Environmental Engineering Reference
In-Depth Information
the desired result is not obtained, the method is adjusted until it
is. This is a trial-and-error method.
In some cases, a combination of methods is used to provide
backup while the system is adjusted. Frequent samples are
taken and tested to ensure that the dechlorination is successful.
If a chemical method is used, the feed amount is adjusted so that
the desired chlorine residual is just attained. This is necessary to
avoid an overfeed situation. Overfeeding some dechlorination
chemicals can have adverse effects on receiving waters.
Calculating the amount of dechlorination chemical necessary
for the volume and concentration present is recommended so
that enough chemical is on-hand to complete the job. The tables
and equations in this chapter are provided to perform these cal-
culations. It is also recommended that some extra chemical (at
least 20 percent) is taken to the site, because some of the calcula-
tions are approximate and local conditions may affect the exact
amount needed.
Frequent chlorine residual testing is needed when employing
this method. Use reliable, certified test methods and field test
kits for this purpose. Personnel must be trained in the proper use
of this test equipment. Regulatory agencies may require docu-
mentation of the test results and other applicable information
such as the initial volume, the chlorine residual, the dechlorina-
tion method used, and the amount of dechlorination chemical.
Nonchemical Methods
These dechlorination methods are generally used for dis-
charges where the amount of water is limited. The reason for this
is that most of these methods require some time to affect the
chlorine reduction. They are not generally suitable for continu-
ous flow treatment. The most common nonchemical methods and
estimates of dechlorination effectiveness are given in Table 7-1.
Chemical Dechlorination Methods
Common reducing agents used in the field include sodium
bisulfite (NaHSO
3
), sodium sulfite (Na
2
SO
3
), sodium
metabisulfite (Na
2
S
2
O
5
), sodium thiosulfate (Na
2
S
2
O
3
), calcium
thiosulfate (CaS
2
O
3
), ascorbic acid (Vitamin C), and sodium
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