Chlorine Consumption by Suspended Solids Fractionated Based on Settleability Using Sequential Centrifugal Force and Dissolved Organic Matter

Suspended solids (SS) and dissolved organic matter (DOM) in source water consume chlorine during disinfection, yet how this varies among SS fractions with different settling characteristics remains poorly understood. This study investigated chlorine consumption by SS fractions from Kiso River water using sequential centrifugation based on settleability. Raw water was fractionated into five settled SS fractions (F1–F5), an unsettled fine SS and DOM (W5), and a DOM fraction (W6) from 0.2 µm filtration of W5. Fractions F1–F3 reached complete consumption within 12 h, whereas F4 and F5 showed slower, sustained consumption over the full 72 h period. In contrast, W5 and W6 showed minimal chlorine consumption. The rapid and subsequent slower consumption phases were described by first-order and zero-order models, respectively. Normalization by particle number, total particle surface area, and turbidity showed that lower-settleability particles exhibited higher apparent chlorine reactivity per particle despite lower particle number and total particle surface area. Rate parameters normalized by dissolved organic carbon (DOC) concentration showed lower apparent DOM reactivity per unit DOC. These results demonstrate that particle settleability influences chlorine consumption and highlight particulate-associated constituents as important contributors to chlorine demand, with implications for prioritizing removal of fine, low-settleability particles in treatment.