Break Point Chlorination (exp:13) - assignment - Q-A

Break Point Chlorination

(1) Q: Why chlorination is necessary?
Ans: chlorination is necessary for any treatment of domestic water because , the chlorination of supplies and polluted water serves primarily to destroy or deactivate disease producing microorganisms. A secondary benefit is the overall improvement in water quality resulting from the reaction of chlorine with ammonia, iron, manganese, sulfide and organic substances.
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(2) Q: What is dechlonation and why it is necessary?
Ans: This is required when residual chlorine is so high that water is rendered aesthetically or otherwise objectionable. Dechlorination is the partial or complete reduction of residual chlorine in water by chemical or physical means, e.g. SO2, activated carbon or aeration.
SO2 +Cl2 + H2O = 2HCl +H2SO4
C + Cl2 + H2O = HCl +CO2
Dechlonation is necessary because, chlorination may produce adverse effects. Taste and odor characteristics may be changed, phenol and other organic compounds present in water supply may be intensified. Combined chlorine formed on chlorination of ammonia or amine bearing waters adversely affects some aquatic life. To fulfill the primary purpose of chlorination and to minimize any adverse effects, it is essential that proper treatment procedures be used with a foreknowledge of the limitation of analytical determination.
(3) Q: What is Break point chlorination?
Ans: Some free available chlorine is desired as residual in water supply system to safe guard the water from the water from any contamination that may occur through leaky joints or cracks in the pipes. Accomplishment of the desired residual chlorine depends upon the characteristics of water.
If the water is free of ammonia or other compounds that may reacts with chlorine, the application of chlorine will yield free available chlorine of same concentration. This is denoted by no demand or zero demand line.
If the water contain only such inorganic ,matter like H2S, Fe, Mn etc, that reduces chlorine then an application of chlorine would yield residual of less concentration than that applied. The bellow the zero demand line denotes this.
If the water contains ammonia that results in the formation of co,bined available chlorine residual, it must be destroyed before a free chlorine residual is obtained. This destruction is brought about by application of excess chlorine. Initially, when molar ratio of chlorine to ammonia remains bellow 1, formation of monochloramines predominates with only a small consumption of the total available chlorine. As the dosage of chlorine is increased the production of dichloramiens is enhanced with corresponding decomposition to nitrogen with compensating consumption of available chlorine and formation of hydrogen chloride. The probable reaction in the destruction of ammonia or chloramines are:
2NH3 +3Cl2 =N2 + 6HCl
4NH2Cl +5CL2 +H2O =N2 + N2O +10HCl
2NHCl3 + HOCl + H2O = 2NO2 +5HCl
at the end of destruction of the chloramines N2, N2O, NO2 etc are mainly gaseous products. HCl being produced as the pH gradually decreases with the application of chlorine and its consumption in the destruction of ammonia decreases till a minimum residue is reached. Amount of chlorine used to reach the minimum free available chlorine residue is called “Break point dose” or “break point chlorination”. At this point the curve bends to a minimum value. Further chlorination above this point gives as equal amount of free available chlorine and the curve become parallel to “no demand line”.

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