Biodegradation of oil tank bottom sludge using a new two-phase composting process: kinetics and effect of different bulking agents

In this study, the effect of adding various bulking agents on the performance of a two-phase composting method in bioremediation of petroleum compounds from oily sludge was surveyed. The utilized system included the primary stage composting (PSC) using 6 windrow piles and the secondary stage composting (SSC) using 6 in-vessel bioreactors. Various amounts and combinations of immature compost, mature compost, and green waste were blended with oil tank bottom sludge (OTBS). Then, the mixture was added to the PSC and SSC experiments and composted for a period of 12 and 6 weeks, respectively. The results showed that the degradation percentages of total petroleum hydrocarbons in the PSC and SSC experiments were 63.56–84.50% and 57.59–88.95%, respectively. The removal efficiency reached through the two-phase system was higher than that of the single-stage process. Biodegradation was fitted to the first- and second-order kinetic with the rate constants of 0.076–0.389 day−1 and 0.006–0.176 g kg−1 day−1, respectively. The bacteria identified from the composting mixtures were Bacillus sp., Pseudomonas sp., Staphylococcus sp., and Klebsiella sp. It was concluded that the two-stage composting system, as an inexpensive and efficient method, is practically applicable for remediation of OTBS.