Monitoring of Element Changes During in-Vessel Composting for Removal of Total Petroleum Hydrocarbons from Oily Acidic Sludge

Background: Oilyacidic sludge (OAS) from re-refineries has a high concentration of total petroleum hydrocarbons (TPH), elements, and heavy metals. Objectives: This research investigated the monitoring of elements during in-vessel composting of OAS with urban immature compost. Methods: This experimental study was conducted as a batch mode in a plastic container (500 mL). The ratios of OAS to compost were 1:0 (as control),1:5 to 100 (as dry basis) at a C:N:P ratio of 100:5:1, and with 45% - 65% moisture content for 70 days. The elements Zn, Cu, Fe, Mo, B, Mn, Al, Pb, Cr, Sn, Ni, Li, V, As, Cd, Co, and Hg were monitored every week. Results: The highest and lowest total petroleum hydrocarbons (TPH) removals were observed in mixing ratios of 1:5 (71.6%) and1:100 (48.6%), respectively. The amount of Zn, Cu, Fe, Al, and B in OAS was very high. Element analyses after mixing OAS with immature compost showed that the initial concentrations of the elements had decreased. ANOVA test results showed that the changes in time were not effective on the average concentrations of elements (P = 0.99). Conclusions: According to the obtained results, the mixing ratio of 10:1 can be selected as the preferred choice for removing TPH and reduce elements' concentrations.