In-line initial mixing has been proposed for the rapid and complete dispersion of a coagulant within a second before the metal hydroxide precipitate has formed. Pump diffusion flash mixing (PDFM) is an effective device for the quick dispersion of hydrolyzing metal salts. In this study, attempts were made to understand the removal characteristics of natural organic matter (NOM), which was influenced by the initial type of mixing for chemical coagulation and; thereby, propose an appropriate method for coagulant dosage optimization for the operation of PDFM, and reduce the disinfection by-product formation potentials (DBPFPs) due to NOM. The dissolved organic carbon (DOC) and turbidity were more effectively reduced by PDFM than by conventional rapid mixing (CRM), due to the rapid and uniform dispersion of coagulant. Although the hydrophilic fraction in haloacetic acid precursor sites is enhanced compared to the hydrophobic fraction, which comprises a major faction of the bulk NOM from Han River water, the hydrophilic fraction was more effectively removed by PDFM than CRM. Therefore, PDFM is an effective and innovative process for the reduction of DBPFPs, which also requires a lower coagulant dosage than CRM. When PDFM is used for the initial mixing, originative method is needed to decide on an optimal coagulant dosage as that determined by a jar-test results in overdosing. The optimal coagulant dosage point in the operation of PDFM can be decided using a streaming potential detector (SPD). (c) 2006 Published by Elsevier B.V.