Numerical simulation of the transport and dispersion of powdered activated carbon in raw water transmission pipelines

Powdered activated carbon (PAC) dosing in raw water transmission pipelines is a promising emergency strategy for enhancing pollutant control before conventional treatment. In this study, a three-dimensional CFD-DPM model was established to investigate the transport and dispersion of PAC particles in a pressurized pipeline. The effects of velocity ratio, particle density, and injection position were evaluated systematically. The results showed that the velocity ratio v*, defined as the ratio of injection velocity to main-flow velocity, was the dominant factor governing PAC dispersion. As the velocity ratio increased, the particle distribution evolved from local aggregation to cross-sectional spreading and then to relatively sufficient mixing. When v* < 1, PAC particles remained concentrated near the injection zone, resulting in limited dispersion. When 1 ≤ v* ≤ 2.5, cross-sectional spreading increased significantly. At v* = 5.0, PAC particles occupied a large proportion of the pipe cross-section within a short downstream distance, and the dispersion ratio stabilized at approximately 50%. In contrast, particle density and injection position had only minor effects on the overall dispersion pattern, mainly influencing local spreading and the initial cross-sectional distribution.