Electrocoagulation (EC) is a mature technology for many types of industrial waste water treatments. The technology can remove metal ions, colloidal suspensions, fine particles, and soluble inorganic pollutants from aqueous solution by introducing highly charged polymeric metal hydroxide species. In this paper, EC is explored as a treatment option for concentrated ARD water with focus in silica removal. Fe metal plates were employed as sacrificial electrodes. The experiments systematically investigate the influence of pH, dissolved oxygen (DO), current density (CD), and duration on the formation of green rusts (GRs) and silica removal. Batch tests have proved the concept that EC is a viable approach for silica removal from 80% RO reject water. Silica can be reduced from ~14 ppm down to ~3.0 ppm under 25mA/cm2 current density for 60 minute or 12.5 mA/cm2 for 120mins. Such level of silica removal can alleviate membrane fouling during subsequent NF/RO treatment. The DO level in solution has an important influence on pH and water chemistry, governs ferrous or ferric or ferrous/ferric mixed GR generation, and impacts the removal of silica in RO rejects from ARD water. EC with ferrous green rust formation under lowest DO level is best for silica removal.