Investigation of different configurations of microbial fuel cells for the treatment of oilfield produced water

Produced water (PW) is the largest waste stream in the oil production process: it contains light polar and aliphatic hydrocarbons, production process compounds, dissolved gases, anions and cations. Disposal of PW is subjected to strict legislations. Oil producing countries are focused on finding effective and economic methods for its treatment. Some physical and chemical methods have been used for treatment of PW and biological treatments have been proven to efficiently remove dissolved hydrocarbon compounds. Coupling of anaerobic biological treatment with electrochemical technology in microbial fuel cells (MFCs) can in principle lead to the production of clean water and electric energy. The suitability of MFCs for improving the treatment of PW was investigated in the present work. For the first time, the simple design of single chamber MFCs fed with real PW (PW-MFCs) was studied, in different configurations: (i) with and without membrane; (ii) with and without Pt cathodic catalyst. The results demonstrate the effectiveness of the membraneless configuration without chemical catalyst at the cathode. Even though the electrical output of PW-MFCs was very low (3 mW m−2), it is currently the best reported performance. Furthermore, almost complete hydrocarbon degradation was achieved for each fed-cycle (96.6 ± 1.94%). The Coulombic efficiency was limited by the difficulty to obtain strict anaerobic condition at the anode, since the biocathode of PW-MFCs remained more permeable to oxygen than in acetate-fed MFCs. The DNA sequencing of operating anodic biofilm detected electroactive Desulfobulbaceae mixed to aerobic biodegraders (Burkholderiales) likely through cycling sulfur compounds, which enriched from the PW initial pool in the hypersaline environment. Above all, the results pointed to the practical possibility of using a MFC to enhance and monitor the PW biodegradation process. In fact, the MFC electrical output indicated the occurrence of anaerobic degradation, while the electrochemical parameters of cathode (Tafel slope) resulted correlated to aerobic degradation, suggesting the possibility to design an on-line sensor of the biotechnological industrial treatments of PW.