AWWA WQTC65718

The U.S. Environmental Protection Agency (USEPA) promulgated the Arsenic Rule inJanuary 2006, which reduced the federal arsenic maximum contaminant level (MCL) fordrinking water from 0.050 mg/L (50 µg/L) to 0.010 mg/L (10 µg/L). The USEPA identified thefollowing 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 comparedcoagulation/filtration and adsorption which have been the most common technologies to removearsenic from groundwater.Coagulation/filtration relies upon an electrostatic attraction between the ferric hydroxide flocsurfaces and negatively-charged dissolved arsenic molecules. Ferric chloride hydrolyzes inwater to form ferric hydroxide floc, which have a net positive surface charge at pH values lessthan 8.0. The ferric flocs remove arsenic using a combination of co-precipitation andadsorption. The arsenic-bound ferric flocs are then removed from the water using adownstream filtration step, such as pressurized sand filters. The two main parameters thatcontrol the efficiency of this process are the ferric coagulation dose and the hydraulic loading onthe filter media. An inadequate dosage would result in arsenic "leaking" through the filter intothe finished water while excessive coagulant dosing would result in higher chemical costs andshortened filter run times. In a similar vein, reduced hydraulic loading would meanconstructing filters that are larger than required while hydraulic loadings higher than requiredwould 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 theUS. The IBS media is essentially the ferric coagulant floc in a solid particulate form. Water ispassed through columns packed with this media. As the water travels by and around theindividual particles, dissolved arsenic in the water is adsorbed onto the particle surface and isremoved from the water. Since the media is based on iron, it has the same limitation ascoagulation/filtration with regards to competing anions. IBS manufacturers typicallyrecommend 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 fullscalepacked-bed adsorption systems using much smaller adsorption units and flowrate(Westerhoff et al, 2006). Includes 3 references, tables, figures.

Product Details

Edition:

Vol. - No.

Published:

11/01/2007

Number of Pages:

13

File Size:

1 file , 210 KB