## 4 Bioremediation of PAH-Contaminated Soil by Composting
### Effect of Organic Co-Substrate Stability  
Even though various organic co-substrates/amendments have demonstrated their viability in the composting of PAH-contaminated soils, composition of these materials varies significantly in the sources and stages of decomposition [25; 59; 78; 95; 96], which as a consequence influences the removal rate in different ways [59; 78; 97; 59]. The selected organic co-substrates for the bioremediation process should contribute in improving and overcoming any deficiencies or limitations that influence the process performance and efficiency. Accordingly, selection of the most suitable organic co-substrates represents as a major challenge in such studies [59; 95]. In the bioremediation of PAH-contaminated soils, organic matter stability is of particular importance, as this parameter is directly correlated to the organic substrates' composition and biological activity [98]. Various studies have pointed out that the fate of PAHs is dependent on the quality and nature of the amended organic matter [25; 69; 78; 93]. Bioremediation of PAH-contaminated soils with more stable compost has proved to be more effective than with less stable or fresh organic amendments [25; 78; 97]. In this context, the preference of these substrates related to the presence of humic substances was found to form a considerable part of stable compost and was proportional with its degree of stability [25; 78]. This humic matter was documented to enhance the organic compounds' bioavailability [78; 97]. During the composting process, organic co-substrates provided nutrients for microorganisms [99]; meanwhile, humic matter evolution is expected to facilitate the microbial accessibility to PAHs. This behavior is established as a result of decreasing humic matter binding affinity and increasing of the heterogeneity of binding sites, closer to soil humic matter, which is conducive to microbial accessibility to PAHs [97; 100]. Additionally, stable compost contains low biodegradable organic matter content and a higher concentration of organic macromolecules [101], which are believed to enhance the biodegradation of the contaminant. The presence of easily degradable organic matter is assumed to reduce the process efficiency, as microbial cultures prefer to use easily degradable organic matter and thus decrease or retard utilization of the contaminant. Another important point in this item is represented by effect of the potential working surface area. In this regard, less degraded organic matter generally has coarse fractions (>5 mm), whereas most humified organic matter is generally presented in fine fraction [102]. The finest compost size fraction (<3 mm) with a higher surface area ratio provides more accessibility to microorganisms and releases more nutrients compared to coarse compost fraction [101; 103]. During the bioremediation process, an increase in the content of humic matter from 0.23% to 0.70% was observed, and these changes resulted from the structural changes that occurred in the material composition. Potentiometric titrations of humic acid solution showed increases in the buffering and redox capacities of humic acids [104]. [PERSON] et al. [97] reported that the composting process caused a structural conversion of humic acids from an organic substrate by reducing the aliphatic fraction and increasing the polarity and aromatic polycondensation in a PAH-contaminated soil. This conversion decreased the PAH binding affinity of humic acids, and thus improved PAH-degrading microbial accessibility. Similar results were observed in other studies, which supports the application of stable organic co-substrates [24; 66; 73; 78; 105; 106]. It is worth mentioning that when the same ratio of inorganic fertilizer (Nitrogen (N).Phosphorus (P).Potassium (K)) was compared with organic compost on the bioremediation of diesel-polluted agricultural soil over a two-month period, the results revealed that total petroleum hydrocarbon removal from polluted soil was 71.40 \(\pm\) 5.60% and 93.31 \(\pm\) 3.60% for N.P.K.