Dramatic changes in trihalomethanes of tap water caused by decreasing the chlorine dose from to break-point to zero at a pre-chlorination could be found during several years from starting the advanced treatment in Osaka Prefectural water utilities (from 1998 to 2002). In 1998 (break-point chlorination), chloroform was predominant (an average of 14.3 microg/L) and bromodichloromethane (5.96 microg/L), dibromochloromethane (4.19 microg/L) and bromoform (1.06 microg/L) were in the decreasing order. In contrast, in 1999 (chloramination), dibromochloromethane and bromodichloromethane were abundant trihalomethanes (6.61 and 5.72 microg/L) and followed by chloroform (4.14 microg/L) and bromoform (1.41 microg/L). Trihalomethane distribution in 2002 (non-chlorination) was almost the same as that in 1999.
