Environmental Engineering Reference
In-Depth Information
chloride or nitrate. This is of particular advantage in deluoridation of brackish waters
with high total dissolved solids (TDS). A large percentage of feed volume, nearly 20%,
is generally wasted. Elazhar et al. (2009) reported that rejection of F is quite high (~98%
for an initial F concentration of 2.32 mg/L) and costs are comparable between existing
NF drinking water treatment plants and an NF facility designed for selective luoride
removal.
17.4.3 Electrodialysis
Electrodialysis is similar to the RO process for separating ions, but uses an electrical gra-
dient to separate ions through semipermeable membranes rather than using pressure.
Negatively charged ions (such as F
−
) migrate toward positively charged anodes, and
are prevented from further migration due to a negatively charged compensating cation
exchange. In this way, the main stream going through the exchange loses its contaminants.
Depending on initial water quality, electrodialysis requires pretreatment to reduce fouling
on the anodes. Waterworks in the United States is reported to use this technology (Amor
et al., 1998).
17.4.4 Donnan Dialysis
Donnan dialysis is an irreversible ion-exchange equilibrium nonporous membrane-type
separation process. Experiments have shown it to be highly eficient but expensive (Ayoob
et al., 2008). Hichour et al. (1999) studied the Donnan dialysis process in a counter-current
low system in which the anion-exchange membrane was incorporated with sodium chlo-
ride and the feed used was 0.001 M NaF together with other sodium salts. Fluoride was
migrated into the receiver as other ions migrated into the feed. This technique was later
used for removal of luoride from simulated solutions containing high-luoride African
groundwaters with >30 mg/L luoride, while other ions present with the luoride in the
feed could also be brought to <1.5 mg/L (Hichour et al., 2000). Subsequently, dialysis was
combined with adsorption, with the addition of aluminum oxide and zirconium oxide to
the receiver to force the Donnan equilibrium in the direction of luoride low out of the
feed. In this way, it was possible to maintain a low of feed while leaving the receiving
solution in place or renewing only in several batches. The cation composition was found to
remain unchanged, whereas anions, with exception of chloride, were partially eliminated
and substituted by chloride ions, resulting in a residual luoride concentration of 1.5 mg/L
(Garmes et al., 2002).
17.5 Distillation
Distillation is a physical process that can be used to remove the solid impurities from
high-salinity feed water by evaporation followed by condensation. Distillation devices
convert liquid water to steam and collect the condensed steam for application. This pro-
cess, however, requires signiicant energy considering the higher latent heat of water.
A domestic distillation process has been reported to remove up to 99% of luoride from
water (Brown and Aaron, 1991); however, much lower removal has also been reported.
The effectiveness of distillation is not inluenced by the pH of the water, while unlike
