Offline bioregeneration of spent activated carbon loaded with real Produced Water and its adsorption capacity for benzene and toluene

Abstract

The disposal of water produced during the petroleum extraction (Produced Water) is a relevant issue due to the occurrence of contaminants. Adsorption on activated carbon is one of the best available technologies for the removal of synthetic organic chemicals from water. However, the replacement and disposal of exhausted carbon is quite expensive and the spent carbon may have to be handled as hazardous waste. The bioregeneration of spent carbon could be a feasible solution; however, hypersaline wastewaters, like Produced Water, are often recalcitrant to biological process due to the strong inhibition by salts (mainly NaCl), elevated temperature, and presence in solution of biocides. In this study, adsorption kinetics, isotherms, and rapid small-scale column tests have been performed to select the type of granular activated carbon (GAC) with the best adsorption capacity of target monoaromatic compounds (benzene and toluene). Continuous-flow pre-loaded GAC biological regeneration experiments were conducted with both synthetic and actual hypersaline wastewaters (oily Produced Water), using solutions containing selected micro-organisms. GAC adsorption was found very effective to remove target compounds (benzene and toluene) from both the synthetic hypersaline water and the real Produced Water. A preferential adsorption of toluene was observed from batch and dynamic adsorption experiments. This study demonstrates that GAC loaded with either synthetic or real Produced Water can be regenerated by offline bioregeneration. Indeed, about 57% and 50% of the GAC regeneration capacities were achieved for benzene and toluene, respectively, during experiment with real Produced Water. The genetic characterization of the isolated bacteria has shown the presence of species which are well known for the degradation of hydrocarbons. The maximum values of optical density, CFU, and CO2, indicating the highest biomass growth, have been found simultaneously with the maximum bioavailability of benzene and toluene. These results clearly demonstrate that biological regeneration occurs.