(3) Superchlorination and dechlorination. (See following caution.) When
hydrogen cyanide is the contaminant and the water is cold (70oF or below) or when one
of the nerve agents is the contaminant, two pretreatment tanks should be used. Raw
water is pumped into both tanks and superchlorinated by the addition of 100-ppm
available chlorine from high strength calcium hypochlorite. (The calcium hypochlorite is
added to the tanks prior to starting to fill them with raw water.) The water is then
dechlorinated by the active carbon treatment outlined in (1) above. By the use of two
tanks, a continuous quantity of pretreated water can be made available to the
coagulator in much the same manner as two or more floc tanks are used in the batch
process of normal water treatment. It is important that the residue of activated carbon in
the pretreatment tanks be kept in suspension. Pumps should be used for this purpose if
necessary. The carbon is removed by coagulation and diatomite filtration. If the
residual is inadequate, post-chlorination is necessary.
CAUTION:
Calcium hypochlorite, a powerful oxidizer, and activated carbon, a
reducing agent, wiII react violently, possibly resulting in fire or explosion,
if mixed with one another in slurry form. The following safety
precautions should be observed:
(a) Do not store activated carbon and calcium hypochlorite near one
another.
(b) Do not use the same paiI to prepare slurries of these two
substances.
(c)
Keep both chemicals dry untiI actually ready for use.
MD0160
6-33