Abstract: Sediment flocculation in water significantly impacts river dredging and water environment improvement; accelerating sediment flocculation through artificial interventions is of great importance for water treatment and environmental enhancement. This study focuses on the characteristics of bubble motion during electrocoagulation and its effects on sediment flocculation. We design a laboratory experiment and use the particle image velocimetry (PIV) technology to continuously observe the movement of bubbles under 9 different current densities, collecting 5,500 image frames. Sediment particle size and solution turbidity are observed under various conditions, with 4,860 particle size data and 162 turbidity data collected. Comparative analysis is made on sediment particle size variations and solution turbidity evolution under different conditions. The results indicate bubble-driven convection is a crucial mechanism for flow field evolution and flocculation during electrocoagulation. At low electric current densities (e.g., 10 A/m2), the bubble generation rate is relatively low, resulting in a stable flow field that favors floc stability. However, excessively high current densities (e.g., 50 A/m2) can enhance flow field turbulence, disrupting the flocs formed previously and reducing flocculation efficiency. The number and distribution of vortices significantly influence the formation and growth of flocs. The ratio D/V is demonstrated to be an important indicator of flocculation effectiveness, with both D/V and removal efficiency reaching their peaks at a current density of 40 A/m2. Flotation and flocculation settling are the main factors for turbidity reduction, with a critical particle size of 37.2 μm. This study reveals the impact of flow field disturbances caused by bubble motion on flocculation efficiency, laying a basis for further optimizing electrocoagulation performance.

Key words: sediment flocculation, electrocoagulation, bubble motion, particle image velocimetry (PIV), particle size, turbidity, flocculation effect