Odor produced by breakpoint chlorination in a drinking water purification process was researched. Ammonia and six amino acids (glycine, alanine, valine, leucine, arginine, proline) were chlorinated and production of odor and its change were investigated. An intense odor which was different to the odor of residual free chlorine was detected after the chlorination of ammonia. The production of the odor was affected by pH and chlorine dose rate. While the intense odor was not produced in the breakpoint chlorination at pH 8.3, a weak odor before the breakpoint and the intense odor after the breakpoint were produced at pH 6.5. At pH 3.0, the intense odor was detected even in a sample of low chlorine dose rate. An identification of the intense odor substance was done using a gas chromatograph-mass spectrometer and trichloramine or a dimer of dichloramine was suspected as the odor causing substance. In chlorination of the six amino acids, the intense odor and its change were different according to each amino acid. Production of chlorinated intermediate and final products which had a different odor were suspected as one of the reasons.

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