Engr. Okoh Elechi1, Dr. Zekeni Robert Yelebe2, Engr. Bernard Oruabena3
1Department of Chemical Engineering, Niger Delta University, Amassoma, Nigeria
Email: [email protected] of Chemical Engineering, Niger Delta University, Amassoma, Nigeria
Email: [email protected] of Civil Engineering, Federal Polytechnic, Ekowe, Nigeria
Email: [email protected] — the emergence of crude oil products as a potent primary source of energy for homes and industries has elicited profound negative impacts on the environment as a harbinger of soil, water and air pollution. The impacts cannot be overlooked and therefore these ubiquitous environmental contaminants need to be removed from the environment after a spill as they are potent immunotoxicants and carcinogenic. Prolonged exposure to contaminated environment might result in an increased risk of cancer, kidney and liver damage.

Whether crude oil and its products enter the environment by accident or by negligence the effect on human and plant health has a serious health implication that need to be curtailed. The application of bioremediation in the removal of crude oil contaminants from the environment has gained worldwide acceptance as the process is deemed efficient, is low cost and requires little or no technical knowledge to operate and has no overall negative consequence on the environment. But the effectiveness of the bioremediation system is constrained by the properties of the contaminants, the soil matrix as well as the environmental factors. There is need to use technology to reduce the treatment time of remediation by accelerating the rate of degradation through the optimization of those parameters that affects the process negatively. This article provides a review of the process of crude oil contaminated land remediation employing a biostimulation strategy to accelerate the process, highlighting, and optimizing those environmental factors that affects the bioremediation process.

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Keywords — Biostimulation; Crude oil; Nutrients; Remediation; Response Surface Methodology; Soil Matrix.

IntroductionCrude oil derived products are a major source of energy for homes and industries and their entrance into the environments either by accident or sheer negligence comes with adverse effect on humans, plants and animal health as the contaminants are carcinogenic CITATION Mar01 l 1033 1. The complex nature of crude oil is exemplified by it being a non-homogenous mixture of aliphatic, branched and aromatic hydrocarbons, including the volatile components of gasoline, petrol, kerosene, lubricating oil, and solid asphaltene residues CITATION SEA10 l 1033 m Ban032, 3.
The aromatic hydrocarbons are carcinogenic and highly toxic and they contribute in no small measure to environmental pollution. Different methods of removal of crude oil products from the environment exists but presently employing the biological treatment of bioremediation is the most potent, popular and cost effective strategy. Bioremediation is a technology based on the science of biodegradation which is environmentally acceptable, and cost effective method that exploits the various degradative abilities of microorganisms in the environment to convert organic contaminants to non-toxic products such CO2 and water by mineralization CITATION Aga18 l 1033 m Gid08 4, 5. But the effectiveness of the bioremediation system is constrained by the properties of the contaminants, the soil matrix as well as the environmental factors. To achieve optimum biodegradation there is need to optimize the soil parameters and reduce the treatment time of the bioremediation systems by accelerating the rate achieved through employing a wide variety of technologies.

The evaluation, selection and rate of remediation determination for the effective remediation of crude oil contaminated soils system will require a careful consideration of the contaminated sites and the soil characteristics CITATION Son06 l 1033 6 as soil matrix has a considerable influence on the removal of hydrocarbon from soil. It also involves a good selection of the biostimulation strategy and proper assessment of the influence of environmental parameters on the rate of degradation and the optimization of these factors for maximum petroleum hydrocarbon removal by employing an effective statistical approach. There are numerous crude oil contaminated sites all over the world with the Niger Delta oil producing communities in Nigeria harbouring a substantial number of these sites. Adequate knowledge on what constituted the operative factors in bioremediation process is necessary in making decision to assist the biodegradation efficacy.

Crude oil as a soil contaminantThe frequency and degree of soil contamination by crude oil and petroleum products is a pervasive problem that is universally felt and the consequences are extremely high CITATION Sam15 l 1033 7. The most extensive and the most devastating soil and groundwater contamination ever experienced is that caused by crude oil spills, described by Baiyu et al. as a potential threat to human, and ecosystem health CITATION Bai12 l 1033 8. The aromatic hydrocarbons, benzene, toluene, ethylbenzene, and xylene are compounds with one or more fused aromatic rings found in crude oil are of particular environmental concern because there are potential carcinogens or may be transformed into carcinogens by microbial metabolism when crude oil is spilled CITATION Mar06 l 1033 9.

When Crude oil enters into the soil environment, it alters the physical and chemical properties of the soil resulting to changes in soil characteristics. Crude oil in soil may also create nutrient deficiency, inhibit seed germination and cause restricted growth or demises of plants on contact CITATION Chi13 l 1033 10. Crude oil spilled on land prevents water absorption by the soil; and spills on grassland and agricultural land have the effect of choking off plant life. Spilled hydrocarbons could be held in voids in the soil forming a large bank of residual saturation which may lead to a long contamination of ground water if not removed CITATION Abd12 l 1033 11.. The best response priorities to spill in the environment is to prevent the oil from leaching into the ground water or entering waterways as runoff and to return the soil to productive use as quickly as possible.

Depending on the degree of contamination and remediation measures taken, the soils impacted by crude oil may remain unsuitable for crop growth for a very long time. The sustainability of the soil is of an immense interest and concern to us because of the direct reliance of our existence on it. It is therefore essential that soil quality, fertility and productivity should be continually maintained and monitored. Oil pollution associated with ground water is also a problem for such places as the Niger Delta that relies mostly on ground water resource for its water supply.

Fate of crude oil in the soilSeveral studies have examined the fate of hydrocarbon in the soil environment and other ecosystems CITATION Sch99 m Whi99 l 1033 12, 13. When crude oil enters the environment, it is subject to many physical, chemical and biological changes that contribute to its loss or alteration CITATION Xue01 l 1033 14. Portion of the crude oil mass will volatilize and some portions will solubilize as components of soil vapour and groundwater. The volatilization and solubilization tends to make the remaining mass of crude oil denser, less mobile and more difficult to biodegrade CITATION API96 l 1033 15. Such physical, chemical and biological changes also include biodegradation, evaporation, adsorption, penetration, migration and release and these weathering processes alter the properties of the crude oil in such a way as to affects the method of removal from the environment. According to Atlas RM, “microorganisms in the ecosystem have the ability to utilize the hydrocarbons as the sole source of carbon and energy and such microorganisms are widely distributed in nature” CITATION Ant06 l 1033 m Ron8116, 17.
The degree to which petroleum hydrocarbons are retained in the soil is determined by the properties of the liquid as well as the soil parameters CITATION Ant06 l 1033 16. The most important crude oil properties include the API gravity and viscosity. The soil environmental factors that limit the rate of hydrocarbon biodegradation include temperature, soil pH, moisture content, soil texture, sorption, bioavailability, salinity, contaminant concentration and the presence of microbial toxins CITATION Ron81 l 1033 m Cho10 m Lab0317, 18, 19. Other factors that affect biodegradation include chemical composition of the crude, volume of the released crude oil, its physical state, volatility, pH, Biochemical Oxygen Demand (BOD) etc. The behavior of crude oil pollutant in the environment also depends upon a variety of other processes and properties.

1. Chemical processes, e.g. hydrolysis, oxidation and reduction.

2. Physical or transport processes and properties, e.g. advection, dispersion and diffusion, volatilization.

3. Biological processes, e.g.: bioaccumulation, biotransformation, biodegradation, and toxicity and
4. Combined environmental factors.

Technologies for crude oil removal from the soilTechnologies commonly employed in soil remediation include mechanical, burying, evaporation, dispersion and washing CITATION Das111 l 1033 m Joh05 m Lea9020, 21, 22. These technologies have limited effectiveness CITATION Das112 l 1033 23 they rarely achieve complete cleanup of oil spills and generally, they are expensive and can lead to incomplete decomposition of contaminants CITATION Vid01 l 1033 24. They only transfer the contaminant from one environmental compartment to another CITATION Xue01 l 1033 14. Because of the limitations of the physiochemical methods, great deal of literature has reported that bioremediation technologies are alternative to the methodsCITATION Sey11 l 1033 25. Bioremediation is the most popular and cost – effective biotechnology strategy that is increasingly being studied and implemented among the other different methods to detoxify or degrade environmental pollutants from the soil to innocuous substances.

Bioremediation techniques are essentially destructive technique, easily implemented at low cost, effectively inexpensive CITATION Xue01 l 1033 14 and directed toward simulating the growth of microorganisms that uses the crude oil contaminants as food and energy source by creating favourable environments for the microorganisms to thrive. Biodegradation of crude oil contaminant by natural population of microorganisms in soil represent one of the primary mechanism by which crude oil can be eliminated from the soil environment and is governed by physico-chemical factors (e.g. nutrient, pH, temperature, moisture content) as well as the soil factors.
Leahy and Colwell have reviewed extensively the requirements for optimal microbial growth under a variety of circumstances CITATION Lea90 l 1033 22, and the degradation trails for crude oil have equally been detailed by so many researchers CITATION Das111 l 1033 m Xin1520, 26. Furthermore, the influence of soil parameters and crude oil physical interactions such as mass transport CITATION Eri35 l 1033 27 sorption and desorption CITATION Eve92 m JHu l 1033 28, 29 on the remediation rate is also well documented in literature too.

Bioremediation as a process of crude oil removal from the soil environmentBioremediation defined as a grouping of technologies that use bacteria or fungi to degrade or transform hazardous contaminants to materials such as carbon dioxide, water, inorganic salts, microbial biomass, and other byproducts that may be less hazardous than the parent materials CITATION Jim06 m PAg13 l 1033 30, 31. The bioremediation of crude oil contaminated soils is best defined within the context of biodegradation, a naturally occurring process. Biodegradation is a component of oil weathering and is a natural process whereby bacteria or other microorganisms alter and break down organic molecules into other substances, eventually producing fatty acids and carbon dioxide. Bioremediation is the acceleration of this process through the addition of external microbial populations CITATION Tim96 l 1033 m Agn1032, 33, through the stimulation of indigenous populations by nutrients addition CITATION JTD76 l 1033 34 or through manipulation of the contaminated media using techniques such as pH, moisture, aeration or temperature control CITATION Ron81 m Ric96 l 1033 17, 35. Examples of bioremediation technologies include biostimulation, bioaugumentation, bioventing, bioreactor, phytoremediation, composting etc.

Bioremediation will not always a win all solution; as it has its limits and is yet to be regarded as a mature technology and the range of contaminants on which it is effective is limited. Additionally, the time scales required to achieve a complete cleanup of contaminated soil is relatively long, and the residual contaminant levels achievable may not always be appropriate CITATION Shi12 l 1033 m Nye9336, 37.

Factors affecting the bioremediation of crude oil from the soilIn the evaluation of the potential of microorganisms to remove crude oil from a contaminated soil, the use of properly selected populations of microbes and the maintenance of environmental conditions that are conducive to their metabolism, is an important means of optimizing the biological treatment of crude oil polluted soils CITATION Placeholder1 l 1033 38. For microbes to grow, they require suitable set of environmental factors which includes the physical and chemical parameters such as the pH of the soil, temperature, soil moisture, soil nutrients and available water content CITATION Ron81 l 1033 m Xue01 m Mar00 17, 14, 39.
Okoh, 2006 reported that heavier crude oils are generally much more difficult to biodegrade than lighter ones CITATION Ant06 l 1033 16. Studies have also shown that several agro-technical methods including tilling and loosening, watering and the addition of organic materials (straw, compost, cow dung etc.) and mineral fertilizer could decrease the contamination level by facilitating the oxidation of the easily degradable petroleum components, as well as contributing to an increase in microbial activity CITATION Reg08 m Vas01 l 1033 40, 41. However, a number of limiting factors have been recognized to affect the biodegradation of petroleum hydrocarbons.

Some of these factors include microorganism type, nutrients availability, soil pH, temperature, moisture content of the soil, oxygen availability, other soil properties, and contaminant concentration CITATION Aga10 m Lab03 m Ron81 l 1033 42, 19, 17. So many researchers have concluded that the disappearance of crude oil from the soil could be accelerated by the addition of nutrients, such as nitrogen or phosphorus or both. The main requirements for a successful biodegradation process are therefore an energy source and a carbon source. The environmental parameters do not affect the rate of bioremediation separately but they interact and the rate of biodegradation usually responds to the most limiting factor CITATION Xue01 l 1033 14. In summary, (a) The intrinsic ability of the microflora at the site, (b) Characteristics of the crude oil (c) Availability of nutrient and other environmental factors as (a) Effect of temperature (b) Soil moisture content, (c) Soil pH and (d) Soil nutrient are the major factors affecting the degradation of crude oil in soil.

Effect of Soil Texture on crude oil remediation
The soil is classified into four main categories as coarse material, sand, clay and silt and contains moisture and air. The soil matrix has a big influence on the removal of petroleum hydrocarbon from a crude oil contaminated soil. The amount of pore spaces in the soil is dependent on the soil type and degree of compaction. Clay soils have a higher degree of pore spaces than sandy soil but may not allow the delivery of nutrients in an efficient manner because of the physical size of the pores CITATION Mic91 l 1033 43. Regularly large quantities of spilled oil could be held in voids in the soil forming a large bank of residual saturation which could lead to the continual contamination of groundwater if not removed on time, The soil type, the composition and inherent biodegradability of the crude oil pollutant, is the most important consideration when the suitability of a bioremediation approach is to be appraised.
One of the results of bench scale studies crude oil polluted environment remediation is biostimulation which involves the addition of rate-limiting nutrients to accelerate the biodegradation process. According to Venosa et al. the main purpose of bench-scale treatability studies is to determine the type, concentration, and frequency of addition of amendments needed for maximum stimulation in the field CITATION Ven96 l 1033 44.

Other studies on the biostimulation experiments have shown that addition of growth limiting nutrients, namely nitrogen and phosphorus, has enhanced the rate of oil biodegradation. However, the optimal nutrient types and concentrations vary widely depending on the oil properties and the environmental conditions CITATION Xue01 l 1033 14.

Another challenge associated with biostimulation in oil-contaminated coastal areas is maintaining optimal nutrient concentrations in contact with the oil.

Therefore, the effectiveness of biostimulation depends on the nutrient concentration in the crude oil polluted environment (Atlas and Bartha, 1992; Bragg et al., 1994). The concentration of nutrients in the soil environment should be maintained at a level high enough to facilitate bacterial growth as excessive concentration of nutrient can induce toxic response as well CITATION Xue01 l 1033 14.

Amendments Necessary for Effective Bioremediation Application
The effectiveness of bioremediation technology is highly affected by the types of soil, since important soil characteristic are related to the soil type, and the soil is the medium in which treatment will take place, so its parameters must have to be evaluated. As stated before the successful application of bioremediation technology to crude oil contaminated soil requires knowledge of the characteristics of the site as well as the parameters that affect the microbial biodegradation of pollutants CITATION Lab03 l 1033 19. However, the ability to optimize these soil parameters must be taken into consideration. There are several soil factors that have significant effects on the biodegradation of crude oil contaminants in soil and these factors include nutrients, water content, temperature, and soil pHCITATION Placeholder2 l 1033 45. Biodegradation of contaminants in the soil can therefore be enhanced by making these environmental factors optimum for the required reactions.
Nutrient Amendment
Nutrient addition has been proven as an effective strategy to enhance crude oil biodegradation. In the soil environment, the growth of petroleum-hydrocarbon-utilizing cells is limited if mineral nutrients, especially Nitrogen (N) and Phosphorus (P), are in short supply CITATION Ant061 l 1033 46 and in order to prevent the nutrient limitations and accelerate the degradation of crude oil in soil, the addition of urea phosphate, organic fertilizers and animal dungs, NPK fertilizer, ammonium and phosphate salts have been extensively studied by several researchers CITATION Jos90 l 1033 m Mar01 m SEA1047, 1, 2. Nutrient is added at concentrations that approaches a stoichiometric ratio of C: N: P of 100:5:1 based on the organic carbon content of the feed CITATION Eve92 l 1033 28 and the proper manipulation of the N : P ratio coupled with the proper loading and application strategies will result in the enrichment of the microbial population and increase remediation rate.

Soil pHThe pH of the soil has an effect on the biodegradability of hydrocarbons pollutants and is highly variable ranging from 2.5 in mine spoils to 11 in alkaline deserts. It affects the solubility and consequently the availability of many constituents of the soil and it is highly variable over a wide range. Most optimal microbial activity is enhanced by a pH close to neutrality, with fungi being more tolerant than bacteria in acidic condition CITATION Ron81 l 1033 17. The optimum pH for rapid decomposition of wastes and residues is usually in the range of 6.5 to 8.5 CITATION Eve92 l 1033 28. A soil pH of 7.8 should be close to the optimum CITATION JTD76 l 1033 34. Soil pH not only affects the growth of microorganisms, but also has a tremendous effect on the availability of nutrients, mobility of metals, rate of abiotic transformation of organic waste constituents, and the soil structure CITATION Sam16 l 1033 48. Soil pH can be adjusted by addition of chemical reagents. For acidic soils, agriculture lime may be used to raise the pH; aluminum sulfate or ferrous sulfate or sulfur may be used to lower the pH of alkaline soils.
Soil Moisture ContentSoil moisture content or the amount of water it can hold is an important variable that is assessed during soil characterization CITATION Ban03 l 1033 3. Soil moisture impacts the effectiveness of bioremediation technologies by movement of air. The maintenance of moisture content at ultimate level is very important in the study of the remediation of crude oil impacted soil. Water serves as a transport medium through which nutrients and organic constituents pass into the microbial cell and waste products pass out of the cell CITATION Pet94 l 1033 49. The activity of bacteria in remediation process is highest in the presence of moisture CITATION JRB4b l 1033 50.Too much water or too little water can be detrimental to an aerobic bioremediation operation CITATION Lui17 l 1033 51. Saturation of the soil with water will inhibit oxygen infiltration, and dry conditions will slow down the microbial activity or even stop the biodegradation process. A desirable range that will allow the bacteria to get air and water is between 70 to 80% of field water holding capacity. A soil is said to be at field capacity when soil micropores are filled with water which facilitates the diffusion of soluble substrate and macropores are filled with air which makes O2 diffusion easier CITATION Luo06 l 1033 52. The water holding capacity depends upon the nature of the soil. Table 1 provides a general soil moisture characteristic for two types of soils. And if the moisture content is maintained at optimum levels, studies by Fredrickson and Hicks shows that generally clay soil biodegradation rates are higher than sandy soil CITATION Fre87 l 1033 53.
Table SEQ Table * ARABIC 1: soil characteristics for effective bioremediation treatment
Soil type Water application rate Moisture holding capacity Permeability Field capacity (% by weight) Wilting point (% by weight)
Sandy 10-12 inches High Low 9-25 3-10
Clayey 8-9 inches Low High 38-43 25-28
Oxygen SupplyOxygen supply is severely restricted when the soil becomes saturated and oxygen is consumed faster than it can be replaced making the soil anaerobic CITATION Placeholder2 l 1033 45. Tilling may be applied as an effective means of aeration to accelerate the crude oil removal from a contaminated soil environment.

Methods Used in Monitoring Oil Bioremediation
The biodegradation rate of hydrocarbons in contaminated environments may be indirectly measured by respirometric techniques CITATION Son06 m Lui17 l 1033 m Eve926, 51, 28, where the carbon dioxide formed from microbial respiration is taken as a measure of the degree of removal of the crude oil pollutant in the soil. Carbon dioxide, measurements by respirometry, is a common method in bench-scale biodegradation studies CITATION Cyn l 1033 54. A respirometer with oxygen and carbon dioxide (CO2) sensors and biometer flasks is used to measure microbial respiration rates of soil samples using this technique.
Another primary measure of the success of bioremediation treatments is reduction in the concentrations of spilled oils and target oil constituents in particular. Since components of petroleum degrade at different rates, it is difficult and misleading to speak in terms of an overall biodegradation rate rather the method of TPH is best employed in the determination of crude oil contamination of soil resulting in a sum parameter that does not give the concentration of a specific compound. In so many instances evaluation using the nonspecific methods of oil analysis to measure the Total Petroleum Hydrocarbon (TPH) degradation rate has been used to establish the rate of biodegradation for crude oil polluted soils. Other techniques of oil analysis that employ the specific method have been developed and used in petroleum hydrocarbon analysis, which include gravimetric methods, infrared spectroscopy (IR), gas chromatography/flame ionization detection (GC/FID), gas chromatography-mass spectrometry (GC/MS), and thin-layer chromatography-flame ionization detection (TLC-FID) CITATION Xue01 l 1033 14. Environmental conditions, particularly nutrient concentrations, may also be monitored for evaluating the effects and rates of bioremediation.

Rate of remediation (Kinetics of crude oil biodegradation)For soils contaminated by crude oil in which the biodegradability of the oil is unknown, it is basic to undertake a laboratory investigation of the kinetics of biodegradation of that soil contaminantCITATION Placeholder2 l 1033 45 in order to determine the rate and duration of bioremediation treatment. The rate at which microorganisms can remove organic compounds from the soil can be expressed mathematically to approximate the time required for remediation. According to Yelebe et al. though much work have been done in bioremediation studies and practice, yet little is known about the kinetics of bioremediation and how it affects the performance of several available bioremediation options CITATION ZRY15 l 1033 55 including the effect of soil heterogeneity. The knowledge of the kinetics of oil biodegradation under different environmental conditions is important for assessing the potential fate of targeted compounds, evaluating the efficacy of bioremediation, and determining appropriate strategies necessary to enhance crude oil biodegradation CITATION Xue01 l 1033 14.
In the determination of the rate of degradation it is appropriate that the minimum set of factors or variables that need to be considered in assessing the rate of degradation of a compound be included in the determination of the rate equation.
Environmental factors such as temperature, pH, soil moisture content and nutrients concentration appear to be the key parameters that can be incorporated into most of the models to predict biodegradation rates of crude oil in the environment. There are so many rate studies of crude oil biodegradation rate conducted under laboratory conditions and very few kinetic studies under field conditions. Yelebe et al (2015), Oyoh and Osoka (2007), based on certain assumptions developed some models which they fitted to experimental data from NPK fertilizer enhanced bioremediation CITATION Oyo07 l 1033 m ZRY15 56, 55. The model considerations included scenarios where, (a) the microbial growth was exponential and yield constant, (b) microbial growth was exponential but yield not constant and (c) growth is logistic and yield is constant. Song et al. developed a base model based on CO2 accumulation data as:
Vct=?tb ……………………… (1)
Where Vc is the cumulative volume (?l) of CO2 produced through microbial respiration, t is the incubation time (hour), and a and b are coefficients that are functions of various physical and chemical parameters. And differentiating equation 1 to obtain the bioremediation rate at any point in time as CITATION Son06 l 1033 6:
The Exxon Valdez monitoring program developed a multiple regression model based on field studies conducted by researchers from Exxon CITATION Xue01 l 1033 14. The best-fitting model was expressed as:
Cht=?1-pt?e?rt+?t? ……. (3)
where Ch(t) is the time-varying hopane-normalized concentration of an analyte, p is the polar fraction of the oil, r is the ratio of the average residual nitrogen concentration to oil loading, and ? is the assumed multiplicative error term, while ?, ?, ?, and ? are fitting parameters determined from the multiple regression analysis.

Venosa et al. (1996) also developed from field data first-order biodegradation rate constants for resolvable alkanes and important two- and three-ring PAH groups present in light crude oil. This relationship was expressed as:
AH=AH0e-kt …………………………….(4)
Where (A/H) is the time-varying hopane-normalized concentration of an analyte, (A/H)0 is that quantity at time zero, and k is the first-order biodegradation rate constant for an analyte.

Statistical analysisNumerous authors such as CITATION Opt m Pra16 m Sam12 l 1033 m Ola15 m Kal1157, 58, 59, 60, 61 and so many others have proposed different mathematical models based on statistical analysis to predict the migration or fate of crude oil contaminant in soils; however most of these models fail to include the complete set of soil parameters nor consider the effect of soil heterogeneity that are closely associated with microbial activities. Most presume that biodegradation to be a simple first order process and propose equations to treat it as such. Aghamiri et al. employed a design of experiment (DOE) technique based on the Taguchi method to optimize bioremediation of crude oil in contaminated soils and obtained a set of optimum conditions for the 8 factors investigated and about 68% of crude oil removed after 20 days experimentationCITATION Sey11 l 1033 25.
Agarry, (2018) investigated the effects of inorganic and organic fertilizers and activated carbon on bioremediation of soil contaminated with weathered crude oil using the Box Behnken design (BBD) of Response Surface Methodology (RSM) at three levels and three factors of inorganic NPK fertilizer, Activated Carbon and Organic Fertilizer as variables.

The work concluded that biostimulation using inorganic fertilizer as well as activated carbon results in the significant enhancement of the degradation of crude oil in soil and the Response Surface Methodology as a reliable and powerful tool for modelling and optimization of the crude oil biodegradation process CITATION Aga18 l 1033 4.

Also, Golam Taki CITATION par l 1033 62 also employed the Box-Behnken design method of Response Surface Methodology to optimize some set of operating parameters in order to remove and recover crude oil from contaminated soil using subcritical water extraction process and attested to the usefulness of the Box-Behnken Design method.

None of this works considered the effects of soil factors and soil heterogeneity, which are known to play an important role in the bioremediation of crude oil contaminated soil CITATION Son06 l 1033 6.
According to works of Sturman P et al and Song Jin et al. results and correlations developed in a laboratory setting do not always accurately represent what will happen in the field, as there are variations caused by soil heterogeneity, soil pH, soil moisture content, temperature and aging of contaminants CITATION Stu95 l 1033 m Son0663, 6. Holcomb and Pigott concluded that in all remedial scenarios, corrective action plans using bioremediation should be evaluated on a site-by-site basis because microbial action is controlled by site conditions CITATION Eri17 l 1033 64. There is therefore this need to relate the environmental conditions and soil types as they affects the bioremediation of petroleum hydrocarbon contaminants in soils. The optimization of the soil conditions involved in the kinetics of the process such as aeration, pH, addition of nutrients, soil moisture, and temperature control CITATION Mar00 l 1033 m Box139, 65 and the accurate measurement of biodegradation rates is necessary for successful, cost effective application of bioremediation.

ConclusionSignificant evolution in bioremediation practices, a multi-disciplinary perspective based on integrated scientific principle has been witnessed over the years focusing primarily on the impact of hazardous chemicals on the environment and human health. Bioremediation of petroleum, contaminated soils is a viable and cost-effective technology and the ease of application on crude oil contaminated soils make it more efficient than many other conventional methods and result in considerable savings in costs.
Although the technology is simple and economical, easily deployed but attempts to integrate information on rates from laboratory studies with field scale practices are hampered by tremendous diversity in measurement techniques, soils and environmental conditions, as well as, the quality and quantity of the crude oil product in the soil environment.
Applying the various methodologies elucidated to increase the rate or extent of the biodegradation process, its success and efficiency in the reduction of crude oil contaminants in soil depends on the ability to optimize the various physical, chemical, and biological conditions in the contaminated environment.

It is also by far the most elegant and pure cleanup solution, resulting in the transformation of toxic compounds to carbon dioxide and water.
The use of engineering modeling techniques will help the remediation engineering communities to understand the microbial community dynamics structure and assists in providing the needed insight into details of bioremediation which will be facilitated to make the bioremediation technology safer, reliable and help to transform bioremediation from a mere practice into a science.

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