When the reduced maximum contaminant level (MCL) of the new arsenic rule went into effect in early 2006, many water
purveyors were forced to take actions to meet the new arsenic rule. For water that also
contains other containments exceeding the primary MCL or secondary standard, simultaneous
compliance could be very costly. The objective of this project was to determine the most cost
effective approach that removes arsenic and excess fluoride from groundwater.
Coyote Springs Development area is a master planned community north of Las Vegas NV that
will use groundwater wells as its only potable water source. A 4,000 gpm water treatment plant
is designed for the first phase of the development. The two source water wells contain 14 µg/L
of arsenic and 2.1 mg/L of fluoride, exceeding the respective primary and secondary standard
for these two elements. The mandatory fluoride limit in Nevada is set at 2.0 mg/L. This paper presents results from pilot scale tests conducted to achieve simultaneous compliance with
the arsenic and fluoride standards. Activated alumina (AA) is the U.S. Environmental Protection Agency (USEPA) designated best available
technology (BAT) for arsenic and fluoride removal. However, bench-scale evaluation indicated
that although AA can remove both arsenic and fluoride, its bed life is rather short, especially for
fluoride removal. Although iron based adsorptive media has a much longer bed life for arsenic
removal compared with AA, it can not remove fluoride. Therefore, an additional unit process
must be added for fluoride removal if iron based adsorptive media is selected for arsenic
removal. Pilot test conducted on site using the same water with a proprietary filter media
demonstrated that coagulation filtration with ferric chloride can reduce arsenic by over 90
percent at a dose of 1 mg/L, much lower than demonstrated by bench test and those reported
in literature. Pilot tests were conducted to determine the optimum dose for simultaneous
removal for both arsenic and fluoride as well as to investigate potential additional arsenic
removal mechanisms associated with the proprietary filter media. Results of these pilot tests
indicated that: the proprietary media has adsorption capacity for arsenic and will result in
over estimated arsenic removal by coagulation/filtration if the adsorptive capacity is not taking
into consideration; simultaneous removal of arsenic and fluoride can be achieved by
coagulation/filtration with a mixture of ferric and alum or with alum alone; and, the optimum dose
for simultaneous removal of arsenic and fluoride for the Coyote Springs source water is
determined to be 7.5 ppm alum. Includes tables, figures.
| Edition : | Vol. - No. |
| Number of Pages : | 17 |
| Published : | 06/01/2007 |
