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水力压裂技术是一项油气田增产的重要技术手段[1]。在水力压裂过程中,会产生大量压裂返排液,这些返排液中含有高浓度的有机物、盐类物质、悬浮物、重金属及天然存在的放射性物质等污染物[2],若处理不当直接外排,将会对周围土壤、地表水系[3]、大气环境[4]等造成严重污染。
有机物是压裂返排液的主要污染物,目前大多数研究采用物理化学法去除返排液中的有机物,如混凝沉淀法[5]、Fenton氧化法[6]及臭氧催化氧化法[7]等。虽然物理化学法对返排液中有机物有较好的去除效果,但处理成本较高;另外,反应的副产物如芬顿铁泥等可能会给环境带来二次污染。与物理化学方法相比,生物处理法不仅可以有效去除压裂返排液中有机污染物,同时还具有显著的经济优势[8-9],因此,生物法对压裂返排液的处理效果受到了许多学者的关注。
目前,去除压裂返排液中有机物的生物处理技术主要包括活性污泥法、好氧颗粒污泥法、生物膜法等。由于压裂返排液中含有高浓度有机污染物且水质复杂多变,因此,通常采用组合工艺对返排液进行处理。YANG等[10]采用絮凝-Fenton氧化-SBR组合工艺处理油田压裂返排液,COD去除率达到97.1%;王海蒙[11]采用Fenton氧化-混凝沉降-水解酸化-SBBR-活性炭吸附的组合工艺处理压裂返排液,COD去除率达到99%以上。然而,有研究发现单一生物法对返排液同样具有较好的处理效果。陈翱翔等[9]采用好氧颗粒污泥处理压裂返排液,其COD去除率为74%~81%;KEKACS等[12]采用好氧生物法处理返排液,其COD去除率可达到90%以上。可以看出,目前压裂返排液的生物处理以好氧生物处理法为主。虽然好氧生物处理法可实现对压裂返排液的高效处理,但好氧生物法能耗大、处理成本高。而与好氧工艺相比,厌氧生物处理技术具有回收能源、运行成本低等优势[13],已经成为高浓度有机废水处理的主流工艺。有研究[14-15]表明,厌氧工艺可以有效去除油田采出水中的难降解有机化合物,而有关压裂返排液厌氧生物处理的研究尚未见报道。
本研究采用厌氧颗粒污泥处理压裂返排液,研究了压裂返排液的厌氧生物降解特性,同时监测了甲烷的产量,以评估能源回收潜力;此外,对厌氧处理前后的废水样品进行了表征,以评价有机物组分和分子质量分布的变化,以期为压裂返排液厌氧处理技术的工程应用提供参考。
油田压裂返排液的厌氧处理特性
Anaerobic treatment characteristics of oilfield fracturing flowback fluid
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摘要: 油田压裂返排液中含有高浓度有机物、盐类物质和悬浮物等污染物,如不妥善处置直接外排会对环境产生严重危害。以配制的胍胶压裂返排液为研究对象,采用厌氧颗粒污泥对其进行处理,以COD、TOC去除率及甲烷产量为考察指标,研究了压裂返排液的厌氧生物降解特性。结果表明,压裂返排液经168 h厌氧处理后,COD和TOC由处理前的1 735.2 mg∙L−1和698.5 mg∙L−1 降低至277.2 mg∙L−1和94.9 mg∙L−1,去除率分别为84.0%和86.4%,每克COD能产生688.2 mL的甲烷。采用红外光谱、紫外可见光谱以及三维荧光光谱对压裂返排液厌氧处理过程中有机物的组分及光谱特性进行了分析,发现厌氧生物处理可显著去除压裂返排液中芳香族化合物和类腐殖酸等物质。凝胶渗透色谱分析结果表明,压裂返排液中分子质量小于200 Da的有机物在厌氧处理过程中被优先去除。此外,将厌氧工艺出水进行好氧生物处理,COD去除率仅为22.2%,说明厌氧处理后的压裂返排液可生物降解性较差,需要采用高级氧化等方法进行深度处理。Abstract: Oilfield fracturing flowback fluid contains pollutants such as high concentrations of organic matter, salinity, and suspended solids. If oilfield fracturing flowback fluid is not disposed properly, direct discharge would cause severe harm to the environment. In this study, anaerobic granular sludge was employed to treat the prepared guar gum fracturing flowback fluid, and its anaerobic biodegradation characteristics was explored with COD and TOC removal rates and methane production as the indicators. The results demonstrated that after 168 h treatment, COD and TOC decreased from 1 735.2 mg∙L−1 and 698.5 mg∙L−1 before anaerobic treatment to 277.2 mg∙L−1 and 94.9 mg∙L−1 after anaerobic treatment with the corresponding removal rates of 84.0% and 86.4%, respectively. Meanwhile, the evolution of dissolved organic matter (DOM) in oilfield fracturing flowback fluid during the anaerobic biological treatment was investigated. The DOM was sampled at different treatment stages and characterized through Fourier transform infrared (FTIR) spectroscopy, fluorescence excitation-emission matrix (EEM), and UV-Vis. The results revealed that organic pollutants such as aromatic compounds and humic substances could be significantly removed by anaerobic biological treatment. Gel permeation chromatography (GPC) indicated that the organic matter with molecular weight (MW) less than 0.2 kDa was first removed during the anaerobic treatment. In addition, aerobic biological treatment was used to treat the effluent from anaerobic treatment unit and the COD removal efficiency was only 22.2%, indicating that the fracturing flowback after anaerobic treatment had low biodegradability and the advanced oxidation method would be needed for deep treatment.
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表 1 不同初始浓度下厌氧处理压裂返排液过程中COD的变化
Table 1. COD changes during anaerobic treatment of fracturing wastewater at different initial concentrations
反应器编号 COD/(mg∙L−1) COD去除率/% 反应前 反应后 A 917.5 181.5 80.2 B 2 012.2 321.1 84.0 C 3 632.3 492.3 86.4 D 4 380.7 638.7 85.4 表 2 压裂返排液厌氧处理过程中UV254及SUVA254的变化
Table 2. Changes of UV254 and SUVA254 during anaerobic treatment of fracturing flowback fluid
反应时间/h UV254 SUVA254 0 0.78 0.11 24 0.11 0.05 48 0.10 0.10 168 0.15 0.16 表 3 好氧降解压裂返排液厌氧出水过程中COD的变化情况
Table 3. COD changes during aerobic treatment of the effluent after anaerobic treatment
反应时间/h COD/(mg∙L−1) COD去除率/% 0 170.5 — 4 158.7 6.9 8 146.1 14.3 12 138.3 18.8 24 132.6 22.2 -
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