Chlorination - Types of Chlorination - Definition, Availability of Chlorine

Definition:

It is the method of disinfection by which the micro organisms are killed if chlorine & its components are used. Chlorination serves not only for disinfection, but as an oxidant for other substances (iron, manganese, cyanide, etc) & for taste & odor control in water & wastewater. Other chemical disinfectants include chlorine dioxide, ozone, bromine, iodine. The last two chemicals are generally used for personal application, not for the public water supply.

Types of Chlorination

Plain Chlorination

Chlorination of water relatively free from suspended matter without any other treatment

Pre Chlorination

The application of chlorine to raw water before any other treatment to improve the coagulation & to remove the taste, odor, & color

Post Chlorination

Application of chlorine to treated water after all the other treatment. Dosage chlorine range from 0.25-5.0mg/lit in order to obtain the combine residual chlorine in range of 0.1-0.2mg/lit

Chemical Properties of Chlorine - Chemistry of Chlorination

Free Available Chlorine

Effective chlorine disinfection depends upon its chemical form in water. The influencing factors are pH, temperature, & organic content in water. When chlorine gas dissolved in water, it rapidly hydrolysis to hydrochloric acid (HCL) & hypochlorous acid (HOCL)

Cl₂ + H₂O ? H⁺ + Cl^- + HOCL

The equilibrium constant is KH = [H⁺] [Cl^-] [HOCL]

[Cl_{2 (aq)}] = 4.48 × 10 ^{- 4} at 25 °C

Hypochlorous acid is a weak acid & subject to the further dissociation to hypochlorite ions & hydrogen ions

HOCL ? OCl^- + H⁺

& its acid dissociation constant K_a is K_a = [OCl^-] [H⁺]

HOCL = 3.7 × 10^-8 at 25 ^?C

Combine Available Chlorine

Chlorine reacts with certain dissolved constituent in water, such as ammonia & amino nitrogen compounds to produce the chloramines. These are referred as combined chlorine.

In the presence of ammonium ions, free chlorine reacts in a stepwise manner to form three species.

1. Monochloramine NH2Cl
2. Dichloramine NHCl
3. Trichloramine NCl₃