The fate of total petroleum hydrocarbons during oily sludge composting: a critical review

In this review, studies on petroleum sludge composting have been investigated in the aim to understand the sludge biotransformation and highlighting the points that need more attention. Total petroleum hydrocarbons (TPH) are the major contaminants present in the petroleum sludge they ranged from 0.204 to 521.12 g/kg of dry matter, indicating a potential risk to the environment if not correctly managed. In this case, composting of petroleum sludge with different bulking agents (soil, wood, green wastes and manure) have been used as an environmentally friendly treatment. The TPH removals ranged between 47 and 95% during the composting process. This difference between the TPH removals is due to divergence in TPH concentration, chemical characterization, mixing ratio and applying or not of ameliorated treatment. The enhanced treatment like the bioaugmentation and the biostimulation increased significantly the TPH degradation kinetic which ranged between 0.8 and 2%/day of TPH degradation, while in the unimproved composts it was below 0.66%/day of TPH degradation. Moreover, lack information on physico-chemical parameters in totally or partially of some studies restrain the better understanding of the whole petroleum sludge composting. The humic substances can absorb some hydrocarbons during their formation process, it is required to assess the interactions between these compounds throughout the composting. It is also important to know about the finality of the product from the composting process. In all, more researches are needed on the interactions between TPH and other organic matters, removal mechanisms, the toxicity and the stability of the final products.