## 3 Findings and Discussion
### The Removal of Tetracycline in the CW Treatment System
A post-treatment such as PC should be used to control bacteria and OM releases because of the concern regarding OM release due to the formation of potentially carcinogenic disinfection byproducts, particularly THMs, during chlorination. Indeed, PC is an emerging alternative technology for the removal of organic compounds as well as the inactivation of the bacteria [61; 62; 63]. Therefore, the application of PC is expected as an effective post-treatment process for the removal of the OMs and bacteria released by a biological process in CW. It can be seen from the Figure 9 that the removal efficiencies of the combined system in terms of sCOD, UV-254 and DOC were 80.3, 90.0 and 76.1%, respectively, which were higher than those of CW treatment alone (71.2, 81.8, 73.8%, respectively). The results show that the combination of CW pre-treatment and the photocatalytic post-treatment helped to improve effluent water quality with less chemical consumption that if chlorination were applied after this system. In fact, [PERSON] et al. [17] declared that the photocatalytic process might work more efficiently as a post-treatment step as it requires favorable transparency for radiation penetration. In such case, the colour of the influent which is due to the presence of HA was solved in the CW; thus, its support helps the TiO\({}_{2}\) photocatalysis process to be more successful and the amount of catalyst used in this step will be lower. In addition, due to the formation of OH. during the photocatalytic process, the reduction of UV-254 decreased more efficiently from 0.155 cm\({}^{-1}\) to 0.085 cm\({}^{-1}\). Figure 8: Time-dependent removal of TC over a period of treatment in CW with various initial TC concentrations. Figure 9: The influent, effluent and removal efficiency during the operation of the combined system. However, there was a minor improvement of water quality in the effluent after the treatment of the combined system, especially in the parameter of DOC (which just increased 2.3%). This can be due to the fact that the CW followed by the photocatalytic process might not lead to a significant change in the molecular weight distribution [60]. Actually, the main purpose of using PC as post-treatment in this study was the fact that PC acts as an additional stage for further OM reduction before chlorination and reuse, with the hope that DOC concentration in effluent would be lower than the maximum contaminant level (MCL) for total THMs of drinking water quality standard. For example, [PERSON] et al. reported that formed THM concentrations were above 0.01 mg/L (the guideline for German drinking water) when the concentration of TOC of the raw water exceeded 2 mg/L. In this research, the concentration of DOC in the effluent was recorded at 2.701 \(\pm\) 0.300 mg/L, which may meet the MCL for total THMs egulated by the Taiwan Environmental Protection Administration (Taiwan EPA), which is 0.08 mg/L r.  
In terms of the sCOD parameter, sCOD in the effluent was 7.43 \(\pm\) 0.98 mg/L, which was lower than the acceptable value of 25 mg/L recommended for raw water quality for drinking water by Taiwan EPA. With this sCOD value, the treated water can be considered as a potable water source.