Abstract

In the process of Enhanced Oil Recovery (EOR), carbon dioxide and water are used excessively (8000 ft 3 of CO 2 and 10 bbl of water per 1 bbl oil extracted) to increase the oil production and as a result, a large stream of waste water is generated. The main contaminants of this waste are dissolved gases mainly CO 2 and dissolved petroleum hydrocarbons (referred to as TPH). CO 2 which forms about 70% of the greenhouse gases. is the major cause of global warming and its atmospheric concentration (currently 385 ppm) has been rapidly increasing since the past decades. In a series of batch experiments, the application of several electron donors such as simple volatile fatty acids and mono and disaccharides were investigated to remove CO 2 and TPH from a synthetic waste stream (containing about 200 mg COD/L of TPH and dissolved CO 2 at the saturation level). About 95% of CO 2 and 76% of TPH were removed using formate (2 g/L) and sucrose (2.5 g/L) respectively at a mesophilic range of temperature (about 35°C). In the second phase of this study, a two-step reactor system was used to treat this waste and the system operation was optimized using the method of evolutionary operation (EVOP) factorial design. For the first reactor with CO 2 reduction and CH 4 production as the target parameters, values of pH, temperature and organic loading rate of 2.5, 38°C and 6240 mg COD/L were obtained. The corresponding values for the second reactor were 6.5, 39.5°C and 394 mg COD/L for the TPH removal. The energy balance for the system resulted in the calculated net energy ratio (NER) of 3.7 which showed a sustainable biogas production. The kinetic study of the system showed that degradation of formate and sucrose in both reactors is affected by the presence of petroleum hydrocarbons probably due to their inhibitory effects. Also, it was shown that the original differential equations for the substrate concentration and microbial growth can better predict the kinetic behavior of the system than the simplified models. As the overall conclusion of this study, this method is less complex compared to other competitive methods and it can be easily applied. Moreover, besides its low energy requirements, it can generate CH 4 from CO 2 as a clean source of energy.