# Bioremediation of Hydrocarbon-Polluted Soil: Evaluation of Different Operative Parameters
## 1 Introduction  
Industrial development has caused a dramatic increase in the amount of crude oil used. Consequently, the rapid expansion of crude oil industries has led to the generation of large volumes of hydrocarbons that need attention due to their environmental and health impacts [1]. Pollution by hydrocarbons is associated with different industrial activities which may harm the environment and human health [2]. It is generally accepted that this kind of pollution has an anthropogenic origin, often due to accidental spill from production units and transport pipelines, leakages from storage facilities and underground tanks, mining, human activities in the production, transportation, and improper or illegal behaviours in waste treatment and disposal [3; 4; 5]. Despite several regulations and great care for operational safety, the possibility of the uncontrolled release of petroleum products still exists [6]. Indeed, there is growing public concern about the large volume of hydrocarbons that can be inadvertently or deliberately emitted into the environment [5; 7; 8]. These contaminants can be accumulated in soil due to their low degradation rates, affecting the physical, physiological, and biochemical properties of this valuable resource [9; 10; 11]. Furthermore, hydrocarbon pollution poses serious risks to human health [12; 13; 14; 15], due to the toxic effects (e.g., mutagenicity and carcinogenicity) and the capability to move through thefood chain [16; 17]. Hence, special attention is required not only to remediate this pollution and prevent their threats to humans and ecosystems but also to recover the polluted areas and give them back to local communities for safe use [18; 19; 20]. In soil remediation, the main target is the removal of pollutants to the maximum extent, ensuring that the final concentration will not be higher than the acceptability threshold [11]. Since most of the hydrocarbons are biodegradable, bioremediation processes have attracted widespread interest to clean up contaminated areas degrading hydrocarbons into less toxic forms [11; 21; 22]. In this sense, numerous studies have demonstrated that bioremediation can constitute a highly attractive, pioneering, and environmentally friendly technology, based on the use of microorganisms to degrade specific contaminants [11; 20; 23]. In addition, bioremediation needs simple equipment and low-cost techniques, compared with other chemical and physical methods [24; 25]. Bioremediation is an emerging and sustainable technique that can either occur naturally (natural attenuation) or be enhanced by introducing nutrients (biostimulation) or microorganisms (bioaugmentation) [26; 27]. Biostimulation has proven to be one of the best solutions in terms of easiness and management compared to the achieved results [10; 28]. It consists of the improvement of environmental conditions from a nutritional point of view. In addition, biostimulation presents a noticeable advantage in comparison with other alternatives, because the microorganisms are already adapted to the site conditions [27]. Although different approaches have been developed for the enhancement of microbial growth to increase the hydrocarbon remotion [27], the methods used to evaluate the influence of different operative conditions are not yet standardized. Previous studies have shown how mathematical models can be developed from experimental data [29; 30]. Mathematical techniques especially used in quality control [31; 32] can also be applied successfully to bioremediation processes, setting up the hypotheses and requirements as a statistical experimental array. Various authors agree with the utility of experimental design to evaluate the biodegradation process [5; 11]. This approach constitutes a useful structured method applicable to any of the stages of the process research and development.