## 4 Bioremediation of PAH-Contaminated Soil by Composting
### Effect of the Mixing Ratio  
The suitability of different substrates based on their physiochemical properties is recognized as an important factor in the composting of PAH-contaminated soils. Determining the appropriate quantity to be added to the mixture is also of great importance, since an inappropriate ratio may hamper or inhibit microbial activity and bioavailability [78; 93]. It has been determined that even though microbial metabolism may be temporarily increased using a certain mixing ratio, the long-term inhibition of functionally important organisms may result in the failure of the bioremediation of high-molecular-weight PAHs [78]. The amount of various nutrients, and the ratio of nutrients like C, N, and P in particular, are quite conceivable as being involved in the success of the bioremediation process. Furthermore, determining the minimum quantity of the amendment that could support and maintain the desired activity with a high degradation rate is directly related to process economics [78].  
As reported in the study conducted by [PERSON] et al. [109], a microorganism's selection of nutrients could delay the degradation of pollutants, as normally microorganisms prefer easily degradable materials over resistant ones. This study revealed that that treatments with amendment ratios of 1:1 and 2:1 had average TPH removal rates of 30.7% and 33.3%, respectively, but the amendment ratio of 3:1 had a slower net degradation rate of between 11.6% and 26.8%. An excess of readily degradable carbon might overtake the TPH and act as substrate for the metabolism of microbial degraders. Therefore, the proper amount of amendments should be taken into account in composting to balance the motivating effect on microorganisms and the competing effect with pollutants [109]. Similarly, [PERSON] and [PERSON] [110] concluded that the compost additions combined with earthworms at a ratio of 1:0.5-1:1 (soil/compost, _w/w_) were efficient in enhancing the removal of extractable petroleum hydrocarbons and PAHs. However, when higher ratios of compost (1:2 and 1:4) were used, PAH losses were not advanced, which may indicate that the activity of earthworms were restricted by a higher addition of compost. [PERSON] et al. [66] showed that compost addition is an effective approach for enhancing PAHremoval from soils, especially for higher initial PAH concentrations, but increasing the ratio of compost added does not necessarily help to increase removal.  
Experiments with different ratios of contaminated soil to green waste from 0.6:1 to 0.9:1 have demonstrated that in general, PAH removal is significantly enhanced in reactors increased with green waste until a maximum mixing ratio of 0.7:1 [89]. The same observation was found in [PERSON] et al. [78]. These results imply that low mixing ratios were not sufficient to stimulate the microbial growth; on the other hand, excessive amounts could eventually inhibit the targeted contaminants, as microbial communities prefer to use more available and easily degradable nutrients. Furthermore, co-composting of sediments (S) polluted by PAHs with urban green waste (GW) was performed using two mixing ratios (1:1 and 3:1; S/GW). In the first six months of treatment, the PAH concentrations in the 1:1 and 3:1 ratio scenarios was reduced by 57% and 26%, respectively. Despite the fact that only two mixing ratios were tested, the results again demonstrate that the low mixing ratio (3:1) was not sufficient to enhance the degradation process [94]. When different corn straw ratios (1%, 2%, 4%, or 6% _w_/_w_) were investigated for the remediation of aged PAHs in soils, removal rates were significantly (_p_ < 0.05) enhanced under the 6% ratio, mainly for HMW PAHs. This indicates that the high amendment of corn straw was a potential option for the remediation of PAH-contaminated soils [111].