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Disinfection

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It may be defined as the process of killing pathogens from water, and making it safe for drinking. The filter water still contains some impurities such as bacteria, dissolved inorganic matter, colour, odor, iron, manganese etc. Therefore it is essential to disinfect the water in order to prevent the contamination.

 Criteria of good disinfectant:-

  1. Be capable of destroying the pathogens within the available contact time.
  2. Should not leave reaction products which render water toxic or impart colour.
  3. Be readily available at a reasonable cost.
  4. Be added in such proportions that leaves some residual concentration for protection against contamination.

Methods of disinfection:-

  1. Boiling method (heating of water at 1000C and more for 10-15 minutes to kill pathogens )
  2. Light method (Penetration of sunlight or UV rays for disinfection)
  3. Chemical methods using iodine, bromine, KMNO4, copper and silver ions ozone in gaseous form is also used as an excellent disinfectant.
  4. Chlorination: - It is an universally accepted method. Application of small quantity of chlorine or chlorine compounds to water is called as chlorination. The amount of chlorine required is called as chlorine demand.

 

Action of chlorine:-     Cl2 + H2O --------- HCl + HOCl

                                    HOCl       -----------HCl + O (nascent)

                                    HOCl      ------------H + OCl – (reversible reaction)

Disinfection by chlorine is rapid if pH is less than 7.0 and is less effective if pH is greater than 8.5. It is essential to keep dissociation of HOCl to minimum. HOCl ions are more effective than OCl ions. Therefore it is common practice to add ammonia which forms chloramines.

NH3 + HOCl --------    NH2Cl  + H2O

 pH >7.5                      ( Mono chloramine)

                                    NH2Cl + HOCl -------NHCl2 + H2O

                                    pH – 5-6.5                   (di chloramines)

                                    NHCl2 + HOCl--------- NCl3 + H2O

                                    pH < 4.4                      Nitrogen tri chloramine

 

The dosage of chlorine depends on the quality of water and shall be determined in the laboratory.

 

Types of Chlorination:-

  1. Plain chlorination: - When only chlorine treatment is given to raw water the process is called as Plain chlorination. This method may be used for treating clean  water obtain from reservoirs, lakes, etc. and amount of chlorine required is 0.5ppm.
  2. Pre chlorination: - It is the application of chlorine to water prior to any unit treatment process to control the biological growth in raw water conduits. This would also help when the water quality is poor.
  3. Post chlorination: - Application of chlorine at the end of water purification and before the distribution system. The dose should be such that 0.2 – 0.3 ppm of residual chlorine should be present after a contact period of 20 min.
  4. Double chlorination: - when chlorine is added to raw water at more than one point the process is called as double chlorination.
  5. Dechlorination: - The process of removing excess chlorine from water is called              Dechlorination. Dechlorination agents such as Sodium Disulfate, Sodium Thiosulfate, Sodium Sulfate, Potassium Permanganate, Sulphur Dioxide, and activated carbon are use for this purpose. It can be also achieved by aeration.
  6. Super chlorination: - Application of chlorine beyond break point is called Super chlorination. The water contains excess of free available chlorine which must be removed before it becomes acceptable to consumers. The dose is generally about 0.5ppm to 2.0ppm. It is generally added at the end of filtration and useful if epidemics are spread.
  7. Break point chlorination: - It indicates the amount of chlorine to be added to water. It represents the dose of chlorine beyond which any more addition of chlorine will appear as free residual chlorine. Chlorine, when added to water reacts with ammonical compounds present in it and form chloramines. The fall in residual chlorine will continue with further increase of chlorine dose and after a stage the residual chlorine begins to increase in proportion to the dose of chlorine added. This point at which the free residual chlorine appears is the break point and corresponding dosage is break point dosage.

 

 

Advantages of Break point chlorination:-

a)      It removes taste and odour from water.

b)      It prevents growth of weeds in water.

c)      It destroys all the pathogenic bacteria.

d)     It oxidizes the impurities of water.

e)      It also removes the colour of water imparted by organic matter.

 

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