## 3 Results and Discussion
### Effects of Operating Parameters on Continuous Cr(VI) Removal by Immobilized ChrR Biosystem
The outlet Cr(VI) concentration exceeded the discharge limit of 0.5 mg L\({}^{-1}\) on day 14, day 56, day 63, day 91, day 154, day 189, and day 196. Although the inlet concentration was only \(295\pm 1.7\) mg L\({}^{-1}\) (below the ideal treatment concentration, 300 mg L\({}^{-1}\)) on day 91, the removal efficiency dropped to \(98.52\%\pm 0.04\%\), with an outlet concentration of \(4.37\pm 0.09\) mg L\({}^{-1}\) due to turbidity problems. When a filter device was installed, even with an inlet concentration as  
Figure 4: Effects of (**A**) the liquid retention time, (**B**) temperature, (**C**) Cr(VI) concentration, and (**D**) operation time on the continuous treatment of synthetic Cr(VI)-containing wastewater by the immobilized ChrR biosystem. high as 425 \(\pm\) 10 mg L\({}^{-1}\) (on day 133), the removal efficiency achieved 99.91% \(\pm\) 0.04%, with an outlet concentration of 0.38 \(\pm\) 0.17 mg L\({}^{-1}\). When 408 \(\pm\) 3.75-mg L\({}^{-1}\) Cr(VI) was introduced to the biosystem on day 154, the removal efficiency dropped to 97.86% \(\pm\) 0.27%, with an outlet concentration of 8.73 \(\pm\) 1.16 mg L\({}^{-1}\) because of an insufficient DO supply (3.6 \(\pm\) 0.4 mg L\({}^{-1}\)). Thus, an aeration device and a DO sensor were installed (DO would be controlled at >4.0 mg L\({}^{-1}\)), and the removal efficiency gradually rose and stabilized. With the filter device, aeration device, and DO sensor, the outlet Cr(VI) concentration would meet the industry discharge limit of 0.5 mg L\({}^{-1}\) if the inlet Cr(VI) concentration was below 430 mg L\({}^{-1}\). Figure 5B represents the continuous treatment of actual electroplating wastewater by the immobilized ChrR biosystem. The average Ni\({}^{+}\) concentration, Na\({}^{+}\) concentration, SO\({}_{4}\)\({}^{2-}\) concentration, Cl\({}^{-}\) concentration, SS, BOD, and COD in wastewater were 13.5 \(\pm\) 0.6 mg L\({}^{-1}\), 4.2 \(\pm\) 0.8 mg L\({}^{-1}\), 6.5 \(\pm\) 0.7 mg L\({}^{-1}\), 4.8 \(\pm\) 0.3 mg L\({}^{-1}\), 24 \(\pm\) 2.3 mg L\({}^{-1}\), 42 \(\pm\) 5.6 mg L\({}^{-1}\), and 68 \(\pm\) 2.7 mg L\({}^{-1}\), respectively. According to the previous results, a pH adjustment system was installed to adjust the pH of the water from 5.1 \(\pm\) 0.8 to 7.0 \(\pm\) 0.5. The inlet Cr(VI) concentration varied from 52 to 158 mg L\({}^{-1}\) during the 196-day treatment period. The trend between the changes in the luminescent bacteria toxicity for water quality and the dynamics of the outlet Cr(VI) concentration was consistent. To evaluate the adaptability of the biosystem, the immobilized enzyme replacement was deliberately neglected on the  
Figure 5: Continuous treatment of actual Cr(VI)-containing wastewater by (**a**) the immobilized _Ochrohxtum_ sp. CUST210-1 biosystem and (**b**) immobilized ChrR biosystem. 70 th day, because the replacement frequency was set to a 35-day cycle. The outlet Cr(VI) concentration exceeded the maximum contaminant level of 0.05 mg L\({}^{-1}\) on days 77-98. On day 105, the immobilized ChrR beads were replaced, and the removal efficiency gradually stabilized to \(>\)99.97%. Moreover, the outlet concentration was less than 0.05 mg L\({}^{-1}\). When \(143\pm 0.65\)-mg L\({}^{-1}\) Cr(VI) was fed on day 133, the removal efficiency dropped to 98.52% \(\pm\) 0.05%, with an outlet concentration of \(2.11\pm 0.08\) mg L\({}^{-1}\) because of an insufficient DO supply (\(2.8\pm 0.6\) mg L\({}^{-1}\)). Thus, an aeration device and a DO sensor were installed, and DO in the wastewater was controlled at \(>\)3.0 mg L\({}^{-1}\). The removal efficiency of the immobilized ChrR biosystem gradually stabilized at 99.98-100%, and the outlet Cr(VI) concentration conformed to the maximum contaminant level of 0.05 mg L\({}^{-1}\). Although the maximum contaminant standards for waterbody and wastewater discharges differ at present, developing effective technology to meet future, more stringent standards is worthwhile. In the literature, limited studies have focused on the treatment of actual Cr(VI)-containing wastewater by a continuous biosystem.