Water Chlorination
Zahid Hussain
Service Engineer Engro Polymer and Chemicals Limited
Buckman Laboratories
Introduction
Chlorine is the most commonly used substance for disinfection of water
The addition of chlorine or chlorine compounds to water is called
chlorination. Chlorination is considered to be the single most
important process for preventing the spread of waterborne disease.
How it works

Chlorine deactivates microorganisms through several mechanisms that can
destroy
most biological contaminants:

It causes damage to the cell wall.

It alters the permeability of the cell (the ability to pass water in and out
through the cell wall).

It alters the cell protoplasm.

It inhibits the enzyme activity of the cell so it is unable to use its food to
produce energy.

It inhibits cell reproduction.
Availability

as pure elemental gaseous chlorine (a greenish-yellow gas with a pungent and
irritating odor), which is heavier than air, nonflammable, nonexplosive, and,
when released to the atmosphere, toxic and corrosive .

as solid calcium hypochlorite (HTH), in tablets or granules

as a liquid sodium hypochlorite solution of various strengths. The advantage
of one
form of chlorine over the others for a given water system depends on the
amount of
water to be treated, the confguration of the water system, the local
availability of the
chemicals, and the skill of the operator
Major Advantage
One of the major advantages of using chlorine is the effective residual that it
produces. A residual indicates that disinfection is completed, and the system has
an acceptable bacteriological quality. Maintaining a residual in the distribution
system helps to prevent regrowth of microorganisms that were injured but not
killed during the initial disinfection stage.
DETERMINING CHLORINE DOSAGE (FEED RATE)
The units of milligrams per liter (mg/L) and pounds per day (lb/day) are most
often used to describe the amount of chlorine added or required. Following
equation can be used to calculate either mg/L or lb/day chlorine dosage.
Dose Lb/day= Chlorine PPM X Flow Rate GPM X 8.34 Lb/gal
CALCULATING CHLORINE DOSE, DEMAND, AND RESIDUAL

Chlorine Dose:
The amount of chlorine added to the system. It can be
determined by adding the desired residual for the fnished water to the chlorine
demand of the untreated water. Dosage can be either milligrams per liter (mg/L)
or pounds per day (lb/day). The most common is mg/L.
Chlorine demand (mg/L) = Chlorine dose (mg/L) – Chlorine residual (mg/L)
Chlorine residual

The amount of chlorine (determined by testing) remaining after the demand is satisfied.
Residual, like demand, is based on time. The longer the time after dosage, the lower the
residual will be, until all of the demand has been satisfed. Residual, like dosage and
demand, is expressed in mg/L. The presence of a free residual of at least 0.2 to 0.4 ppm
usually provides a high degree of assurance that disinfection of the water is complete.

Combined residual is the result of combining free chlorine with
nitrogen compounds; combined residuals are also called chloramines. Total
chlorine residual is the mathematical combination of free and combined
residuals. Total residual can be determined directly with standard chlorine
residual test kits
BREAKPOINT CHLORINATION CALCULATIONS

To produce a free chlorine residual, enough chlorine must be added to the
water to produce what is referred to as breakpoint chlorination, the point at
which nearly complete oxidation of nitrogen compounds is reached; any
residual beyond breakpoint is mostly free chlorine. When chlorine is added to
natural waters, the chlorine begins combining with and oxidizing the
chemicals in the water before it begins disinfecting. Although residual
chlorine will be detectable in the water, the chlorine will be in the combined
form with a weak disinfecting power.
BREAKPOINT CHLORINATION CALCULATIONS
To be Continue……..