## 3 Results and Discussion
### Applied Cases  
Several types of wastewater from a pharmaceutical plant, municipal drain, chemical plant, vehicle maintenance, and a farm were sampled for the test. The sampling sites are indicated on the map in Figure 6. The sampled wastewater, composed of certain concentrations of organic compounds, inorganic salts, and other contaminants, were filtrated before the electrochemical degradation. The range of wastewater that can be treated had a pH range of 3.8-6.5. The wastewater that can be degraded had main contents included 1,2,4-triazole, toluene, sulfamide, isopropyl acetate, tert-butyl alcohol, phosphoric acid, and alcohols with salt content below 5%. The wastewater was a mixture of these contents, which resulted in superposing responses for the removal efficiency. The hydrogen evolution in acidic electrolytes is better than that in the alkaline medium in this electrochemical system [9]. The wastewater samples tested in this experiment were conductive due to their acidic properties. The generated hydrogen had a high purity (>85%) by GC measurement. An accelerated reaction rate on the HABDD anode could promote the reaction rate on the FNi cathode [25]. The performances for the COD removal rate and hydrogen production efficiency of the electrochemical system are listed in Table 1. The typical treatment times for the wastewater with the initial COD and pH values are provided for the prediction estimation of the practical running time [26]. The acid species in the wastewater could play an electrolyte role. However, the pH value should not be too low; otherwise, the ion exchange membrane might be destroyed by the high concentration of acidic ions. Figure 5: COD changes on the rotating HABDD (green line) and stationary HABDD (red line) via treatment time. The electrochemical-generated radicals from the HABDD surface contribute to the EO process for wastewater. A typical EO process on the HABDD surface is determined by the mass transfer steps [11; 27]. After each EO circulation, the color in the anodic cell changed from light yellow to almost colorless, indicating the decomposition of the waste organics on the HABDD. Small amounts of foam formed on the anode surface were observed during the treatment process, but they might disappear in the fast-flowing solution. The sampled wastewater for treatment was less than 50 L for each sample due to the design's capacity. The maximum COD of this system was normally lower than 8.0 g/L since much more time would be required. The \(E_{\text{h}}/E_{w}\) values were located between 81-89% because of the complex content of the organic materials and the rotating power needed. The hydrogen production efficiency was normally higher than 80% due to the higher electrochemical performance of the FNi electrode. The decolorization of the wastewater occurred quickly on the rotating HABDD, which can be attributed to the higher mass transportation on the dynamic structure. Referenced from previous reports, the typical wastewater with COD values between 3-9 g/L treated by flat BDD electrodes would cost 6-20 h [9; 28]. The \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Wastewater** & **Initial COD (g/L)** & **Initial pH** & **COD Removal Rate (\%)** & **Treatment Time (h)** & **Hydrogen Concentration (\%)** & **Energy Efficiency** \\ \hline Pharmaceutical plant & 6.52 & 4.26 & 83.4 & 7.2 & 85.5 & 81.8 \\ \hline Municipal drain & 4.66 & 6.22 & 85.2 & 5.4 & 90.3 & 88.4 \\ \hline Chemical plant & 5.87 & 5.11 & 78.1 & 6.5 & 92.1 & 77.6 \\ \hline Vehicle maintenance & 3.85 & 5.47 & 93.3 & 3.8 & 88.1 & 86.5 \\ \hline Farm & 2.28 & 5.84 & 91.3 & 3.5 & 92.5 & 85.2 \\ \hline \hline \end{tabular} Note: The wastewater was sampled from the sites indicated on the map.  
\end{table} 
Table 1: Performance of wastewater degradation and hydrogen production of the HABDD-FNi system.