Magnetic-hydraulic synergy intensifies coagulation: Toward shear-resistant flocs for enhanced water quality and sludge minimization

Most magnetic coagulation (MC) studies employ static sedimentation (during settling, no inflow/outflow in the tank, thus no hydraulic shear). However, engineering practice almost exclusively employs dynamic sedimentation (continuous inflow/outflow causing hydraulic shear), making static-based findings poorly transferable to practice. This study developed a continuous-flow MC process integrating dynamic sedimentation with magnetic field regulation. MC without a magnetic field (MC-NM) and MC with a magnetic field (MC-M) were compared with conventional coagulation (CC) under both static and dynamic conditions. MC-NM outperformed CC by more significant margin under dynamic conditions than under static conditions: turbidity removal increased by 17.55% (vs. +2.81% static), median floc size (D 50 ) increased by 29.63 μm (vs. +27.49 μm), floc shear resistance by 18.67% (vs. +22.37%), and sludge settling ratio decreased by 9.84% (vs. −6.94%). Optimal performance was achieved with MC-M under dynamic conditions at a magnet-beaker distance of 8 cm, yielding effluent turbidity of 0.99 NTU and a maximum D 50 of 102.46 μm (indicating possible chain-like flocs). Multimodal characterization (Fourier transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy with energy dispersive X-ray spectroscopy) indicated the likely formation of magnetic core-shell flocs. Vibrating sample magnetometry showed excellent magnetic particle reusability, with only a 7.09% loss in saturation magnetization after use. The results reveal that MC-NM outperforms CC under dynamic conditions owing to hydraulic-magnetic synergy, which forms more shear-resistant and faster-settling flocs while reducing sludge volume. These findings offer theoretical guidance for designing compact, sludge-minimized MC systems.