Abstract

In this study, the potential of electrocoagulation (EC) as a suitable pretreatment option for shale gas produced water (SGPW) prior to FO was investigated. Specifically, the removal of turbidity, chemical oxygen demand (COD), and three inorganic ions (Ca2+, Cl-, Fe2+), which are known to promote inorganic fouling of the FO membrane, was examined. Experimental work was divided into three parts. The first part focused on the optimization of EC parameters through preliminary experiments (using synthetic SGPW) and Response Surface Methodology (RSM) optimization (using industrial SGPW). The second part explored COD and ion removal efficiency upon the addition of a coagulant aid to EC while the third part analyzed fouling reduction in FO after feed pretreatment. A comparison of contaminant removal and flux in FO for chemical coagulation pretreatment of SGPW was also performed.
In the first part, better COD removal (but lower ion removal) was observed under RSM’s optimum conditions (pH 3.2, time = 35 mins and current density = 45 A/m2) compared to the neutral pH condition from preliminary experiments (pH 7, time = 40 mins and current density = 200 A/m2). The addition of 25 ppm polyacrylic acid (PAA) under neutral pH condition improved COD and ion removal significantly due to the stretched out conformation of polymer and its lesser adsorption propensity. COD, chloride, calcium and iron removals were 69.78%, 52.49%, 36.64% and 61.33% respectively. SGPW pretreatment via EC, prior to FO, led to a 27-37% reduction in flux compared to a 70% flux decline for raw SGPW feed. Final flux at 450 minutes for acidic pH (with no PAA) and neutral pH (with 25 ppm PAA) pretreatments were 3.53 and 5.22 LMH, respectively. COD and ion removal was least when 3000 ppm of alum was employed; however, no significant gypsum fouling (thus higher flux) was observed in FO due to slow nucleation and increased gypsum solubility in the presence of high concentrations of competing ions like magnesium and sodium. Acidic pH condition (with no PAA) is recommended for EC pretreatment of SGPW if only generation of reusable water for fracking operations is required. However, if higher product water quality is needed and secondary membrane treatment can be employed, neutral pH condition (with 25 ppm PAA) is recommended.