Study On Fracturing Fluid Degradation Based On Combined Treatment Process

Fracturing fluid is a special industrial wastewater produced during the extraction process of shale gas.Fracturing fluid has some characteristics,such as high viscosity,high COD concentration,large discharged amount,and poor biodegradability,which will put serious impacts on the environment if the fracturing fluid is not properly treated.The common processing methods used in domestic include micro-electrolysis,Fenton oxidation,ozone oxidation,electro-flocculation.However,there are some disadvantages such as high processing costs,inconvenient to operate,secondary pollution during operation.In this work,a green,efficient treatment technology,chemical oxidation coupled with Electro-Fenton oxidation and flocculation precipitation,was developed for the treatment of fracturing fluid of shale gas.Firstly,the viscosity of wastewater was reduced by the chemical oxidation;and then,advanced degradation was carried out through Electro-Fenton oxidation.Finally,chitosan-modified flocculant was used for flocculation of pollutants.The main results of this thesis were listed as follows:(1)The chemical oxidation experiments were carried out using four chemicals with strong oxidizability,including(NH4)2S2O8,H2O2,KMnO4,and NaClO.Among them,NaClO was the best one for wastewater treatment.The optimum conditions for the use of NaClO were the initial pH of 5,the concentration of 2.0 g/L,the reaction time of 90 min,which were obtained through orthogonal test.Under optimized condition,the viscosity of 1.13 mPa·s,COD of 2982 mg/L,and the COD removal rate of 29% were acquired.(2)After a chemical oxidation of wastewater,Electro-Fenton oxidation was applied for advanced degradation.Iron plate and the graphite felt were employed as anode and cathode materials of Electro-Fenton reactor,respectively.And H2O2 was formed through aeration in the cathode.Firstly,the comparison experiment of Electro-Fenton oxidation and Electro-oxidation was carried out for studying the performance of Electro-Fenton reactor.Then the conditions of the reaction were investigated.The optimal experimental conditions were current density of 40 mA/cm2,the plate spacing of 1.0 cm,the reaction time of 50 min through optimization experiment.And COD of 527mg/L and the removal rate of 82.3% were obtained under optimized condition.(3)A green flocculant prepared by chitosan modification was used to treat the wastewater after treatment by Electro-Fenton.Water solubility of carboxymethyl chitosan was improved.A Fe3O4 was functionalized to carboxymethyl chitosan owing to its easy separation from water.And then quaternary ammonium salt of DMC was connected to chitosan to increase its molecular weight and positive charge.Finally,the cationic flocculent Fe3O4-CMC-PDMC with magnetism was obtained.The effects of various factors on the flocculation efficiency were studied by the turbidity removal efficiency.The results showed that as-prepared flocculant showed advantages in comparison to commercial flocculant.The optimal operating conditions of flocculation evaluated by response surface analysis method were the flocculant with grafting ratio of 234%,the concentration of 5.8 mg/L,and the stirring speed of 103 rpm.After a treatment with the optimal operating conditions,the water turbidity was decreased to 2 NUT,and the removal rate of 96%,COD of 225 mg/L.(4)After a treatment via combined technology,the COD concentration decreased from 4200 mg/L to 225 mg/L,and the total COD removal rate reached 94.6%.The results showed the combined treatment technology proposed in this study is simple without secondary pollution,which provides the technical basis for its industrial application.