A pilot plant to treat chromium‐contaminated groundwater

Abstract

AbstractThe groundwater at a site in California is contaminated with Hexavalent Chromium. Different treatment options were tested. The options tested included: carbon adsorption, ion exchange, chemical treatment, reverse osmosis, and in‐place fixation.Electrochemical treatment was the treatment of choice. Treatment operations were designed with turn down flexibility to allow operation at variable flow rates. Metal reduction is the first treatment step after collection of the groundwater and lowering of the pH to 3 in an on‐line acid tank. Soluble ferrous ions are produced in an off‐line electrochemical cell using sacrificial electrodes, as represented by the following equation: equation image Using a slight excess of the stoichiometric 3.2 pounds of iron per pound of chromium, the ferrous ions are mixed with the contaminated groundwater. In the resulting redox reaction, Hexavalent Chromium is reduced to the insoluble trivalent state and divalent iron is oxidized to its insoluble trivalent state, as represented by the following equation: equation image As stated, a slight excess offerrous ions, adjusted pH, and sufficient residence time drive the reaction to completion. After the reaction is completed, the pH is adjusted to a slightly alkaline state (8) to allow precipitation of the metal ions in a pH adjustment tank. The stream is advanced to a clarifier where a polyelectrolyte polymer is added to promote coagulation of the reduced metal and consequently, settlement at the bottom of the clarifier. Solids are drawn from the bottom of the clarifier and allowed to settle in a gravity thickener followed by a plate and frame filter press. The treated groundwater is pH adjusted to below 8 and polished with multimedia filters and carbon, prior to reinjection to the aquifer.The water is continually checked for total dissolved solids and total chromium concentration. The chrome concentration is continuously kept below 30 PPB; however, the total dissolved solids (TDS) oscillate between 1200 to 2400 PPM, depending on the plant TDS influent concentration which ranges from 400–700 PPM and the chromium influent concentration which ranges between 25,000 PPB and 5000 PPB.