Forms of Chlorination : Factor Affecting the Efficiency of Chlorination

When chlorine is added to water, the following reaction takes place;

Cl₂+H₂O ↔ HOCl +H⁺+ Cl^– (Hydrolysis)

The hypochlorous acid (HOCl) further dissociates into hydrogen ions (H⁺ ) and hypochlorite ions (OCl^–) as indicated below.

HOCl ↔ H+ OCl^– (Ionization)

As per the enzymatic hypothesis, the hypochlorous acid (HOCl) and hypochlorite ions (OCl^– ) penetrate into the cell wall and react with the enzymes and protoplasm resulting in the end of life of the microorganism.

Hypochlorous acid (HOCl) is the more effective disinfectant, which is 80 to 100 times more destructive than hypochlorite ions (OCl^–). The sum of HOCl and OCl^– is called free chlorine residual. At pH value 5.5 chlorine exists in the molecular form if pH is between 5 to 9.5 HOCl and OCl^– are formed.

HOCl is generally formed at the pH value of 5 to 7. Chlorine water is unstable and maybe decompose rapidly during exposure to sunlight. The distribution of HOCl and OCl^–as a function of pH.

  1. Forms of Chlorination  

Depending upon the step of purification at which chlorine is added to water and also upon the desired result of the application of chlorine, which may be of the following forms;

1. Simple or plain Chlorination

2. Pre-chlorination

3. Post chlorination

4. Multiple or double chlorination

5. Breakpoint chlorination

6. Super chlorination

7. De-chlorination

a. Simple or Plain Chlorination  

In simple or plain chlorination only chlorine is added to raw water and no other treatment. and no other treatment is given and supplied to the consumers.

When raw water is relatively clear having low turbidities (not exceeding 10 NTU), plain chlorination has resorted to the usual dosage of 0.5 to 1 ppm.

b. Pre-chlorination  

In pre-chlorination chlorine is applied to raw water initially before any treatment. Chlorine is added before filtration sometimes maybe before sedimentation.

The dosage of chlorine should be such that residual chlorine of 0.1 to 0.5 ppm at the time of entering to filter.

Pre-chlorination may assist or reduce the burden to other units by; controlling the growth of algae in the sedimentation tank, reducing the bacterial load to filter, the interval of cleaning period of filter may prolong, coagulant dosage may reduce in sedimentation with coagulation, eliminating taste and odor from water.

c. Post chlorination

It is the application of chlorine to water after all treatments i.e. chlorine is added to water as it leaves filters and before entering the distribution system.

It is useful for protection against contamination in the distribution system and the dosage is adjusted about 0.1 to 0.2 ppm.

d. Multiple or Double Chlorination  

When raw water is highly contaminated and may contain a large number of bacteria, chlorine is added at two or more points in the purification process.

Pre-chlorination is adopted once before sedimentation and post-chlorination are in which chlorine is added after filtration and before the distribution system.

e. Breakpoint Chlorination

Chlorine and chlorine compounds by virtue of their oxidizing power first react with inorganic materials present in water before any disinfection is accomplished.

The chlorine then performs disinfects bacteria in the water. Subsequently, chlorine oxidized the organic matter present in the water.

Hence, as chlorine is added to water two actions take place;

i) kills bacteria present in water, thus disinfection is accomplished, and

ii) oxidized the organic matter present in water.

When chlorine is added to water and if water has no chlorine demand applied chlorine appears as residual chlorine hence the relation between applied and residual chlorine will be as indicated by line A, having a slope of 45°. However, water generally has some chlorine demand and the relation between applied chlorine and residual chlorine will be indicated by curve B.

As the chlorine is added first it performs the function of destroying bacteria and also readily reacts with oxidizable substances such as iron, manganese, nitrates, sulfides, and organic matter that may be present in water.

During this entire applied chlorine is utilized for killing bacteria and oxidizing substances that may present in water and there is no residual chlorine indicated as stage I.

After meeting immediate demand, the chlorine reacts with compounds such as ammonia, proteins, amino acids, and phenols that may present in water to form chloramines and chloro-derivatives which constitute the combined available chlorine.

At this II stage relation between applied chlorine and residual chlorine is represented by curve B and combined available chlorine is also recorded as residual chlorine.

At this stage increase in applied chlorine also increase residual chlorine and curve B further rises till point C where the amount of residual chlorine is recorded maximum.

Further application of chlorine, there is a sudden decrease in the residual chlorine due to the fact that the added chlorine break down chloramines by changing them to nitrogen compounds, thus reducing the residual chlorine, and also lots of applied chlorine is utilized in the oxidation of organic matter present in water.

This is stage III and is accomplished by the bad smell. This stage is represented by curve CD. At point D taste and bad smell suddenly disappears and oxidation of organic matter is also complete.

Point D on the curve is known as the breakpoint because any chlorine that is added to the water beyond this point breaks in the water and appears as residual chlorine.

The breakthrough, point chlorination of water may be defined as the point on the applied-residual chlorine curve at which all, or nearly all, the residual chlorine is free chlorine.

The application of chlorine in water, with chlorine dosage equal. to or slightly greater than that at of breakpoint occurs is breakpoint chlorination.

Hence, chlorine applied beyond point D results in an increase in residual chlorine as represented by line E the slope of which will be 45° so that the entire applied chlorine will appear as residual chlorine.

Advantages of breakpoint chlorination

1. It can remove manganese, taste, and odor.

2. It will have an adequate bactericidal effect.

3. It contains desired chlorine residual.

4. It can complete the oxidation of ammonia and other compounds.

f. Super chlorination

In super chlorination, chlorine is applied to water beyond the stage of breakpoint chlorination.

Normally a dosage of super chlorination maybe 2 to 3 mg/lit or 0.5 to 2 ppm after breakpoint chlorination. Super chlorination is adopted during the epidemic in a certain area to control the water-borne disease.

g. De-chlorination  

It is the process of removing excess chlorine from water. Excessive chlorine is to be removed from the water to avoid chlorine taste before distribution to consumers.

This may be attained by aeration of water or adding sodium thiosulphate, sodium meta bisulphate, sodium sulphite, ammonia, sulpherdioxde, etc to water.

  2. Factors Affecting the Efficiency of Chlorination  

1. Turbidity

2. Presence of metallic compound

3. Ammonia compounds

4. pH value of water

5. Temperature

6. Time of contact

7. Type, condition, and concentration of microorganisms.

a. Turbidity

Turbidity of the water influences disinfection primarily by influencing the chlorine demand. Turbid water tends to contain particles that react with chlorine, reducing the concentration of chlorine residual which is formed.

b. Presence of metallic compound

Metallic compounds such as iron and manganese utilize more chlorine for oxidation so to increase the effectiveness of chlorination metallic compounds should be removed.

c. Ammonia compounds  

The free chlorine can react with compounds such as ammonia, proteins, amino acids, and phenol to form chloramines and chloro derivatives which obey the combined chlorine path and which are less effective in disinfection.

d. pH of the Water  

Hypochlorous acid (HOCL) is formed effectively in the pH range of 5 to 7 and penetrates into the enzyme system of bacteria and killed them.

When the pH value of c water is less than 5 and greater than 10 generally chlorine remains in molecular form. Hence pH is a major factor that directly affects bactericidal removal efficiency.

e. Temperature of water  

A decrease in the temperature of water results substantial decrease in the killing power of both free and combined chlorine.

If the temperature is lowered, free availability is decreased and efficiency also decreased.

f. Time of Contact

The time period after the application of chlorine required to kill pathogens should be adequate for effective disinfection.

Generally, for disinfection by free chlorine, a contact period of 30 minutes is required while it takes twice of this for a combined chlorine path.

g. Type, Condition, and Concentration of Microorganisms

Bacteria and viruses are microorganisms that commonly occur in water. The enteric pathogenic bacteria are less resistant to chlorine than E-coli bacteria and viruses are more resistant hence dosage and contact time differ with type, condition, and concentration of microorganisms.

This was for the Forms of Chlorination.