We investigated the reducing rate of manganese oxides (MnOs) and organic matter in water by using pre-oxidation and coagulation/sedimentation methods with different chemicals. The reduction rate using NaOCl for organic matter was about 11%, while that of manganese was 12%. The reduction rate using chlorine dioxide (ClO₂) was only 7% for organic matter. However, the rate of manganese was 29% when using ClO₂. Potassium permanganate (KMnO₄) removed organic matter and MnOs more effectively, with a rate of 18 and 71%. Moreover, aluminum sulfate (Al₂(SO₄)₃), ferric chloride (FeCl₃), and polysilicate iron (PSI) worked more effectively, with a reduction rate of 99% and 55% for turbidity and organic matter.