Chlorination Process

Manual control chlorinator , source:Wikimedia Commons

Chlorination is one of the methods used in the treatment of water, and it is simply the addition of chlorine into water for the purpose of killing bacteria and other viruses in drinking water.
Water is disinfected to destroy the pathogenic organisms. Micro organisms are removed from water in various degrees during the processes of settling, addition of chemicals and filtration, but to make the water safe for human consumption, it must be disinfected.

Apart from pathogen destruction, chlorine destroys taste and odour producing compounds, algae and related water blooms. Chlorine also helps in the oxidation of iron, manganese and Hydrogen sulphide . One of the major problems associated with the use of chlorine is its formation of by-products which are carcinogenic.

Chlorine combines with water to form hypochlorous and hydrochloric acids. Hydrochloric acid is neutralized by the alkalinity in the water. Hypochlorous acid becomes dissociated. Now let's look at how chlorine is been applied.

Application of Chlorination

The application of chlorination is mainly on the the control of fungi and slime producing bacteria in pipelines and other water conduits, improvement of coagulation of water and wastewater, conversation of cyanides into cyanates, corrosion control brought about by the destruction of Hydrogen sulphide , BOD reduction by oxidation of organic substances and finally bacteria reduction.

I urge you to read carefully, as I will explain the various methods of chlorination as well as the quantity of water volume and measurement of chlorine to add.

Methods of chlorination

Pre-chlorination, post -chlorination, break-point chlorination and super chlorination as well household disinfection are the methods of chlorination. I will take the aforementioned methods of chlorination one after another as to throw more light on them.

Pre-chlorination is simply the addition of chlorine directly to raw water . Prechlorination improves coagulation and reduces taste and odour by oxidizing organic matter. It also helps to control algae and other organisms which clog filters by destroying them. The dose should be such that prior to filtration, a residual of 0.1 to 0.2 mg/l should be present. Although pre-chlorination is advantageous, it does not eliminate the need for post-chlirination.

In the case of post-chlorination, chlorine is added to the filtered water. The dosage depends on the nature of the water and the contact period required (i.e time required for the chlorine to be in contact with water). The dosage is usually 2 to 3mg/l combined chlorine or 0.2 to 0.5 mg/l free available chlorine for a contact time of above 30 min.

Chlorine demand of water depends on the organic and inorganic substances present. As the dose of chlorine is increased, combed available residual also increases. This will continue until the residual starts decreasing, indicating the oxidation of the chloroorganic compounds. When the oxidation is complete, there will be a further rise in the chlorine residual, and chlorine will be in the form of free available chlorine. The point at which this takes place is called the "break point " since free residual chlorine appears beyond this point. The advantage of chlorination at and beyond break point are that all odour and taste disappear and a high germicidal effect is achieved.

Addition of larger dosages are involved in super chlorination method. This is normally done during epidemics. The factors that affects chlorination are organic matter present, pH value of water, amount of carbon dioxide, temperature, time of contact, amount of chlorine added and presence of various chemicals in the water which react with chlorine.

It is often necessary to disinfect household water especially in rural areas where the source may be of uncertain quality.

Chlorine products suitable for household disinfection are sodium hypochlorine solution (liquid bleach), chlorinated lime (bleaching powder), as calcium hypochlorite (which are used in swimming pools). Application rates of 1 mg/l and 10 mg/l of liquid bleach is effective for clear water and water drawn directly from river or stream. Since chlorine products gradually degenerates once opened, liquid bleach should be bought in small bottles. Powder or tablet chlorine need to be dissolved in small quantities of water before adding to large containers of water.

Use of liquid bleach (5.25% sodium hypochlorite) for water disinfection

For a 50 litre volume of water, add 1 mg/l of available chlorine ,for 10 mg/l of available chlorine add 10.

For a 100 litre volume of water add 2 mg/l to provide 1 mg/l of available chlorine and to provide 10 mg/l of available chlorine add 20 mg/l.

For a 500 litre volume of water add 10 mg/l to provide 1 mg/l of available chlorine and to provide a 10 mg/l of available chlorine add 100 mg/l.

For a 1000 litre volume of water, add 20 mg/l to provide 1 mg/l of available chlorine and to provide 10 mg/l of available chlorine add 200 m/l.

Making a one Percent Solution

For sodium hypochlorite ( liquid bleach ) product concentration by weight of 5% . Amount of water per litre will be 200 ml .

For sodium hypochlorite (liquid bleach) product concentration by weight of 10%. Amount of water litre will be 100ml.

For chlorinated lime (bleaching powder) product concentration by weight of 5%. Amount of water per litre will be 200 g.

For calcium hypochlorite (powder) product concentration weight by 70%. The amount of water per litre will be 14 g.

How to use one Percent Chlorine Solution for Water Disinfection

For a 50 litre volume of water, add 25 mg/l to provide 1 mg/l of available chlorine and to provide 10 mg/l of available chlorine add 250 mg/l.

For a 100 litre volume of water, add 50 mg/l to provide 1 mg/l of available chlorine and to provide 10 mg/l of available chlorine add 500 mg/l.

For a 500 litre volume of water, add 250 mg/l to provide 1 mg/l of available chlorine and to provide 10 mg/l of available chlorine add 2500 mg/l.

For a 1000 litre volume of water, add 500 mg/l to provide 1 mg/l of available chlorine and to provide 10 mg/l of available chlorine add 5000 me /l.

Conclusion

The essence of chlorination is
to kill germs in water and prevent the spread of
typhoid, cholera and dysentery which are waterborne diseases.
Chlorination is a reliable method of water treatment. It is simple and affordable and very easy to use.
As a rule of thumb, I will advise if the water smells strongly of chlorine during the treatment process , then it has been over treated and should be left untouched for a few days until some of the chlorine is dissipated.

Reference

[1] Water chlorination
[2 ] SSWM chlorination
[3 ] Water Professionals-chlorination
[ 4] Chlorination of Drinking Water
[5 ] Drinking Water Treatment - Chlorination

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