AWWA WQTC65718

The U.S. Environmental Protection Agency (USEPA) promulgated the Arsenic Rule in January 2006, which reduced the federal arsenic maximum contaminant level (MCL) for drinking water from 0.050 mg/L (50 µg/L) to 0.010 mg/L (10 µg/L). The USEPA identified the following as Best Available Technologies for achieving compliance with this regulatory level: ion exchange; activated alumina; oxidation/filtration; reverse osmosis; electrodialysis reversal; enhanced coagulation/filtration; and, enhanced lime softening (USEPA, 2001). This study compared coagulation/filtration and adsorption which have been the most common technologies to remove arsenic from groundwater. Coagulation/filtration relies upon an electrostatic attraction between the ferric hydroxide floc surfaces and negatively-charged dissolved arsenic molecules. Ferric chloride hydrolyzes in water to form ferric hydroxide floc, which have a net positive surface charge at pH values less than 8.0. The ferric flocs remove arsenic using a combination of co-precipitation and adsorption. The arsenic-bound ferric flocs are then removed from the water using a downstream filtration step, such as pressurized sand filters. The two main parameters that control the efficiency of this process are the ferric coagulation dose and the hydraulic loading on the filter media. An inadequate dosage would result in arsenic "leaking" through the filter into the finished water while excessive coagulant dosing would result in higher chemical costs and shortened filter run times. In a similar vein, reduced hydraulic loading would mean constructing filters that are larger than required while hydraulic loadings higher than required would cause increased system headlosses and early particulate breakthrough (Fields et al, 2000). Adsorption using iron-based sorbents (IBS) media is a common arsenic treatment process in the US. The IBS media is essentially the ferric coagulant floc in a solid particulate form. Water is passed through columns packed with this media. As the water travels by and around the individual particles, dissolved arsenic in the water is adsorbed onto the particle surface and is removed from the water. Since the media is based on iron, it has the same limitation as coagulation/filtration with regards to competing anions. IBS manufacturers typically recommend a hydraulic loading of 5 gpm/ft² and an empty bed contact time (EBCT) of 5 minutes. Rapid Small Scale Column Test (RSSCT) is a method for developing treatment estimates of fullscale packed-bed adsorption systems using much smaller adsorption units and flowrate (Westerhoff et al, 2006). Includes 3 references, tables, figures.