Application of Ozone to water supplies for disinfection
Disadvantages- Ozone gas is a faint blue gas with pungent odour
- It is an unstable allotropic form of oxygen with each molecule consisting of three oxygen atoms
- It is produced by passing high tension electric current through a stream of air in a closed chamber.
- Because of its instability, ozone readily breaks down into normal oxygen and releases nascent oxygen
- The nascent oxygen so produced is a powerful oxidizing agent and removes organic matter as well as bacteria from water.
- During treatment, ozone gas is manufactured and bubbled through water in a sterilizing chamber and ozonized water is removed through the outlet at the top.
- Approximately 2 to 3 ppm of ozone is generally consumed to obtain a residual ozone of 0.1 ppm after a contact period of ten minutes
- Ozone and chlorine are competitive disinfectants. Chlorine is preferred and universally accepted.
- Since ozone is unstable, nothing remains in water by the time it reaches the distribution system
- Ozone removes colour, taste and odour from water, in addition to removal of bacteria
- Ozonised water is tasty and pleasant unlike chlorinated water which becomes bitter to taste
- It is expensive when compared to chlorination
- Ozone needs electricity for its manufacture and hence it can be used only in places where electricity is available easily and for a low cost
- No residuals can be maintained because it is highly unstable and its use does not ensure safety against possible future contamination
- Using ozone as a disinfectant requires complicated ozone manufacturing apparatus called ozonizer to be installed at the the treatment facility since ozone cannot be supplied in cylinders like chlorine
- Ozone is not as efficient in disinfection as chlorine
- Ultra-Violet (UV) rays are invisible light rays having wave lengths of 1000 to 4000 micro-meters.
- They are found in sunlight but can also be produced by passing electric current through Mercury enclosed in quartz bulbs.
- Mercury vapour lamps enclosed in quartz bulbs can therefore be used as good source of such rays.
- UV rays are highly effective in killing all types of bacteria thereby yielding truly sterilised water
- Water to be treated with UV rays should be less turbid annd low in colour. Preferably the water should be colourless and turbidity should not exceed 15 mg/l.
- In the process of using UV radiation to achieve disinfection, water is passed several times around quartz bulbs emitting UV rays.
- Since the effective depth of penetration of UV rays is around 10cm, the depth of water over the bulbs should not exceed 10cm for efficient disinfection
- Sterilization with UV rays does not impart any additional taste or odour to water since no chemicals are added.
- This method is used extensively for treatment of small quantities of water supply in hospitals and dispensaries for surgical purposes or drinking water purification units installed in homes and offices
- It is a very expensive process and requires technical know-how
- Possibility of interruption in process due to failure of electricity
- It is not feasible to treat large scale water supplies
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