## 3 Results and Discussion
### Applied Cases
Figure 6: Sampling sites in Chengdu urban and suburb areas (Google map) as chemical plant, municipal drain, pharmaceutical plant, vehicle maintenance, and farm from the top to the bottom indicated as triangle shape associated with the pH range. 3D-BDD electrodes show advantages of higher mass transfer efficiency and higher removal rates than the flat BDD electrodes by approximately 90% [17], which was confirmed by our results. The higher applied voltages facilitate the promotion of wastewater degradation efficiency, but the lower voltages decrease the energy consumption of hydrogen evolution. Hence, a moderate voltage applied on the HABDD-FNi electrodes should be carefully chosen. In this study, we chose an operating voltage of 5.0 V in electrolysis because most waste species could be efficiently degraded at this voltage while not consuming much energy. The mass transfer effect was affected by the flow of the ions generated at the HABDD surface, which became more efficient with an increasing flow rate. The EO process's effect on mass transfer ability must be critically evaluated. A comparison of the COD removal ability for municipal wastewater on the HABDD and the BDD (with the same size) was illustrated in Figure 7a. The highest removal performance was located at the flowing rate of 150 mL/min in this system. This was because, on the one hand, under a lower flowing rate, the EO rate in the local area was increased while the total EO rate was decreased due to a lower contact surface area between the electrode surface and the electrolyte. On the other hand, at a higher flow rate, the wastewater could not sufficiently contact the electrode surface before it flowed out. Hence, the flow rate of the wastewater should be selected at a moderate value. In addition, the influence of the flowing rate on the HABDD was much higher than that on the flat BDD. The flow on the flat surface will maintain a low turbulence state like the laminar flow state. Meanwhile, in the HABDD, the cross-flow velocity and turbulence of the flow are generated by a higher turbulent kinetic energy [13]. The COD removal rate on the HABDD was higher than on the flat BDD, demonstrating the promotion effect from the dynamic holed structure. The correlation between the \(E_{\text{h}}\) and COD removal rate on the HABDD electrode was studied. As shown in Figure 7b, after 8 h of treatment time for the phenol wastewater, the \(E_{\text{h}}\) for the hydrogen production decreased from 92% to 75%, while the COD value decreased with the current efficiency, meaning the waste species were consumed. In the treatment process, the current efficiency decreased less than the decreasing COD values. The electrolyte flowing resistance from the anode to the cathode due to the presence of complex contents in the cell might increase the extra power consumption. This whole system is currently under continuous development in our lab. Figure 7: (a) The dependence of COD remove ability of HABDD (green line) and BDD (blue line) on flowing rate; (b) Current efficiency of hydrogen generation (orange line) on FNi, and COD decline (phenol aqueous solution, purple line) versus the working time on the rotating HABDD.