Electro-flocculation pretreatment experiments of shale gas drilling wastewater

Abstract

At present, standard discharge has been adopted in the treatment of shale gas drilling wastewater and the electro-flocculation pretreatment has a good application prospect due to its high efficiency, cleanness, low dosage, etc. To improve its adaptability to drilling wastewater treatment, we conducted experiments to investigate the effects of current densities and reaction time on hardness, turbidity and organic matter removal, and the mechanism of electro-flocculation was further explored and a comparative analysis was made with chemical coagulation and ultrafiltration. The following findings were achieved. (1) The drilling wastewater is rich in dissolved salts, among which the hardness ions are mainly Ca2+, and the Ca2+ concentration varies little at the reaction time of 10 min, but decreases significantly with the increase of reaction time. (2) During the electro-flocculation process, the harness ions are usually removed with suspended matters, and their removal trend is different. Lower current densities and longer reaction time will be good for higher hardness removal rates, while higher current densities can help decrease the turbidity quickly before the reaction time of 10 min (3) The pH value of the wastewater is negatively correlated with the concentration of Ca2+ and iron ions. The TOC decreases with the increase of reaction time, and the larger the current density, the greater the decrease of TOC. A linear correlation is found between ΔTOC and ΔCl– and ΔTurbidity. Compared with chemical coagulation and ultrafiltration, electro-flocculation presents significant removal effects of hardness and turbidity. When the current density is 8 mA/cm2 and the reaction time is 20 min, the removal rates of Ca2+, turbidity and TOC are 53.4%, 98.3%, and 62.7%, respectively. Especially for those macromolecular substances with conjugated double bonds, electro-flocculation has an obvious effect and has other advantages like no chemical dosing, no membrane pollution and short reaction time, and so on.