Environmental Engineering Reference
In-Depth Information
luoride concentration is not uniform within the country, and the data given are based on
the available information. It is also worth noting that the magnitude of the problem is high
in developing countries where a high level of rural poverty prevails.
19.3.1 Fluoride-Free Water: Solution and Challenges
The irst and immediate solution to this problem is to check for alternative water sources
such as rainwater and surface water. The local availability of surface water in suficient
quantity and the suitability of its quality in terms of biological and chemical pollutants
for drinking limits its use in many cases. Rainwater as an alternative source seems to be
an ideal solution since it provides much cleaner water at a minimum cost. However, its
uneven distribution is a big constraint. Thus, the option of using alternative water sources
has its own limitations and may not be always viable.
Using groundwater with low luoride contamination, from different depths around
the same location, can also serve as another solution. However, the concentrations of
luoride usually vary with time in both vertical and horizontal direction. Hence, it is
necessary to test the wells individually on a regular basis to judge the quality of water,
which is not always possible in rural areas because of inancial and technical constraints.
The only viable option available in many contaminated areas is to treat the contami-
nated water.
Several factors must be considered while selecting the treatment of luoride for a par-
ticular site, such as feed-low rate, nature of the solute, solute concentration, water chem-
istry, and the expected removal eficiency. The socioeconomic conditions of the affected
population and the type of water supply source in the affected area should also be consid-
ered while choosing the process. Another important factor to be considered is whether the
affected area requires a centralized treatment system or a large number of independent
treatment facilities. Management of secondary wastewater and the chemical sludge gener-
ated after the treatment are also of prime concern. Although these factors inluence the
selection of the treatment process, in a rural setup the cost and ease of operation should be
given more weightage than others. It is also important to have public participation at the
village level for the success of any technology in villages. A simple and low-cost technol-
ogy may also fail in rural areas, unless the technology its into the rural circumstances and
is well accepted by the locals.
19.3.2 Overview of Defluoridation Technologies
Various deluoridation methods have been developed over the years. Processes adapted
in these technologies include precipitation, ion exchange [52], sorption [53,54], and mem-
brane processes [55] such as electrodialysis [56] and Donnan dialysis [57]. Many of the
processes have been proven successful in laboratory- and pilot-scale studies, but did not
produce desired results in the ield. Coagulation-iltration is a well-documented process
and has been traditionally used to remove many suspended and dissolved solids, includ-
ing luoride, from drinking water. The common chemicals used in this process are metal
salts (Al
2
(SO
4
)
3
·18H
2
O, FeCl
3
, and Fe
2
(SO
4
)
3
·7 H
2
O), lime, or a combination of lime and alum.
In this method, chemicals are added to contaminated water to convert dissolved luoride
into an insoluble form, which is then precipitated. Elimination of luoride may also take
place by adsorption onto insoluble locs, which are coprecipitated. The effective operation
of the coagulation process depends on the selection of a right coagulant, its optimum dose,
and the pH of the system. However, the coagulation method failed in removing luoride
