation: Bioventing: bioventing is a process of stimulating the natural in situbiodregradation of contaminants in soil by providing air or oxygen to existing soil microorganisms.
Bioventing uses low air flow rates to provide only enough oxygen to sustain microbial activity in the vadose zone. Phytoremediation:Phytoremediation is a bioremediation process that uses various types of plants to remove, transfer, stabilize, and destroy contaminants in the soil and groundwater. Although phytoremediation is a cheap and environmentally friendly clean-up process for the biodegradation of soil pollutants, it takes several years Oksana Air sparging: Also known as air stripping and in situ volatilization is used for the treatment of saturated soils and ground water contaminated by volatile organic compounds like petroleum hydrocarbons which is a wild spread problem for the ground water and soil health.
Bioaugmentation:Addition of specific microorganisms to the polluted soil called bioaugmentation. Pollutants are complex molecules and microorganism alone not capable to degrade some pollutant. Ex situ bioremediation:Ex-situ bioremediation describe a process where contaminated soil or water is removed from the environment by biological organism.
Ex-situ bioremediation can use bioreactors and added nutrients to speed up the breakdown of environmental pollutants. This process is better controlled but very costly. There are different techniques of ex situ bioremediation dig and dump, pump and treat, incineration, oxidation and adsorption.
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