Environmental Engineering Reference
In-Depth Information
the treated water. Simultaneous disinfection is achieved with bleaching powder, which
also keeps the system free from undesirable biological growths.
The method was extensively investigated under laboratory conditions and then tested
in the ield. Such ield studies included experiments in buckets and drums (batch type) as
well as in pilot plants (continuous). The technique was seen to be effective in producing
clear water, with luoride within permissible limits. The process is applicable for removal
of a wide range of luoride concentration in water. However, owing to the use of a high
dose of aluminum sulfate as a coagulant, the sulfate ion concentration in treated water
sometimes increases substantially.
17.3.1.2 Chemo-Deluoridation Technique
In chemo-deluoridation (Figure 17.3), the salts of calcium and phosphorous have been
used to reduce the raw water luoride concentration in the range 5-10 to <1 mg/L. The
required dose of chemicals are added in the luoride-contaminated raw water and
mixed with the stick. The chemicals react with each other to form the chemical complex
that sorbs luoride and are precipitated out. After 15-20 min of mixing of the chemicals,
water is allowed to low by gravity into the sand ilter at the rate of 300-400 mL/min.
Filtered water with luoride concentration <1 mg/L is collected in the third plastic con-
tainer and can be used for drinking and cooking purposes. The layer of chemical com-
plex precipitate formed on the sand ilter also removes some luoride from the water
during iltration. After about 1-2 months of operation, the ilter is choked by the forma-
tion of a thick layer of sludge. The Nylobolt cloth kept below the 2-5 cm of the sand layer
is lifted along with the sand, and the sand is washed separately in a tub or bucket. The
Nylobolt cloth is cleaned and replaced on the sand bed. Washed sand is again placed
over the Nylobolt cloth. The ilter is ready to use. The process is free from interference
of any anion concentration in the water and does not affect the palatability of the water.
Under a project sponsored by the Rajiv Gandhi Science and Technology Commission
(government of Maharashtra), 70 deluoridation plants were installed in Sakhara vil-
lage in Yavatmal district in Maharashtra to test the performance of the units. In addition,
FIGURE 17.3
Chemo-deluoridation unit of CSIR-NEERI, India.
