The refining and transportation of crude oil often results in an inevitable accumulation of recalcitrant waste products such as oily sludge. Remediation processes such as landfarming and anaerobic landfilling have been recently ruled as inappropriate disposal methods, due to the length of time it takes for remediation to reach completion, as well as the uncontrollable release of harmful organics into the soil, atmosphere and potentially groundwater. This study focused on a biological treatment method for oily sludge using a plug flow reactor system.
Biosurfactant-producing bacteria, Bacillus cereus, Klebsiella oxytoca and Cronobacter dublinensis were isolated from petroleum-contaminated and uncontaminated soil samples. Optimal growth of the bacteria was observed when incubated at ±30°C and biosurfactant activity was confirmed using the drop-collapse test. Oily sludge was simulated in the laboratory using fumed silica as intermediate compound to form Pickering emulsions. The recovery of oil involved the comparison of a plug flow system and a batch system. The plug flow system achieved up to 85% oil recovery from oily sludge while the batch system only achieved a maximum of 30% elution of the aqueous phase. The effect of waterflooding, in conjunction with biosurfactant-producing bacteria on the batch system had negligible effect while on the plug flow system, an increase of ±10% in percentage recovery was observed. GC-MS analysis of oil before and after the recovery process showed a reduction of concentration of certain compounds, suggesting possible degradation by bacteria or loss due to volatilization. TOC analysis showed a reduction in TOC of residual sludge, after completion of recovery process, suggesting degradation activity by bacteria trapped in the sludge matrix after the completion of the recovery process. Fifteen days after completion of recovery process, residual sludge from the plug flow system had a low TOC value of 1.03mg/L while sludge from the batch system had a much higher value of 7.119mg/L. The high percentage recovery of oil, coupled with the reduction of sludge TOC to negligible values suggest that MEOR is a feasible method for treatment of oily sludge and can be further investigated as a waste treatment procedure in the petroleum industry.