Research On The Enhanced Treatment Of Polymer-flooding Wastewater By Electrocoagulation And Its Mechanism

As a representative technology in enhanced crude oil recovery,polymer-flooding method is widely applied to onshore and offshore oilfields.It can efficiently recover remaining oil by injecting water with a polymer to increase its viscosity during the displacing phase.However,this water has deleterious effects on the operation of wastewater treatment due to its high viscosity,which reduces the efficiency of pumping,and can even destroy pumps.The existing wastewater treatment efficiency is difficult to reach to the standard by chemical coagulation method.To solve this problem,electrocoagulation(EC)is introduced for the treatment of high volumes of the produced water within a short period.In this study,a polymer-flooding wastewater in Gangxi block of Dagang oilfield was investigated.The EC conditions were optimised via the mathematical model between the parameters and water treatment efficiency.The mechanism of EC reaction was also discussed.After that,a three-step process involving slant oil interceptor,electrocoagulation and fine filter with the optimised parameters was used to treat water.The results revealed as follows:(1)By changing experimental conditions,this study compared the effect of EC on polymer-bearing wastewater,thus determining the key parameters and the optimal range of the values of each parameter.An aluminum plate is used as both the cathode and anode in the treatment of the polymer-bearing wastewater(polymer concentration:300 mg/L).It found that the optimal ranges for electric current density,plate distance,and electrolysis time are 20 to 60 mA/cm~2,2.0 to 6.0 cm,and 15 to 25 min,respectively;(2)Using the orthogonal design,support vector machine(SVM)regression algorithm,and response surface methodology(RSM)methods,this paper built three mathematical models for the viscosity reduction rate and COD removal rate with respect to electric current density,plate distance,and electrolysis time.Based on the three models,this paper determined the optimal range of each parameter.By comparing the deviations between the predicted optimal parameter values obtained from the three methods and the actual optimal parameter values,this study found that the RSM model was able to predict the optimal parameter values more accurately than the two other models,with a degree of deviation as low as 2.0%and 2.7%.The RSM model determines that the optimal parameter values are as follows:an electric current density of 45.6 mA/cm2,plate distance of 3.2 cm,and electrolysis time of 22.8 min.Under these conditions,the viscosity reduction rate and COD removal rate are 90.9%and 74.9%,respectively;(3)The process of organic pollutants removal in the EC was revealed from the point of view of the flocculating and electrochemical oxidation mechanisms.By analyzing the composition of the floccule material and that of the organic pollutants before and after electric flocculation,this study explored the reaction mechanism of the electric flocculation treatment of polymer-bearing wastewater.Morphological analyses of plate surfaces,inference of floccule composition,and real-time monitoring of floccule growth environment and size and electrokinetic potential(of the floccule surface),the following reactions occur during the electric flocculation process:1)generation of trivalent aluminum ions through a sacrificial anode;and 2)the trivalent aluminum ion,mainly existing in the form of Al(OH)4-are electrostatically combined with the polyacrylamide contained in the electrolyte solution during the electrolytic process.Real-time monitoring of the electric flocculation process shows that the pH value of the wastewater rises from 8.28 to 9.33,the absolute electrokinetic potential of flocculate particles increases from 40.33 to 41.35 mV,and the average floccule size rises to 12.2 ?m.Under electric flocculation,floccule particles gradually become larger and thus adsorb and remove the pollutants in the wastewater.The contribution of organic components to COD is analyzed to reveal the degradation and removal of pollutants during the electric flocculation process.A water sample was analyzed for 5 min using electrolytic treatment.The COD loss was as low as 95 mg/L,while the test discrepancy of COD was as high as 201.8 mg/L.This shows that oxidation contributes significantly to the removal of COD during this period.After 10 min,COD loss rose from 213 mg/L to 340 mg/L,while the discrepancy of COD declined from 220.6 mg/L to 143.5 mg/L.This may have been due to the contribution of flocculation or to the complete mineralization of some small-molecule organics;(4)Filtration condition was optimized in the batch experiments,including the types of filter material,particle diameter of filter,filling formats,filling height.Then the optimised technological process was conducted in the polymer-flooding wastewater treatment.The optimised conditions of fine filtration were as follows:an average size of 2.0 mm and a filling height of 50 mm for quartz sand cushion and an average size of 1.6-2.0 mm and a filling height of 125 mm for anthracite and walnut shell filler.With the optimised three-step process,the standard water quality for operation and rejection in reservoirs can be achieved(COD 143.5?228 mg/L,viscosity removal efficiency>97%,oil content 0?8.9 mg/L,suspended solids content 1.2?2.8 mg/L,94.27%?98.52%median particle size<3.27).The quantitative evaluation of the effect of EC parameters on the polymer-flooding wastewater treatment was conducted in the study.The operational conditions were optimised.The degradation law of the organic pollutants was revealed via the reaction mechanism.It can give a reference to the application of EC in the wastewater treatment.