Dechlorination of Highly Chlorinated Water - Disinfection of Pipelines and Storage Facilities Field Guide - page 91

Dechlorination Practices Used in the Field
There are a number of nonchemical and chemical dechlorina-
tion methods. The selection of a method depends on site-specific
factors, such as cost, availability of chemicals, containment logis-
tics, availability of specialized equipment, and regulatory
approval.
Nonchemical techniques include retention in holding ponds,
land application, groundwater recharge, discharge through hay
bales and other natural obstructions, and discharge into sanitary
sewers. These methods have the advantage of simplicity because
they avoid the issues connected with the storage, handling, and
safety concerns related to dechlorination chemicals.
Chemicals commonly used for dechorination in the field
include sodium bisulfite, sodium metabisulfite, sodium sulfite,
sodium thiosulfate, calcium thiosulfate, ascorbic acid, and
sodium ascorbate. Sulfur dioxide is a common dechlorination
chemical, but it is not often used in field applications, and its use
is not included in this discussion. Chemicals have advantages
over nonchemical methods because they usually require less time
to affect dechlorination.
Some of the dechlorination chemicals pose potential health
concerns if not handled properly and may cause adverse environ-
mental impacts. For example, sodium bisulfite and sodium meta-
bisulfite are skin, eye, or respiratory tract irritants. Sulfite-based
chemicals can cause water quality concerns by depleting dis-
solved oxygen in receiving streams. Some dechlorination chemi-
cals produce hydrochloric acid and therefore, decrease water pH.
When selecting a chemical for dechlorination, it is important to
consider the by-products of the reaction and to receive approval
from the appropriate regulatory agency.
Treat and Test Field Method
Although the amount of chemical needed for dechlorination
(and to some degree, the effectiveness of nonchemical methods)
can be calculated as indicated in this chapter, as a practical
matter, most operators use the “treat and test” method of dechlori-
nation in the field. The operator chooses a method for dechlorina-
tion (nonchemical or chemical) and then measures the result. If