Sequential oxidation-composting-phytoremediation treatment for the management of oily sludge from petroleum refineries - an ecological approach

Abstract

The oil industry generates huge amounts of sludge during the different stages of crude oil treatment: exploitation, transportation, storage, and refining. These oily sludges are complex in nature, containing water, petroleum hydrocarbons, heavy metals, and solid particles in a stable water-oil emulsion. They are classified as hazardous organic waste, and regulations in our country require treatment before final disposal. The use of combined approaches is a viable option for the treatment of matrices contaminated with hydrocarbons. In this context, the extent of a laboratory-scale sequential remediation treatment of an oily sludge was investigated, applying chemical oxidation, followed by composting, and phytoremediation.The total hydrocarbon concentration (THC) of the sludge was 18% (IR). The aliphatic hydrocarbon concentration by GC-FID was 4714 ppm, with no aromatic hydrocarbons detected. Iron was the most abundant heavy metal quantified, followed by Ni and V. Generalist and PAH-degrading bacterial populations were quantified by qPCR obtaining 1.62.1010 and 2.11.107 gene copy number/g. The 16S rRNA diversity analysis revealed that Gammaproteobacteria was the most abundant class (57%).The oxidative treatment with 3% activated persulfate produced a THC elimination of 31% and a three-log reduction of the generalist bacterial population.The next application of composting did not produce additional removal of THC after one year of treatment. But reduction in the total dissolved carbon (TDC) and increase the E4/E6 ratio were verified in oxidized microcosms, along with an increase in the generalist population.Phytoremediation was carried out by sowing ryegrass seeds in the substrate resulting from the previous composting (composted sludge or composted-oxidized sludge). Non-vegetated controls for both conditions were also performed. The biomass of the ryegrass in the composted-oxidized substrate was 67% higher than in the composted-non-oxidized substrate. Regardless of initial sludge oxidation, no hydrocarbon elimination was recorded by IR or GC-FID after phytoremediation. Nevertheless, compared to their non-planted controls, a greater increase in the TDC and E4/E6 ratio was observed in oxidized microcosms. In addition, both the hydrolase and dehydrogenase activities evidenced the effect of the plant. A beta diversity analysis was performed to compare the effect of biological treatments on the bacterial communities of oxidized and non-oxidized microcosms, evidencing that the treatment phase could have significant effects on the structure of the bacterial community.The results obtained showed that the proposed sequential treatment significantly reduced THC levels, resulting in a plant growth-promoting substrate. However, the techniques used in this study did not allow for revealing the hydrocarbon transformations produced by the biological treatments. A subsequent analysis using FT-IR spectrophotometry will allow us to delve into these changes.