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Disinfection/Chlorination in Water Treatment

Disinfection/Chlorination in Water Treatment

 Disinfection is a process whereby micro organisms are eliminated through the use of disinfectants, such as chlorine, or ozone,  e.t.c. Disinfection can be carried out on a variety of products or things on which these disinfectants are suitable for use. For the purpose of this study we shall limit our scope to disinfection in water only. In water treatment chlorine is the most preferred disinfectant because it is not only the cheapest disinfectant, but, it has been proven by experiment and experience to give a very good effective results, it is also very effective against micro organisms such as bacteria and some viruses but, not very effective against protozoans especially Cryptosporidium. Chlorine also provides a residual protection for water during and after treatment, which prevent recontamination, which other disinfectants such as ozone and U.V cannot provide. For Chlorination to be very effective, it requires a contact time of at least 30 minutes or more.
Calcium Hypochlorites

 The Chemical Principle of Chlorination in Water

When chlorine is added to water and dissolve, it quickly react with water to form equilibrium mixture of chlorine, hypochlorus acid(HOCl) and hydrochloric acid(HCl)
    Cl2+H2O➡HOCl+HCl

In acidic solution, the major species are chlorine and hypochlorus acid(HOCl) while in alkaline solution we have ClO- while very small concentration of of ClO2, ClO3- ClO4- can also be found.

When calcium hypochlorite( Ca(OCl)2 is dissolve in water, it result into hypochlorous acid (HOCl) and calcium hydroxide Ca(OH)2

Ca(OCl)2 + 2H2O ➡ 2HOCl + Ca(OH)2

Sometimes, hypochlorite salts(Ca(OCl)2) are substituted for chlorine gas as a disinfectant, but, hypochlorites are safer to handle than gaseous chlorine. The two chemical species of chlorine form in water HOCl and OCl- are known as free available chlorine. Free available chlorine is very effective in killing bacteria. In the present of ammonia (NH3), monochloroamine, dichloramine, and trichloramine are formed.
NH4 + HOCl ➡NH2Cl(monochloramine) + H2O + H+

NH2Cl + HOCl ➡NHCl2(dichloramine) + H2O

NHCl2 + HOCl ➡ NCl3(trichloramine) + H2O.

The chloramines are called combined available chlorine, disinfection by chlorination usually provides for the formation of combined available chlorine which although is a weak disinfectant than free available chlorine, is more readily retained in the water as disinfectant through the water treatment line and water distribution channel. When ammonia is in excess in the water it becomes a problem, as it will increase the chlorine demand of the Water.

Automatic Chlorinator



Chlorine dioxide

Chlorine dioxide (ClO2) is an effective water disinfectant because it does not produce impurities such as trihalomethane in water treatment. In acidic and neutral water respectively, the two half reaction for chlorine dioxide acting oxidants are as followings.

ClO2 + 4H+ +5e- ↔Cl- + 2H2O
ClO2 + e- ↔ClO2-.

Chlorine dioxide in a neutral pH range large exist as a molecular chlorine dioxide until a reducing agent is introduced. Chlorine dioxide is a gas that is violently reactive with organic matter and explosive when exposed to light, and also do not chlorinate or oxidized ammonia or other nitrogen containing compounds, which have raised some health concern over it's degradation byproducts such as ClO2- and ClO3-,  for this reactive nature chlorine dioxide is not the best choice for disinfectant.

Shock Chlorination is a process that involves pouring large amount of calcium hypochlorites or sodium hypochlorite either in powder or liquid form into a well water or swimming pool e.t.c to eliminate or reduce bacteria and algae residue in the water. This practice is most common at home or community where their source of water is open well water and where water treatment plant is not install. The process must be timed between 4-24 hours for effectiveness.

Disinfection by Chlorination also have it's own disadvantage, one of the problem associated with the use of chlorine is that it react with naturally occurring organic and inorganic compounds in the water to form Disinfectant Bye Products (DBPs) such as trihalomethanes(THMs) and Haloacetic acids(HAAs). The major DBPs form as a result of Chlorination are two types which are Bromoform and Dibromochloromethane. These DBPs are of health concern, they are chemical hazards and are carcinogenic. For example Bromoform in high dosage slows down brain activities with the symptoms of such as sedition or sleepiness. Chronic exposure to DBPs can result in liver and kidney cancer, heart disease, unconsciousness and death. But, according to world health organization this health concern are only realistic for chronic exposure over a very long period of time, so the risk is very small when compared to non chlorinated/disinfected water that can result in immediate sickness or death.


 Residual Chlorine
The term Residual chlorine is also know as free or excess chlorine, it is the chlorine that is left over or excess and is available to inactivate or eliminate micro organisms. Free chlorine is the measure of potability of water. It provides protection for water and control reinfection or recontamination.

The present of free residual chlorine in water indicates that;
1. Sufficient amount of chlorine was added to the water initially to eliminate micro organisms that can cause disease.

2. The water is also protected from recontamination during supply and storage.

When chlorine is added to water, some of the chlorine react first with the organic and inorganic substances and metals present in the water, the chlorine used in this oxidative reactions is term chlorine demand of the water. The chlorine demand of water is only the measure of the amount of chlorine used for oxidation purposes and not for disinfection. After the chlorine demand of the water is met, the remaining chlorine is called total chlorine, total chlorine is the chlorine available for disinfection. This total chlorine is also divided into two namely; combine chlorine and free chlorine.


1. Combine chlorine is the amount of chlorine that have reacted with inorganic(nitrate, e.t.c) and organic nitrogen containing molecules(urea e.t.c) to make weak disinfectants that are available for disinfection.

2. Free residual chlorine is the excess chlorine that left over and is available to inactivate pathogenic micro organisms.

Thus, total chlorine equal to the sum total of combine chlorine and free chlorine measurement.
TCl=CCl+FCl

The concentration of free residual chlorine  can be determine by using any of the following test methods
1. Orthotolidine test
2. Diethyl-P-Phenylene-Diamine (DPD) test
3. Chloroscope.
4. Colorimeter
5. Spectrophotometer

DPD tablet




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