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To analyze the effect of flow characteristics on electrochemical water softening, characteristics of flow fields in the vicinity of vertical plate electrodes in a bench-scale electrolysis cell for electrochemical water softening were visualized using particle image velocimetry technology, and the hardness drop values under different process conditions were measured. Properly increasing the current density or reducing the electrode spacing can increase the average flow velocity in the electrode gap. Excessive current density will cause bubble accumulation, form a bubble vortex, interfere with the orderly flow of surrounding liquid and reduce mass transfer efficiency. When the electrode spacing is 120 mm, the highest water softening efficiency measured at the current density of 60 A/m2 is 16.56%. When the current density is 50 A/m2, the highest average speed measured at the electrode spacing of 60 mm is 0.00169 m/s, but the highest water softening efficiency measured at the electrode spacing of 90 mm is 23.3%.The circulation efficiency in the electrode gap of a semi-closed structure is lower than that of a free convection structure. The behavior of bubbles is the key to flow and mass transfer. It is important to consider its influence on bubble behavior when optimizing electrochemical parameters. HIGHLIGHTS Effect of flow characteristics between vertical plate electrodes on water softening efficiency.; The main flow patterns of mass transfer between vertical plate electrodes.; Effect of bubble behavior on flow patterns between vertical plate electrodes.; Optimization of flow mass transfer efficiency by current density, electrode spacing and electrode position.; Trend of velocity distribution between vertical plate electrodes.;

In order to promote the application of electrochemical water softening technology in industrial circulating cooling water systems, electric field type, cathode structure and solution residence time are selected for optimization analysis of an electrochemical water softening device. The experimental results show that the water softening performance per unit area of mesh cathode is better than that of a plate cathode. In addition, the softening amount per unit area of the mesh cathode can be further increased when the high-frequency (HF) power supply is applied. When the HF power supply is applied, the softening amount per unit area is 158.58 g/m2·h−1 more than when the direct current power supply is applied. In order to explore the growth mechanism of calcium carbonate, micro-analysis technology and high-speed bubble photography technology are used. The results show that the bubbles escape along the longitudinal direction of the electrode plate, and the main growth direction of calcium carbonate growth is consistent with the escape direction of the bubble; that is, the bubbles grow along the longitudinal direction of the electrode plate. The special structure of the diamond-shaped mesh cathode facilitates the growth of calcium carbonate crystals. HIGHLIGHTS The high-frequency power supply can further increase the descaling amount per unit area of the mesh cathode.; The special structure of the diamond-shaped mesh cathode facilitates the growth of calcium carbonate crystals.; The maximum desalination rate of this experimental device is 604.5 g/m2 h−1.;

Electrochemical water treatment technology can be used for hard water softening, but its removal efficiency and energy consumption problems hinder its application. The effects of electrolysis voltage and cathode materials on efficiency, energy consumption and scale crystal form of electrochemical water treatment technology were studied experimentally. The experimental results show that electrochemical water treatment can effectively remove more than 40% of the TDS (total dissolved solids) in the influent. The electrolysis voltage has a great influence on the removal rate of hardness ions. The optimum electrolytic voltage for the titanium plate cathode is 25 V. At this electrolytic voltage, the treatment capacity of the titanium plate cathode is 16% higher than that of the copper plate cathode, and the energy consumption is 16% lower than that of the copper plate. Titanium plates are more efficient than copper plates and consume less energy. The effects of electrochemical water treatment on the scale crystal form were investigated by SEM (scanning electron microscopy) and XRD (X-ray diffraction). Electrochemical water treatment increases the mass fraction of aragonite in the calcium carbonate scale and changed the microscopic surface structure of calcium carbonate scale. HIGHLIGHTS The descaling efficiency of titanium plate as a cathode is higher than that of copper plate.; The unit energy consumption of titanium plate is lower than that of copper plate.; The best efficiency reduces the TDS by more than 40%.; Electrochemical reaction makes the microstructure of calcium carbonate irregular.; The crystal type of CaCO3 will change from calcite to aragonite as the electrochemical reaction proceeds.;

Electrochemical water softening has been widely used in industrial circulating cooling water systems; however, their low deposition efficiency is the main drawback that limits usage in medium to large enterprises. In this work, the effect of different parameters on the hardness removal efficiency and energy consumption of the electrochemical water softening system is experimentally studied, and the performance of water softening applied by high frequency electric fields and direct current electric fields are comparative analyzed. The impact factors of the electrochemical water softening system are as follows: initial feed concentration of solute, magnitude of voltage, inter-electrode distance, area of cathode and frequency of power supply. To improve the analysis efficiency, the L25 (55) orthogonal table is used to investigate the five different factors at five levels. The experimental results are shown that the initial feed concentration of solute is the most significant factor affecting the hardness removal efficiency. The optimal combination for water softening in the group applied by high frequency electric field and direct current electric field are A3B2C1D4E3 and A2B5C3D1 respectively. The energy utilization of the device applied by high frequency electric field is 3.2 times that applied by direct current electric field. The practice shows that direct current electric fields have a better softening effect, and are is more suitable for scaling ion removal. Particle image velocimetry (PIV) was used to observe the flow field induced by the electrolysis and found that the vertical and horizontal velocities of the flow field at low voltage are conducive to the migration of scaled ions to the cathode, and then the electrolytic reaction and deposition reaction synergy effect is the optimal. HIGHLIGHTS Orthogonal experiments were used to optimize electrochemical water softening.; The initial concentration determines the electrochemical softening rate.; PIV technology is used to analyze mass transfer process.; Water softening performance depends on the synergistic reaction of electrolysis and mass transfer.; Water softening performance is enhanced by high frequency electric field.;