## 3 Results and Discussion
### Effects of Operating Parameters on Continuous Cr(VI) Removal by Immobilized ChrR Biosystem  
Figure 4A depicts the effect of the LRT on the continuous treatment of synthetic Cr(VI)-containing wastewater by the immobilized enzyme biosystem. The removal efficiency increased with the LRT, and the removal capacity decreased as the LRT increased. Optimal LRT occurred at 1 h--at which point, the removal efficiency and capacity of the biosystem achieved 100% and 100 \(\pm\) 1.2 mg-Cr(VI) L\({}^{-1}\) h\({}^{-1}\), respectively. Thus, the operating LRT of the experiment was 1 h. Figure 4B portrays the effect of the operating temperature on the continuous treatment of synthetic Cr(VI)-containing wastewater. The ChrR biosystem exhibited a high removal efficiency (\(>\)98.20% \(\pm\) 0.2%) over a wide range of temperatures (25 \({}^{\circ}\)C-40 \({}^{\circ}\)C), indicating the immobilized ChrR had high stability in a wide temperature range. Although the efficiency of 99.65% \(\pm\) 0.07% at 25 \({}^{\circ}\)C seemed satisfactory, the outlet Cr(VI) concentration of 0.35 \(\pm\) 0.07 mg L\({}^{-1}\) exceeded the maximum contaminant level of 0.05 mg L\({}^{-1}\) for fisheries, drinking, ground, and surface waters. A higher temperature (e.g., 45 \({}^{\circ}\)C) could impair the function of ChrR and dramatically reduce the Cr(VI) removal efficiency [44]. Thus, the operating temperature of the experiment with the immobilized ChrR biosystem was controlled at 35 \({}^{\circ}\)C. Figure 4C presents the effect of the inlet Cr(VI) concentration on the continuous treatment of synthetic Cr(VI)-containing wastewater. The removal efficiency was excellent (\(>\)99.96%) at 25-150 mg L\({}^{-1}\). The maximum outlet Cr(VI) concentration was only 0.045 mg L\({}^{-1}\), even at 150-mg L\({}^{-1}\) Cr(VI) feeding, which meets industry effluent standards and maximum contaminant levels. Figure 4D displays the  
effect of the operating time on the continuous treatment of the synthetic wastewater. The removal efficiency initially maintained a high level but gradually decreased with the increased operating time. After 35 days of operation, the efficiency curve exhibited an obvious turn. On day 42, the outlet Cr(VI) concentration was 3.68 \(\pm\) 0.4 mg L\({}^{-1}\), exceeding maximum contaminant levels. Thus, regular replacement (or frequency of use) during a 35-day cycle of immobilized ChrR beads should be considered to maintain high removal capabilities in a long-term operation. To our knowledge, this is first report of an immobilized ChrR biosystem applied to continuous Cr(VI) removal. Figure 3: Effects of (**A**) the liquid retention time, (**B**) pH, (**C**) temperature, and (**D**) Cr(VI) concentration on the continuous treatment of synthetic Cr(VI)-containing wastewater by the immobilized _Oclrobactrum_ sp. CUST210-1 biosystem. Continuous Treatment of Actual Cr(VI)-Containing Wastewater by Immobilized Orchroachtrum sp. CUST210-1 Biosystem and Immobilized ChrR Biosystem  
Actual Cr(VI)-containing wastewater derived from tannery or electroplating wastewater was continuously treated by our biosystems for 196 days. Average S\({}^{2-}\) concentration, suspended solid (SS), BOD, and COD in the tannery wastewater were \(2.5\pm 0.6\) mg L\({}^{-1}\), \(303\pm 23\) mg L\({}^{-1}\), \(158\pm 8\) mg L\({}^{-1}\), and \(261\pm 15\) mg L\({}^{-1}\), respectively. According to the previous results, the inlet pH of \(9.2\pm 0.6\) was adjusted to pH \(7.0\pm 0.5\) by a pH adjustment system. Figure 5A depicts the continuous treatment of actual tannery wastewater by the immobilized _Orchroachtrum_ sp. CUST210-1 biosystem. The inlet Cr(VI) concentration varied from 284 to 471 mg L\({}^{-1}\) during the 196-day treatment period. The change of luminescent _V. fischeri_ toxicity (luminescence inhibition) in response to the water quality and the dynamic of the outlet Cr(VI) concentration were similar. This suggested that the residual Cr(VI) concentration was associated with the toxicity of the wastewater.