Chlorination Chemicals - Disinfection of Pipelines and Storage Facilities Field Guide - page 7

Chapter 2

Chlorination Chemicals

Disinfection of water mains and storage facilities is most
often performed using one of several forms of chlorine: liquid
chlorine, sodium hypochlorite solution, or calcium hypochlorite
granules or tablets. All chemicals used for the disinfection of
water mains and storage facilities should be certified to NSF/
ANSI Standard 60: Drinking Water Treatment Chemicals-
Health Effects and should satisfy the requirements of applicable
ANSI/AWWA standards. There are other possible disinfectants;
however, this field guide is limited exclusively to a discussion of
chlorination methods.
Chlorine reacts with water to form hypochlorous acid, among
other products, shown in the equations below (Eq. 2-1, 2-2, 2-3).
This acid can dissociate in water resulting in hypochlorite ion
(Eq. 2-4). Hypochlorous acid and hypochlorite ion together are
known as free chlorine residual. Hypochlorous acid is 100 times
more effective as a disinfectant than hypochlorite ion. The rela-
tive amounts of these two compounds in water are largely deter-
mined by the pH (Figure 2-1). Low pH (acid) conditions favor the
formation of the preferred hypochlorous acid.

Liquid chlorine reaction with water

Cl 2
chlorine

H 2 O
water

HOCl 1
hypoclorous acid

HCl 1
hydrocloric acid

(Eq. 2-1)

+ → +

Sodium hypochlorite reaction with water

NaOCl
sodium
hypochlorite

H 2 O
water

HOCl
hypochlorous
acid

NaOH
sodium
hydroxide

(Eq. 2-2)

+ → +

7