| Produced water (PW) is the largest wastewater stream in the oil and gas exploration process. Due to its serious environmental impacts, many countries have implemented very stringent regulatory standards for the produced water discharges. Meanwhile, the oil and gas industry all over the world is motivated to explore efficacious and economical produced water treatment technology. In China, oil and grease (O&G) and chemical oxygen demand (COD) are main integrated parameters used to evaluate organics contamination level of produced water. However, The data of chemical composition contributable to COD of oilfield produced water should be investigated in order to assess the efficiency and suitability of a wastewater treatment process and removal technology.First aim of this work was to characterize the chemical composition contributable to COD of a typical produced water treatment process, which consists of oil separation, floatation, bio-contact oxidation and sedimentation. The results showed that the majority of COD was originated from oil and grease in raw wastewater. The COD contributed by O&G decreased greatly with continuous treatment and its final percentage was less than 12% of measured COD in effluent. Other chemicals, such as organic acids, low molecular weight carbonyl compounds, volatile phenols, reductive anions, metals and TDP were not the main sources of COD due to their low concentrations in the whole treatment process, and their contribution to COD was no more than 9% at all sampling spots. It was noteworthy that a large proportion of components contributable to residual COD were still unknown after biological treatment process, and the COD contributed by these components was even more than 50% of measured COD no matter whether the estimated COD of ammonia is neglected or not. The results also indicated that the treatment process was very effective for removal of O&G, but the final effluent needs additional treatments to remove unknown soluble compounds (eg., SMP andOCs et a/.) due to its large contribution to COD when aim is to enhance removal efficiency of the treatment system. In addition, the floatation unit and sedimentation unit should be improved.Based on the in situ monitoring and laboratory simulation experiment, degradation of a mixture of three strains of bacterium (WS-3) on oil and COD were evaluated. The result showed that WS-3 can remove petroleum hydrocarbon efficiently, and the biodegradation is the main way to remove oil pollutants in the aeration biological process. This indicated that it is a feasible method for produced water treatment with mixed oil-degradative bacteria.Based on the analysis of main processes of the wastewater treatment system in Gudao Oilfield, an integrated kinetic model was established to predict the fate of O&G. The result showed a good consistency between simulated value and measured value of O&G at steady state condition. The total removal percentages of O&G and COD calculated by kinetic model were 91.2% and 58.7% respectively, which were very close to actual removal percentages of 95.2% and 61.9% respectively. It was indicated that the contribution of each process to O&G removal percentage decreased in the order of oil separation > biodegradation > floatation > volatilization > air stripping > sedimentation, and oil separation, biodegradation and floatation played important roles in removal of O&G, accounting for 80.1%. Other process, such as volatilization, air stripping and sedimentation were not the main removal ways. With the kinetic model, the load with respect to O&G in Gudao oilfield wastewater treatment system was predicted, and the result showed that the system has a capacity to accept high concentration O&G up to 135 g/m3. The kinetic model also can be used to evaluate other biological treatment systems similar to Gudao oilfield's.
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