Faecal Coliform Association with Settleable Suspended Sediments of Different Sizes in Surface Waters

This study aimed to assess how particle size and sediment concentration influence the association of faecal coliforms (FC) with suspended sediments in surface waters. Suspended sediments (<62 μm) were fractionated into four size classes and incubated with FC suspensions. The observed attachment behavior was well captured by the linear partition model, with particle size identified as the dominant controlling factor. Specifically, the mass-normalized associated FC concentration increased markedly as particle size decreased; however, when normalized by specific surface area, this size dependence disappeared. Based on these findings, a formula linking the partition coefficient to particle size was derived and robustly validated. Furthermore, the particle-associated fraction increased significantly with both higher sediment concentrations and finer particle sizes. Collectively, these results demonstrate that the proposed formula effectively captures the influence of particle size on FC–sediment interactions. At the same time, the associated fraction is jointly governed by sediment concentration and the size-dependent partition coefficient.