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Removal of oil hydrocarbons from petroleum produced water by indigenous methanogenic oil degrading microbial communities

Ezennubia, Valentine C.
Plugging of membrane filters by oil (mostly n-alkanes) constitutes one of the main barriers to treating produced water (PW) for its integration into industrial and agricultural uses. Based on a previous study showing that the combined supply of protein-rich matter and CO₂ stimulates the activity of indigenous oil-degrading microbial communities, here we study the feasibility of using this stimulation method to remove dissolved oil hydrocarbons from petroleum-produced water PW in storage tanks. The experimental procedure consisted of vial experiments using actual PW collected from the Stillwater and Cushing oilfields of Oklahoma, USA. Experiments aimed to determine the kinetics and degree of oil hydrocarbon degradation, test the effect of pH and ORP, and elucidate the mechanism of oil degradation by the discovered stimulation method. Results of gas chromatography (GC) analysis of the initial and final composition of dissolved oil hydrocarbons, UV spectrophotometric analysis of total dissolved oil hydrocarbon degradation, and GC analysis of CO₂ production show that the discovered stimulation method works at mesophilic and thermophilic temperatures, isolated soy protein can be used as a substitute for yeast extract, and that HCl can be used aa s substitute for Na₂S to reduce the oxidation-reduction potential (ORP) of PW to optimum levels for oil hydrocarbon degradation. Our results support the hypothesis that CO₂ supplied as NaHCO₃ boosts the stimulating effect of protein-rich matter by removing H₂, which constitutes a thermodynamic barrier for the degradation of oil hydrocarbons. Further studies are needed to understand differences in the microbial community dynamics in PW supplied with the protein-rich matter, CO₂, and reducing agents at thermophilic and mesophilic temperatures. Initial oil concentrations (0.8-1.9 mg-oil/L) decreased by 40-90% within 7- 35 days. The specific rate coefficient and half-saturation constant to represent the kinetics of oil hydrocarbon degradation are 0.614 day-1 and 5.7 mg-oil/L, respectively.