Civil Engineering Reference
In-Depth Information
Fluoride removal is complicated by the presence of other ions in the water that
compete with fluoride for removal. The design of defluoridation systems requires lab-
oratory and pilot-scale work prior to the design of full-scale treatment systems.
Two methods of defluoridation have found practical application. One involves pas-
sage of water through defluoridation media such as bone meal, bone char, ion-exchange
resins, or activated alumina. The second method involves the addition of chemicals
such as lime or alum prior to rapid mixing, flocculation, and sedimentation in a wa-
terworks, for the removal of fluoride only or the concurrent removal of fluoride and
other ions (e.g., calcium and magnesium removal for water softening).
The following discussion presents past experience with media filter and chemical
addition defluoridation systems.
Defluoridation Media
The uptake of fluoride onto the surface of bone was first reported by Smith and Smith
in 1937.
11
They suggested that fluoride was removed by ion exchange in which the
carbonate radical of the apatite comprising bone [i.e., Ca(PO
4
)
6
CaCO
3
] was replaced
by fluoride to form an insoluble fluorapatite, according to:
Ca(PO )
CaCO
2F
Ca(PO )
CaF
↓
CO
(20-1)
2
46
3
46
2
3
Similarly, bone char or tricalcium phosphate (Ca
3
(PO
4
)
2
), produced by carbonizing
bone at temperatures of 2012 to 2912
F (1100 to 1600
C), has been used for defluor-
idation. When exhausted, the column is regenerated by application of a 1.0 percent
solution of caustic soda, which converts the fluorapatite to hydroxyapatite (Ca(PO
4
)
2
Ca(OH)
2
). The fluoride is removed as soluble sodium fluoride. The caustic is followed
by a rinse and then an acid wash to lower the pH.
12
In the regenerated form, the
hydroxyl radical becomes the exchange anion in the defluoridation reaction.
13
While
bone char has been used successfully for full-scale defluoridation, over the past several
decades it has been supported by the availability of more cost-effective sorption me-
dia.
14
Paired cationic and anionic exchange resin beds have also been used for defluori-
dation. In this process, illustrated by Fig. 20-6, water first passes through a cationic
resin (R
) bed, which exchanges sodium with hydrogen to form the equivalent acid:
2NaF
HR
HF
Na R
(20-2)
2
2
2
2
The hydrogen fluoride is then removed during passage through the anionic bed (R
):
2R
HF
2R HF
(20-3)
22
Periodically, the resins are regenerated with acid and alkaline solutions. Synthetic ion
exchange resins for defluoridation are available from several manufacturers.
The most widely used defluoridation method involves beds of granular activated
alumina (Al
2
O
3
). Activated alumina is useful for controlling a number of drinking
water constituents.
15
A discussion of the sorptive properties of activated alumina and
the design criteria for sorption columns is provided in Chapter 18—''Ion Exchange
and Activated Alumina Sorption.''
