Civil Engineering Reference
In-Depth Information
OH
H AsO
Si(OH)
F
SO
HCO
Cl
2
2
4
3
4
3
Activated alumina can be regenerated with HCl, H
2
SO
4
, alum, or NaOH. The use of
NaOH, followed by a neutralization step to remove residual NaOH from the bed,
appears to be the most practical approach to regeneration.
Defluoridation of Water Supplies
The beneficial effect of fluoride is its ability to prevent dental cavities when an opti-
mum amount is present in drinking water. However, on the negative side, long-term
consumption of water containing excessive amounts of fluoride can lead to fluorosis
of the teeth and bones. Fluoride commonly occurs in the earth's crust as fluorspace
(CaF
2
), cryolite (Na
3
AlF
6
), and fluorapatite ((Ca
10
F
2
PO
4
)
6
) and ranks 13th among the
elements in order of abundance. Fluoride is present in seawater at a concentration of
approximately 1.4 mg / L, and concentrations as high as 9-10 mg / L are not uncommon
in some groundwaters.
According to the Safe Drinking Water Act (SDWA), all waters that contain more
than the allowable maximum fluoride concentration must be defluorinated prior to use
as public water supplies. At least 125 U.S. communities reportedly use groundwater
supplies with 4.0 mg / L or more of fluorides.
8
Most of these communities are located
in Arizona, Colorado, Illinois, Iowa, New Mexico, Ohio, Oklahoma, California, South
Dakota, and Texas.
If the provisions of the SDWA are strictly enforced, many municipalities must either
install defluoridation facilities or switch to low-fluoride sources of supply. Since so
few treatment facilities have been built, no single technique has emerged as the best
approach for defluoridation. The most promising approach to defluoridation of munic-
ipal water supplies appears to be removal with packed beds of granular activated
alumina. Defluoridation with an activated system involves four separate modes: treat-
ment, back-wash, regeneration, and neutralization. To describe system operation, it is
convenient to start by assuming that the bed is in an exhausted state. Sodium hygroxide
has been widely used in practice, and this appears to be the best choice of regenerant.
The following sequence of operations describes the procedure to regenerate an ex-
hausted bed of activated alumina.
9
1. An upflow backwash at 8-9 gpm / ft
2
with raw water. This backwash expands
the bed and removes any suspended solids that might have been trapped in the
bed. Normal backwash time is approximately 10 min.
2. An upflow regeneration step employing a 1 percent (by weight) NaOH at 2.5
gpm/ft
2
for approximately 35 min.
3. An upflow rinse at 5.0 gpm / ft
2
for approximately 30 min.
4. A final regeneration step in the downflow direction, using 1 percent NaOH at
2.5 gpm / ft
2
for approximately 35 min.
This series of operations strips fluoride from the bed and restores the removal capacity
of the activated alumina. However, following these operations the entire bed is in the
pH range of 12.5 to 13 as a result of the caustic solution used for regeneration. Fluoride
removal by activated alumina is strongly pH dependent with optimum removal at pH
