| The amount of produced water is increased with the development of the oilfield, especially in its late period. There has been a considerable amount of the produced water discharged into the environment. The Bohai Sea is not only a environmentally sensitive area, but also the national pollution controlling area. The discharge standard in this area is rising higher but the produced water treatments face many technical difficulties. This paper selected the Bohai Sea and typical produced water treatments as study objects, and used liquid chromatography, Fourier transform infrared, GC-MS and other detection equipment to analyze. The goal of this work is to examine the transformation mechanisms of produced water pollutants, identify degration indicators and compositions, The results show below :(1) The typical Bohai produced water can be divided into two groups, the thin oil sewage and the thickened oil sewage. The COD of both sewages is greater than 200 mg/L, and the salinity is greater than 2000 mg/L, but the oil concentration of thickened oil sewage was significantly higher than that of the thin oil sewage. The organic pollutants in the thin oil sewage are mainly alkanes, while the organic pollutants in the thickened oil sewage are mainly fused ring.(2) The removal ratio of oil in aeration tank, bactivated carbon filtration tank, and in the flotation as the main physical removal process can be up to 92%, which achieved the national level one discharge standards; COD removal ratio after the two treatments were 67% and 49.2% respectively, failed to meet the national discharge standards.(3) 67.9% of the COD is contributed by dissolved organic matters, most of which are small molecules(< 3 kDa). These organic matters were mainly removed by biological activated carbon filters. Meanwhile, larger molecular weight microorganism metabolites, dispersed oil, emulsified oil existed in the form of saturated hydrocarbons were produced and can be removed by flotation. For thin oil and thickened oil sewages, the effluent COD were mainly dissolved organic substances containing aromatic matters with benzene ring, esters, organic acids and other oxygen-containing organic acid.To improve the removal efficiency of the selected organic matters and optimize discharge processes through the compare between the biological and physico-chemical treatments. technical optimization processes were used: high-temperature hydrolysis-bio- oxidation as a biological optimization treatment. The optimum conditions were: residence time of acidification segment was 15 hours, aerobic biological contact oxidation residence time was 7.5 hours, dissolved oxygen concentration is 7.0 mg/L, residence time of bio-filter is 1.5 hours, dissolved oxygen concentration is 5.5 mg/L, the effluent COD is 63 mg/L, and the removal rate could be up to 74.8%. Alkanes smaller than the C20 were fully degrated. Cyclo-alkanes and aromatic hydrocarbons were partially degrated. Proportions of esters and amines declined. As a optimization oxidation treatment, the optimum conditions of electro-coagulation flotation were: the anode was made of carbon / tin oxide electrode, the current was 6.0 mA/cm2, the voltage was 6.7 V, the electrolysis time was 20 min, the effluent COD was 88 mg/L, and the removal rate could be up to 77.1%. The polycyclic aromatic, heterocyclic matters can be effectively degraded into low molecular weight substances. |
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