| The "Shale Gas Development Plan(2016-2020)" formulated by the National Energy Administration pointed out that "developing hydraulic fracturing flow-back water(HFFW)of shale gas treatment and disposal technology" is one of the main tasks of shale gas development.At present,the domestic shale gas fracturing flow-back fluid is treated and reused first,and finally the harmless treatment is carried out.In this paper,the pollution characteristics of the Changning HFFW of shale gas in Sichuan were analyzed,and the pollution characteristics evaluation and comprehensive ecological toxicity study were carried out.The influencing factors of coagulation,adsorption and combined adsorption-coagulation treatment of HFFW of shale gas were investigated respectively,and the process conditions were optimized.The differences in the removal of organic pollutants in the HFFW by adsorption,coagulation and combined adsorption-coagulation process were discussed.The main results were as following:(1)The water quality characteristics of HFFW between wells and wells in the same area showed differences.The HFFW was neutral to weakly alkali,high salt and high ammonia nitrogen organic wastewater.According to the Arliekin classification,the HFFW was classified as ClⅢNa.Except for the highest concentration of Na and K,the alkaline earth metal was ambundant in HFFW of shale gas.Stiff-Davis saturation index method was calculated that the HFFW has a calcium carbonate scaling tendency.The HFFW was mainly composed of DOM with molecular weight less than 1 kDa,and the volatile organic compounds were complex,mainly composed of 6 major categories such as alkanes,aromatic hydrocarbons,halogenated hydrocarbons,alcohols,esters and ketones.Through the calculation and analysis of the weight distribution of pollutants,it was clear that the main pollution factors in the fracturing flow-back fluid are TOC and chloride ions.All the HFFW samples collected had relatively large aquatic ecological toxicity effect and poor biodegradability.(2)The effect of coagulation on the removal of pollutants in the fHFFW was studied.The results showed that the dosing quantity of coagulant was 150～200 mg/L,pH range was 6～8,the conditions of stirring conditions was 300 r/min,1 min for fast stirring,then 200 r/min for slow stirring,5 min for slow stirring,and static time was 15～20 min.The removal rates of TOC and turbidity reached 36.09%and 43.12%respectively.The removal rate of coagulation to DOM of less than 1 KDa was 8.1%,and the removal rate of DOM of 0.45-0.2 μm was 3%.Coagulation can remove some aromatic hydrocarbons,and the ester removal effect was better,and dibutyl phthalate was completely removed.(3)The effect of adsorption on the removal of pollutants in the HFFW was studied.The results showed that the dosage of diatomite DM-2 was 8 g/L,the solution was weakly acidic,and the reaction time was 30 min.The removal rates of TOC and turbidity reached 40.01%and 54.41%.In the adsorption process of diatomite DM-2,the removal rate of DOM less than 1 KDa was 31%,which had a sTab.ile removal effect on halogenated hydrocarbons and low carbon alcohols.(4)The PAC and DM-2 combined process was carried out to study the effect of pollutants in the HFFW.The results showed that when PAC dosage was 200 mg/L and diatomite DM-2 dosage was 8 g/L and pH value was 7,adding diatomaceous earth DM-2 for 30 minutes and then PAC adding,the removal rate of TOC reached 57.21%and turbidity was reduced by 93.53%.Compared with the single treatment process,the coupled adsorption and coagulation process can remove more than 30%of DOM less than 1 kDa,and reduce the number of organic compounds by 21.Among them,the saturated linear alkanes and dibutyl phthalate below C20 were completely removed,and the alcohols can be partially eliminated. |
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