Chlorination of water treatment equipment  is practiced for the purposes listed above and the various needs may he satisfied simultaneously. Chlorine is classified according to its point of application in water treatment equipment and its end result.

1.  Water Treatment plain Chlorination
2.  Water Treatment pre Chlorination
3.  Water Treatment Post Chlorination

Water Treatment Plain Chlorination

In some cities surface waters are used with no other treatment than chlorination, although in some of these cities long storage is also given. In such cases, chlorination is extremely important as the principal if not the only safeguard against disease. Such otherwise untreated waters are likely to be rather high in inorganic matter and require high dosages and long contact periods for maximum safety. The chlorine may be added to the water in the pipe leading from an impounding reservoir to the city. For disinfection alone a dose of 0.5 ing/I or more may be required to obtain a combined available residual in the city distribution system.

 Water Treatment Pre Chlorinarian

This is the application of chlorine before any other ltreatment. The chlorine may be added in the suction pipes of raw-water pumps or to the water as it enters the mixing chamber. Its use in this manner has several advantages. It may improve coagulation and will reduce tastes and odors caused by organic sludge in the sedimentation tank. By reducing algae and other organisms it may keep the filter sand cleaner and increase the length of filter runs. Its range of effective action will, of course, depend upon the maintenance of a residual through the units of the plant. Frequently the dosage is such that a combined available residual of 0.1 to 0.5 mg/1 goes to the filters. The combination of prechlorination with fmstchlorination may be advisable or even necessary if’ the raw water is very highly polluted.

Water Treatment Post Chlorinarion

 This usually refers to the addition of chlorine to the water after all other treatments. The chlorine may be added in the suction line of the service pump, but it is preferable to add it in the filter effluent pipe or in the clear well so that an adequate contact time will be assured. This should be at least 30 minutes before any of the water is consumed if only postchlorination is given. Dosage will depend upon the character of the water and may be 0.25 to 0.5 mg/I in order to obtain a combined available residue of 0.1 to 0.2 mgil as the water leaves the plant. Greater residuals will probably be needed if it is desired to hold a disinfecting effect throughout the distribution system. This is considered desirable since it affords protection against contamination front cross connections and prevents organic growths in mains and their resulting odors.

Disinfect Water with  Chlorine in Water Treatment Process
For our domestic and industrial purpose, need water which is safe, clear and free from disease-causing micro organisms. They also desire and necessary use of water which is soft, free from unpleasant smell, tastes and odors, and does not discolor pipe and plumbing equipments or corrosion metals. Industry required treatment water that will not interfere with its processes. Recently there has been increasing concern about the presence of minute quantities of organic material, particularly chlorinated hydrocarbons, which are thought to be causative agents of a variety of diseases. Such contaminants are known to be present in main Water Treatment main supplies although their effect upon health is unknown. Standard disinfection water treatment equipment practice using chlorine contributes to the production of these compounds.

Chlorine in Water

Disinfection of water is the killing of disease-causing microorganisms that it may contain. In the water treatment  process, bacteria are  reduced in number. Complete sterilization, however, is not ordinarily obtained nor necessary. Chlorine in its various forms has been widely used in disinfecting water in Swimming water treatment, drinking water treatment and waste water treatment fields. It is cheap, reliable and presents no great difficulty in handling.

Chlorine is a very active element and when added in  water Treatment as free chlorine it will combine with organic and inorganic matter and oxidize some organic and inorganic compounds. Free available chlorine reacts with ammonia and many organic amines to form chloramines. The chlorine in water in chemical combination with ammonia or other nitrogenous compounds which modify its rate of bactericidal action is known as the combined available chlorine. The chlorine demand of water is the difference between the amount of chlorine present as a residual, either free or combined, after some designated period.

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