## 4 Bioremediation of PAH-Contaminated Soil by Composting
### Effect of Temperature  
Providing optimum temperature is an intrinsic factor for the successful biodegradation of PAH. The importance of this factor stems from its influence on the metabolic activity, bioavailability, solubility, and diffusion rate of the target contaminate [82]. It is noteworthy that the solubility of PAHs increases with temperature, which ultimately increases the bioavailability of the PAH molecules. However, increasing temperature is associated with decreasing oxygen solubility, which on turn reduces the metabolic activity of aerobic microorganisms. Furthermore, and to a certain extent, the specified temperature range will determine the types of dominant microorganisms and their enzymatic activity that will undertake the degradation [73]. The successive stages during the normal composting process (mesophilic phase, thermophilic phase and curing phase) are expected to be accompanied by specific populations of bacteria, and different effects on contaminants are found with different stages of compost product. The biodegradation of PAHs occurs over a wide temperature range, and microorganisms have found to be adapted to biodegrade PAHs at extreme temperature conditions. Under mesophilic and thermophilic temperatures ranges, it has been found that the enzymatic activity of microorganisms increases, which helps in increasing the rate of hydrocarbon degradation. However, it should be underlined that a great amount of research has been directed to focus on the process behavior under mesophilic conditions, as it is believed that a wide spectrum of microbial communities could be present in active roles under these temperatures, and thus reasonable degradation rates could be achieved [78, 83, 84, 85, 86]. During in-vessel composting of pyrene-contaminated soil, composting temperature affected the prevailing of some microbial groups over others, and the predominant bacterial community changed over time. The degradation of pyrene was dominated by \(\alpha\)-, \(\beta\)-, and \(\gamma\)-Proteobacteria, as well as _Actinobacteria_, at 38 \({}^{\circ}\)C during 14 days of composting, and then _Streptomyces_ at 55 \({}^{\circ}\)C. Later, at 70 \({}^{\circ}\)C and after 42 days of composting, _Actinobacter_ and _Thermobifida_ occupied leading position. Finally, _Thermobifida_ and _Streptomyces_ flourished after 60 days of composting at 38 \({}^{\circ}\)C [87]. Concerning the temperature effect, [PERSON] et al. [74] claimed that degradation rates of 89% and 59% for three-ring and four-ring PAHs, respectively, were achieved in reactors under mesophilic temperatures. In contrast, reactors displaying a thermophilic range ended with 71% and 41% removal for the same pollutants, respectively, during the bioremediation process. The addition of compost significantly promoted the removal of PAHs and alkanes up to 88% after 50 days of incubation under mesophilic temperatures (28 \({}^{\circ}\)C), compared to the natural biodegradation of hydrocarbons in soils without compost [67]. Additionally, the composting of PAH-contaminated soils under different conditions and different organic substrates were found to perform better under mesophilic conditions [23, 25, 78, 88, 89], LMW-PAH concentrations, such as naphthalene, acenaphthylene, acenaphthene, fluorene, anthracene, and phenanthrene, were decreased by an average of 89% at 38 \({}^{\circ}\)C, which is twice that compared to the concentration reduction at 55 \({}^{\circ}\)C, which was an average of 45%. Simultaneously, no big difference was observed concerning HMW PAHs, including fluoranthene, pyrene, benzo[a]anthracene, and chrysene, where the removal rate was by an average of 67% at 38 \({}^{\circ}\)C, compared to 69% at 55 \({}^{\circ}\)C. Nevertheless, a high temperature was considered adverse to microbial activity, and volatilization was the leading mechanism of PAH removal [88].