Disinfection: Necessity and methods

DISINFECTION

• Filtered water normally contains harmful disease causing microorganisms. These microorganisms should be killed to make water safe for drinking. The chemicals used for killing these microorganisms are called disinfectants and the process is known as disinfection or sterilization.
• The process of disinfection is important as consumption of contaminated water may cause spread of communicable diseases and lead to outbreak of an epidemic. The process of disinfection of water is evident by the fact that raw water can be supplied for consumption without complete treatment and simple disinfection.
• Disinfection not only removes microorganisms from water at the plant but also ensures killing them even at a later stage in the distribution system. Chlorine is the most commonly used disinfectant throughout the world.

Minor methods of disinfection are:

• Boiling of water
• Treatment with excess lime
• Treatment with ozone
• Treatment with iodine and bromine
• Treatment with ultra-violet rays
• Treatment with potassium permanganate
• Treatment with silver or Electro-katadyn process

• Bacteria in water can be destroyed by boiling it for a long time. However, it is not possible to boil huge amounts of water for public water supplies. Moreover, it kills bacteria only for that instant but cannot take care of future contamination. In case of water borne epidemics, the public is advised to consume water only after boiling
• Lime is usually used for water softening. However, excess lime raises the pH of water making it extremely alkaline which kills 99.5 to 99.9% bacteria. However, excess lime from water must be removed before supplying it for consumption by techniques like recarbonation. Even this method cannot protect water from possible dangers of recontamination
• Ozone is a faint blue gas of pungent odour and an excellent disinfectant.Ozone is extremely unstable and breaks down into oxygen and nascent oxygen. Nascent oxygen is a powerful oxidising agent and removes organic matter and bacteria from water.
• Addition of iodine or bromine to water helps by killing pathogenic bacteria. Quantity of these disinfectants is limited to 8 ppm and contact period of 5 minutes. These disinfectants are available as pills and not used for treating large scale public supplies but may be used to treat small water supplies
• Treatment of water with ultra-violet (UV) rays is an effective method for disinfection. UV rays are found in sunlight but can also be produced by passing electric current through mercury enclosed in quartz bulbs.These rays are highly effective in killing all microorganisms, thereby yielding truly sterilized water. This method does not impart taste or odour to water as no chemicals are added. It is very expensive and not used extensively
• Potassium permanganate is a popular disinfectant. In addition to killing microorganisms it also helps in oxidising taste, killing algae, removing colour and iron from water. Addition of Potassium Permanganate to water produces pink colour. Disappearace of pink colour indicates presence of organic matter. Potassium Permanganate as a disinfectant is cheap, handy and useful. However, it is of limited use against disease causing microorganisms. Moreover, with passage of time, it produces a dark brown precipitate. Due to these reasons, it is not satisfactory and not recommended.
• The electro-katadyn process of disinfection introduces metallic silver ions in water by passing it through a tube containing solid silver electrodes connected to D.C. (1.5V). The introduced silver ions have a strong germicidal action and thus act as a disinfectant. As the method involves using silver, it is prohibitively expensive and not used for treating water supplies.

Chlorination

• Chlorine in its various forms is universally used for disinfecting public water supplies.
• It is:
• Cheap
• Reliable
• Easy to handle
• Easily measurable
• Capable of providing residual disinfecting effects for long periods ensuring complete protection against future recontamination of water in distribution system.
• Disinfecting action of chlorine
• Formation of hypochlorous acid kills most microorganisms
• Free available chlorine (Hypochlorous acid) is extremely effectrive at pH slightly less than 7
• Chlorine immediately react with ammonia in water to form various chloramines
• Chloramines formed are stable and posses disinfecting properties
• When added chlorine has consumed all ammonia available in water, it appears as free chlorine
• Combined chlorine with ammonia in the form of chloramines is called combined chlorine