Dissolved organic matter that affects the increase in disinfection by-products (DBPs) in Oita City was suggested to be caused by the forests and paddy fields in the Oita River basin. Multiple regression analysis of DBPs concentration confirmed that E260 contributed significantly. As a result of examining this point, a highly predictive model of trichloroacetic acid concentration at the edge of the water supply area was developed from the UV value of the filtered water of the water purification plant. By examining the conditions for reducing the UV value of filtered water in water purification treatment, it was confirmed that the preferential use of acid to reduce the UV value of filtered water may lead to higher chemical costs. In addition, it was also confirmed that overinjection of 20 mg/L of coagulant works well or if further reduction is required, pH adjustment with acid and combined use of 5 mg/L of powdered activated carbon were cost effective. As a result of reflecting these conditions in the water purification plant, DBPs met the water quality standards and chemical costs were reduced.

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