According to earlier results 14 in the stage of rapid mixing magnesium

According to earlier results 14 in the stage of rapid

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m for 30L/h and 60 L/h, respectively. According to earlier results [14], in the stage of rapid mixing, magnesium hydroxide was formed and particles grew in a very short time. A suitable period of rapid mixing was necessary for good coagulation. Reactive orange removal efficiency reached 89% and 83% for 30 L/h and 60 L/h, respectively. The retention time in each unit for 60L/h was shorter than that of 30 L/h, especially in the rapid mixer of 1 min retention time. This is too short to form magnesium hydroxide-reactive orange flocs. Table 4. Average size in different process units. Flow (L/h) Average Size (μm) Rapid Mixer Flocculation Basin Sedimentation Tank 30 8.06 7.89 11.21 60 7.25 7.52 10.47 Figure 4 clearly indicates floc formation and growth in the magnesium hydroxide-reactive orange continuous coagulation system. In the rapid mixer and flocculation basin, average floc size remained stable and flocs aggregated to relatively large flocs in the sedimentation tank. These two figures showed the same trends that the magnesium hydroxide coagulation process was similar to the precipitation process. When magnesium ion is added to the alkaline solution in rapid mixing period, the reaction crystallization process will happen rapidly. Magnesium hydroxide coagulation includes magnesium hydroxide nucleation and a combination of reactive orange into flocs. 250 300 350 84 86 88 90 Removal efficiency Zeta potential Zeta potential(mV) Removal efficiency(%) Rapid mixing (rpm) -24 -20 -16 -12 -8 -4 0 Figure 3. Effects of rapid mixing on removal efficiency and zeta potential. 3.2. Effect of Flow on Coagulation Performance For continuous coagulation reaction, the flow determines the retention time of reactants in each reactor. The increase of flow indicates the decrease of retention time. In order to investigate the effect of flow on coagulation performance, continuous experiments were performed under magnesium ion 250 mg/L, rapid mixing speed 300rpm with 30 L/h and 60 L/h. The retention time in the rapid mixing tank was 2min and 1min for flow of 30 L/h and 60 L/h, respectively. As shown in Table 4 , the average floc size decreased with increasing influent flow for each operation unit. As for the rapid mixer, the average floc size was 8.06 and 7.25 μ m for 30L/h and 60 L/h, respectively. According to earlier results [ 14 ], in the stage of rapid mixing, magnesium hydroxide was formed and particles grew in a very short time. A suitable period of rapid mixing was necessary for good coagulation. Reactive orange removal efficiency reached 89% and 83% for 30 L/h and 60 L/h, respectively. The retention time in each unit for 60L/h was shorter than that of 30 L/h, especially in the rapid mixer of 1 min retention time. This is too short to form magnesium hydroxide-reactive orange flocs. Table 4. Average size in different process units. Flow (L/h) Average Size ( μ m) Rapid Mixer Flocculation Basin Sedimentation Tank 30 8.06 7.89 11.21 60 7.25 7.52 10.47 Figure 4 clearly indicates floc formation and growth in the magnesium hydroxide-reactive orange continuous coagulation system. In the rapid mixer and flocculation basin, average floc size remained stable and flocs aggregated to relatively large flocs in the sedimentation tank. These two figures showed
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