Control of trichloramine formation by two-step chlorination in water purification processes

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.