Health Hygiene & Sanitation

Lesson 26 : Water Purification

Purification Of Water On A large Scale


Criteria of potential disinfectants for water supplies:

  • capacity to destroy pathogenic organisms present
  • not influenced by physical and chemical properties of water (temperature, pH and mineral constituents)
  • No residual products of reaction which render the water toxic or impart colour and make it unpotable
  • Ready availability at reasonable cost
  • Possess the property of leaving residual concentration to deal with possible recontamination
  • Be amenable to detection by practical, rapid and simple analytical techniques in the small concentration ranges to permit the control of the efficiency of the disinfection process.


Chlorination is used synonymous with water purification. It is supplement to sand filtration. Chlorine kills pathogenic bacteria but it has no effect on spores and certain viruses like polio, viral hepatitis except in high doses.

Action of Chlorine:

When chlorine is added to water, there is formation of hydrochloric and hypochlorous acids. The hydrochloric acid is neutralized by the alkalinity of the water. The hypochlorous acid ionizes to form hydrogen ions and hypochlorite ions, as follows:-

Hp + CI2 = HCI + HOCI

HOC! = H + OC!

Methods of Chlorination

To disinfect large bodies of water, chlorine is applied as 1) chlorine gas (2) chloramine or (3) perchloron. Chlorine gas is cheap, quick in action, efficient and easy to apply.
Demerit of chlorine as a disinfectant is the recent discovery that chlorination can result in the formation of many "halogenated compounds" some of which are either known or suspected carcinogens, hence other agents are tried as disinfectants.

Other Agents of Disinfection:

bromine, bromine-chloride, iodine and chlorine dioxide. But ‘ozone’ and ‘ultra-violet irradiation' are better.

  1. Ozonation: Ozone is a relatively unstable gas. It is a powerful oxidizing agent. It eliminates undesirable odour, taste, colour and removes all chlorine from the water. Most importantly, ozone has a strong virucidal effect. It inactivates viruses in a matter of seconds. This has prompted many municipalities to consider ozone for potable water treatment.
  2. The drawback of ozone is that after it has done its job, it decomposes and disappears. There is no residual germicidal effect. The ozone dosage required for potable water treatment varies from 0.2 to 1.5 mg per litre.

  3. Ultraviolet irradiation: UV irradiation is effective against most microorganisms known to contaminate water supplies including viruses.

  4. The method involves exposure of a film of water upto about 120 mm thick to one or several quartz mercury vapour arc lamps emitting ultraviolet radiation at a wavelength in the range of 200 to 295nm. The water should be free from turbidity and suspended or colloidal constituents for efficient disinfection.

    The advantages are that the exposure is for short period, no foreign matter introduced and no taste and odour produced. Over Exposure does not result in any harmful effects. The disadvantages are that no residual effect is available and there is a lack of a rapid field test for assessing the treatment efficiency, and the required apparatus is expensive.

Last modified: Wednesday, 25 April 2012, 9:39 AM