January 2016
a cotton gin international technical publication
Chlorination
Summary
Beginning with its use in the town of Maidstone, England in 1897, chlorination has been the
most commonly used disinfection technique for public drinking water. Water Chlorination is
used to kill certain bacteria and other microbes in water as chlorine is highly toxic. Recently,
however, many water treatment plants have changed their disinfection approaches because of
regulation changes concerning disinfection byproducts. Today, chlorination is the most popular
method of disinfection and is used for water treatment all over the world.
What Is Chlorination?
Microorganisms can be found in raw water from rivers, lakes and groundwater. While not all
microorganisms are harmful, there are some that may cause diseases in humans. In order to
battle waterborne diseases, disinfection methods are used for safe potable water.
Chlorine is a yellow-green gas under standard conditions and is highly corrosive. Chlorine will
react with organic substances, such as petrochemicals, which will produce a dangerous
explosive. Therefore, chlorine must be kept away from all organic chemical sources, protected
from sunlight, moisture and high temperatures.
Chlorine Safety Training is highly important when handling chlorine. Chlorine forms harmful
acids when it reacts with moisture. It can burn moist body surfaces such as the eyes, nose,
throat, lungs, and wet skin. [For more information on Chlorine safety, see our training schedule
on “Chlorine Safe Work Practices”]
Forms of Chlorine
P.O. Box 1241 Brookshire, TX 77423 Tel: 281.375.5535
www.cottongininternational.com
Volume 01 - Issue 01
Page 1 of 4
Chlorine is supplied as a gas, liquid and a solid. The gas is 100% elemental chlorine (Cl2). It is
supplied in 150 lb. cylinders (10 inches in diameter and 55 inches tall) and in 2,000 lb. (ton)
containers (30 inches in diameter and 82 inches tall). The liquid is sodium hypochlorite
(NaOCL) commonly used as laundry bleach offers 5 to 15 percent available chlorine. The solid
is calcium hypochlorite [Ca(OCL)2], available in granular form or as tablets and provides 65%
available chlorine; this is the most stable form of chlorine. Both calcium hypochlorite and
sodium can be used to treat water using a liquid solution tank with a small chemical feeder
pump.
Storing Chlorine
Drinking water systems should have at least a one or two week supply of chlorine on hand,
most water treatment plants keep a 30-day reserve on hand. Chlorine may be stored indoors or
outdoors, though shading from sunlight is recommended for outdoor storage. Storage areas
should be away from HVAC intakes, as chlorine gas could be distributed throughout a building
in case of a leak.
Separate chlorine storage areas from incompatible materials, especially ammonia, sulfur
dioxide, and hydrocarbons like fuels and oils. Facility storage of bulk chlorine should have a
well-maintained chlorine gas detector installed.
Chlorine Reactions in Water
Chlorine is only slightly soluble in water. Chlorine
mixed with water forms hypochlorous and
hypochlorite ions, see Equation 1 below. The
hypochlorous ion is a more effective disinfectant and
is formed in greater concentration at lower pH values.
At pH 7.3, the hypchlorous and hypochlorite ions are
present in equal numbers. The hypochlorite ion
predominates above pH 8.3 and is not as effective as
a disinfectant. For this reason, better disinfection
occurs at a lower pH. The relationship between
different forms of chlorine over a range of pH is
shown in Figure 1.
Cl2 + H2O à HOCl + HCl
Figure 1 - ionization curve of HOCL as a function of pH
Equation 1 - Chlorine reaction with water
Chlorine Demand
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Because chlorine is a strong oxidant, it combines with many
other substances in water including inorganics, such as ferrous
iron, hydrogen sulfide, and ammonia. This reaction is
instantaneous and no other disinfection occurs until the
chlorine has combined with the organic and inorganic
substances present in the water.
total residual (mg/L) =
combined residual (mg/L)
+ free residual (mg/L)
Equation 2 - Total Residual Chlorine
On especially important feature of disinfection using chlorine is the ease of overdosing to create
a “residual” concentration. Total residual chlorine is the sum of the combined and fee residuals
as shown in Equation 2. There is a constant danger that safe water leaving a treatment plant
may become contaminated later. There may be breaks in water mains, loss of pressure that
permits and inward leak, or plumbing errors. This residual chlorine concentration provides a
degree of protection right to the water spigot. With free available chlorine, a typical residual is
from 0.1 to 0.5 ppm. Because chlorinated organic compounds are less effective, a typical
residual is 2 ppm for combined chlorine.
P.O. Box 1241 Brookshire, TX 77423 Tel: 281.375.5535
www.cottongininternational.com
Chlorine Dose
Chlorine dosage is the amount of chlorine that is added to the water. When chlorine is added it
chemically combines with minerals and organics in the water, changing the chemical makeup
reducing its effectiveness for bacteria, viruses, and
parasites. Proper dosage means adding enough
chlorine Dose (mg/L) =
chlorine to allow for it to react with substances in the
chlorine demand (mg/L) +
water while still having some leftover to disinfect; See
chlorine residual (mg/L)
Equation 3 to calculate Chlorine Dosage.
The contact time and dose are very important to
achieve proper disinfection. A contact time of 30
minutes is a minimum and contact time may need to be increased at lower temperatures or
higher pH values to achieve suitable disinfection.
Equation 3 - Chlorine Dosage
Chlorination Equipment
The majority of disinfection systems installed consist of either gas chlorinator or
hypochlorination.
Gas equipment consists of 150 lb. or 2,000 lb. chlorine cylinders. Chlorine cylinders are never
totally empty and for that reason “empty” tanks must be carefully stored until the manufacturer
picks them up to re-fill. The tanks have a fusible plug that melts at 158oF to 165oF. this fusible
plug prevents the rupture of the tank at high temperatures. The top valves on a chlorine
cylinder produce gaseous chlorine to feed into a water system.
Chlorine leaks may
be detected using a
rag soaked in
ammonia. To find a
leak, pass the
ammonia-soaked rag
slowly over the
chlorine piping. If a
leak is present, the
ammonia will
combine with the
chlorine and form a
whit cloud using
ammonia spray
bottle is not
recommended.
Figure 2 – Hypochlorinator
P.O. Box 1241 Brookshire, TX 77423 Tel: 281.375.5535
www.cottongininternational.com
Page 3 of 4
Liquid chlorine
(calcium hypochlorite or sodium hypochlorite) is more commonly used to treat homes and water
well owners. The hypochlorinatior is composed of a solution tank, chemical metering pump,
storage tank and piping – See Figure 2 above for the basic set-up. This system is less
expensive than a gas feeder and the maintenance for the liquid feeder is much easer to
perform, however, requires frequent attention.
Chlorine Testing
The method used for testing chlorine residual in water employs a chemical known as DPD (N,Ndiethyl-p-phenylenediamine). Previous methods involved the use of OT (Orthotolide) and
starch-potassium iodide. OT is now known to cause cancer therefore it is not recommended.
The DPD method requires the addition of DPD to a sample of water then measuring the
intensity of color production in a colorimeter also know as a chlorine comparator – See Figure 3.
Chlorine samples must be collected immediately. No holding time or sample preservation is
suitable for chlorine samples. Agitation and sunlight will destroy chlorine in the sample.
Drinking water should be tested for both free and
combined chlorine residuals. The test should be taken
from several locations along the water system to
ensure adequate chlorination is taking place.
Chlorination provides disinfection to protect drinking
water supplies from pathogens. For small drinking
water systems, chlorination is the least expensive form
of disinfection available either in gaseous, solid, or
liquid form, however, chlorine is a dangerous and
corrosive chemical that requires special handling and
storage. Consider attending a chlorine safety training
to minimize the potential of an accident while handling,
storing and dosing to provide safe drinking water.
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Figure 3 - Chlorine Comparator Kit
P.O. Box 1241 Brookshire, TX 77423 Tel: 281.375.5535
www.cottongininternational.com