Abstract/Summary

Oil sludges are composed mainly of crude oil, water and sediments. These are hazardous wastes from petroleum extraction and refining processes, and the worldwide generation of oil sludges is approximately 60 million tonnes per year. Treatment of oil sludges to date has been focused on physicochemical and biological remediation. Oil recovery methods including oil sludge washing with surfactants and co-solvents have also been applied for reusing the oil. However, there is a need to optimise the oil recovery in this process. The main aim of this research was to assess whether the addition of surfactants (Triton X-100 and X-114, Tween 80, sodium dodecyl sulphate, and rhamnolipid) and the co-solvents (n-pentane, nhexane, cyclohexane, toluene and iso-octane) in the oil sludge washing enhances the oil recovery and reduces the burden of hydrocarbon contamination. Specifically, three oil sludge washing parameters were considered: surfactant to oil sludge ratio, surfactant type and surfactant concentration. Also, the influence of the co-solvent type and ratio to oil sludge was investigated. Oil sludges from different sources were analysed, and the toxicity of the residuals from oil sludge washing was assessed with the impact on the soil microbial respiration (dehydrogenase activity test) and ryegrass germination. Rhamnolipid, Triton X-100 and Triton X-114 had the highest oil recovery rates (50 – 70%) compared to SDS and T80. These values were higher compared to other studies (30 – 40%). It was demonstrated that the ratio of surfactant to oil sludge factor had a high impact on the oil sludge washing. Particularly, it was found that the surfactant concentration did not have an effect on the oil recovery, and the addition of surfactant was not significantly different in most of the oil sludges analysed. Only one sludge had a highly significant oil recovery rate when surfactants were used. Cyclohexane, as a more benign co-solvent, was confirmed to have similar oil recovery values to toluene; approximately 75% of recovered oil was obtained with each co-solvent. This work has confirmed that oil sludge washing was an efficient pretreatment method which can reduce the organic contaminant. According to the oil hydrocarbon fractions analysed, the recovered oil had the potential to be reused as a feedstock for light fuel production. The oil sludge washing residuals had an adverse impact on the soil microbiota activity (percentage decrease of 40%), and ryegrass germination. However, some dehydrogenase activity by the soil bacteria and a germination higher than 70% were detected implying that bioremediation techniques can be applied to treat the oil sludge washing residuals further if necessary. Based on these studies, a systematic approach to the extraction of oil from sludges was proposed at both laboratory and large scales. First, a quick bench scale experiment can be done to assess the oil recovery rates with surfactant and without surfactant at a low and high surfactant to oil sludge ratios (e.g. 1:1 and 5:1). By doing this first assay, it can be established if the surfactant is needed or not. If the surfactant is not required, the costs can be reduced. For this first assay, the surfactant can be added at lower concentrations because the results of this thesis showed no significant difference in the surfactant concentrations. The proposed application of this method to a large scale mentioned the possibility of adapting surfactant and co-solvent recycling systems to reuse these reagents in more cycles of oil sludge washing. The residual water obtained from the surfactant recycling step and the sediments at the bottom layer of the oil sludge washing tank can be mixed and considered as oil sludge washing residuals. Finally, these residuals can be further treated if needed with the landfarming and phytoremediation combined method in a designated area. Moreover, the use of soybeans was proposed as the phytoremediator species because these plants can also be used for biodiesel production purposes. Even though the oil sludge washing is a low-cost process compared to other treatments, the cost of applying the surfactant and solvent recycling systems is high due to the expensive equipment. In fact, it was found that about 70% of the total cost of the proposed method at a large scale goes towards these recycling systems. Indeed, it is important to consider the surfactant and co-solvent recovery steps carefully. However, if the proposed method is used on a frequent basis, the investment may be recuperated due to the profit obtained with the use of recovered oil as a feedstock for fuel production. In addition, if the phytoremediation with soybeans of the oil sludge washing residuals is implemented, the production of biodiesel can be a profitable source.