Civil Engineering Reference
In-Depth Information
Water Inlet
Ejector
Ejector
Acid
Tank
Soda
Ash Tank
Support B
ed
Anion
Cation
Defluoridated
Water Outlet
Fig. 20-6.
Paired cationic and anionic exchange resin beds (From Culp, Gordon, and Williams,
Robert,
Handbook of Public Water Systems.
Copyright
1986 by John Wiley & Sons, Inc.
Reprinted by permission of John Wiley & Sons, Inc.)
Chemical Addition for Defluoridation
Lime and alum have been used successfully for fluoride removal. The defluoridation
system generally consists of lime or alum addition to a rapid mix chamber, followed
by flocculation and sedimentation.
Alum was one of the first chemicals investigated for use in removing fluoride from
drinking water supplies.
12
When added to water, alum reacts with the alkalinity in the
water to produce insoluble aluminum hydroxide, according to the following equation:
Al (SO )
14H O
3Ca(HCO )
2Al(OH)
3CaSO
14H O
6CO
2
4 3
2
3 2
3
4
2
2
(20-4)
Rabosky and Miller suggest that fluoride is removed by adsorption onto the Al(OH)
3
particles.
16
Figure 20-7 presents data for Scott et al. and Culp and Stoltenberg on
fluoride removal at various alum dosages.
17,18
The latter reported on the lowering of
the fluoride concentration of a soft, highly mineralized water from 3.6 to 1.0 mg / L
by the addition of 315 mg / L alum followed by 30 minutes of flocculation.
Finally, fluorides have been observed to be removed during lime softening of drink-
ing water. Fluoride precipitation occurs according to the following reaction:
Ca(OH)
2HF
CaF
↓
2H O
(20-5)
2
2
2
Theoretical solubility calculations suggest that an effluent fluoride concentration of
approximately 7.8 mg / L can be achieved by lime precipitation.
12
However, much lower
fluoride concentrations have been observed. Scott et al. reported the defluoridation
during lime softening could have been described by the following equation:
17
F
0.07
F
(Mg)
(20-6)
0.5
r
i
