Environmental Engineering Reference
In-Depth Information
17.3.1 Coagulation-Precipitation-Based Defluoridation Techniques
These methods involve the addition of soluble chemicals (precipitants) to the water lead-
ing to precipitation. Fluoride is then removed either by precipitation, coprecipitation, or
adsorption onto the formed precipitate. Precipitation or coprecipitation has been studied
or practiced using a range of precipitants, including calcium oxide, magnesium oxide, cal-
cium chloride, monosodium phosphate, alum, and an “alum and lime mixture.”
Precipitation is recognized as one of the cheapest and well-established methods of luo-
ride removal from water (Ayoob et al., 2008; Steenbergen et al., 2011). The method involves
stirring or mixing in one of the constituents mentioned above, allowing the precipitate
time to settle; ageing; and decanting or separation. Although some of these processes are
established for household and small-community-scale operation, and the chemicals are
easily and often locally available, there are certain limitations associated with precipita-
tion-based processes. Some of the limitations of precipitation-based deluoridation tech-
nologies include large quantity of sludge generation, uncontrolled pH of treated water,
dificulty in establishing dose requirement, and higher dose requirements for higher F
concentrations (Ayoob et al., 2008). Optimum dose of precipitant must be ensured as some
of the aluminum-based precipitants could pose water quality issues with serious health
impacts.
17.3.1.1 Nalgonda Technique
A comprehensive research program has been carried out (1960-1970) at the CSIR-NEERI in
Nagpur, India, to develop appropriate methods for deluoridation of drinking water. As an
important outcome of this program, it was concluded that the Nalgonda technique could
be preferable at all levels because of the low price and ease of handling (Bulusu et al., 1979).
The Nalgonda technique has been introduced in Indian villages and studied at pilot scale
in, e.g., Kenya, Senegal, and Tanzania (Gitonga, 1984; Lagaude et al., 1988; Gumbo, 1987;
Rao et al., 2010). The Nalgonda technology is still used in countries like Kenya, Ethiopia,
and Tanzania, and the technology is modiied by using local resources and knowledge.
It has been applied in India at different levels. On a household scale, it is introduced in
buckets or drums and at a community scale in ill-and-draw plants. For larger communi-
ties, a waterworks-like low system is developed, where the various processes of mixing,
locculation, and sedimentation are separated in different compartments (NEERI, 1987).
The aluminum sulfate- and lime-based coagulation-locculation sedimentation was the
principle for deluoridation in the Nalgonda technique. It was developed for the low-cost
use at all levels in India (household, village community, and waterworks). It is based on the
combined use of alum and lime in a two-step process and has been claimed as one of the
most effective techniques for luoride removal. In the Nalgonda technique, two chemicals,
alum (aluminum sulfate or calcium aluminum sulfate) and lime (calcium oxide), are added
to and rapidly mixed with the luoride-contaminated water. Induced by a subsequent
gentle stirring, “cotton wool”-like loc develops (aluminum hydroxides) and is subject to
removal by simple settling. The chemical reaction involving luorides and aluminum spe-
cies is complex. It is a combination of two steps involving polyhydroxy aluminum spe-
cies complexation with luorides and their adsorption on polymeric aluminum hydroxides
(loc). Besides luorides, turbidity, color, odor, pesticides, and organics are also claimed to
be removed. The bacterial load is also reduced signiicantly. All these are achieved by the
adsorption on the loc. Lime or sodium carbonate ensures the adequate alkalinity for an
effective hydrolysis of aluminum salts, so that the residual aluminum does not remain in
