Engineered Treatment of As-Laden Regeneration Brine from Ion Exchange Processes

Abstract

Arsenic (As) contamination of drinking water sources has been one of the most challenging global environmental issues. In the United States, the newly revised maximum contaminant level (MCL) of 10 µg/L requires thousands of utilities to either modify their existing treatment systems or adopt new As-removal technologies. While ion exchange (IX) is one of the EPA approved best available technologies for As removal, IX processes generate large volumes of As-laden regeneration brine due to lack of As selectivity. The resultant liquid process waste residuals require costly additional treatment and disposal. Addition of ferric chloride has been well-documented and commonly used to remove As from aqueous solution via co-precipitation and adsorption. Previous studies have determined the optimal pH and Fe/As molar ratio for treating drinking water. In this study ferric chloride addition was investigated as a cost-effective means to treat As-laden spent regeneration brine, where arsenic, sulfate, bicarbonate, and chloride concentrations (300 mg/L, 605 mg/L, 305 mg/L, and 24 g/L, respectively) were orders of magnitude greater than typical drinking water treatment levels. Batch tests revealed that nearly 100% of the As in spent brine can be removed with a Fe/As molar ratio of 2 at pH 6 and 7. Furthermore, column tests indicated that treated brine can be reused for regenerating a polymeric ligand exchanger and nearly 100% of the resin’s capacity can be recovered. It has been estimated that millions of tons of As-bearing sludge are annually introduced as waste residuals from water treatment processes. This study determined the optimum conditions to yield the most stable process waste residuals. The EPA TCLP and California WET tests were employed to determine the leachable As in the brine treatment residuals. When the brine was treated using an Fe/As molar ratio of 5 and 20, the resultant As-laden sludge easily passed the TCLP and WET respectively, both with a limit of 5 mg/L As. Addition of 90 mM calcium decreased leachable As by 80% while adding 210 mM calcium increased As leached by 60% suggesting an optimum range of calcium addition for further stabilization of the treatment residuals. Calcium addition to the brine treatment process also decreased the chemical costs by 18% and reduced the mass of sludge produced by 20%. Furthermore, a dry aging period of 98 days had significant effects on extractable As, increasing extracted As by 54% in residuals formed at a Fe/As ratio of 5 while decreasing extracted As by 70% in residuals formed by Fe/As of 10.