Chlorinous odour in drinking water is of great concern in Japan. Some water utilities use trichloramine (NCl3) as an index of chlorinous odour and are attempting to control its levels in drinking water. In the present study, the effects of two-step chlorination, involving addition of chlorine twice, on NCl3 control were investigated. The results regarding ammonia (NH3), glycine solutions and raw waters at water purification plants (WPPs) indicated that NCl3-formation potentials (FPs) were reduced by two-step chlorination when NH3 was a primary NCl3 precursor and the 1st chlorine addition was set at an excess breakpoint (BP). However, no effect on NCl3-FP was observed when the 1st chlorine addition was set below BP. Two-step chlorination was not effective for NCl3 control regardless of the amounts of the 1st chlorine addition when organic nitrogen compounds were the primary NCl3 precursors. Moreover, the NCl3-FPs in raw water with relatively high NH3 were reduced at actual WPPs when two-step chlorination was applied.

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