# Analysis of the Water Quality Improvement in Urban Stream Using MIKE 21 FM
## 3 Selection and Current Status of the Target Area
### Comparative Analysis of the Observed Water Quality Concentration  
The water quality results of the Gulpocheon upstream are presented in Figure 4. Based on the comparison the pH and DO as shown in Figure 4a, it was observed that the dissolved oxygen is affected by the seasonal effect. The DO is high in the winter season and low in the summer season. Since March 2019, when the ozone treatment facility was completed, the DO concentration was generally higher than the other years compared, suggesting a more stable water quality. The pH was within 6.5 to 8, and the pH change due to the discharge of reclaimed water was insignificant. Figure 4: Water quality comparison of the upstream of Gulpocheon [23]. Figure 4b shows the COD and BOD concentration changes. After April 2019, the concentration of COD and BOD was lower than 8 mg/L and 3 mg/L, respectively. As it shows the lowest concentration of COD and BOD since April 2016, it can be concluded that there is water quality improvement in all seasons after the discharge of the ozone-treated reclaimed water. As the concentrations of SS, TN, and TP rapidly increase, depending on the seasonal effect and pollutant loading period, they can be used as criteria to declare water pollution. Before supplying the ozone-treated reclaimed water, the SS concentration exceeded 25 mg/L in October 2016 and December 2018, corresponding to the'slightly bad' rank of the water quality environment standard of the stream water. The TP concentration rapidly increased in the summer and exceeded 0.2 mg/L, the 'normal' rank. Since April 2019, the TP and SS items were more satisfactory than the 'normal' rank. They did not show significant deviation across different seasons. The water quality analysis results of the Gulpocheon downstream are presented in Figure 5. On comparing the results of pH and DO, the DO showed the lowest value of 5.2 mg/L in November, which corresponds to the 'good' rank the stream water quality standard. These are better results than the ones obtained before providing the reclaimed water. The pH varied between 6.5 to 8.0, and no significant change was observed. The COD and BOD concentrations were stable after April 2019. Based on the results, it can be concluded that there is water quality improvement after the discharge of the ozone-treated reclaimed wastewater. After completing the ozone treatment facility, the SS and TP showed more stable concentrations. The SS showed a continuous decreasing tendency, and the concentration of TP was found to be 0.29 mg/L in November. All showed water quality of 'normal' rank except'slightly bad' according to the stream water quality environment standard. The concentration of TN was satisfactory, with no significant deviation across the whole analysis period. Based on the significant water quality measurement results, the base flow of the ozone treatment facility supplied into the Gulpocheon upstream and Gyesancheon showed water quality improvement in terms of the DO, BOD, COD, and SS concentrations in the target area. By comparing the measured concentrations of the major water quality factors, it was established that the water quality concentration is consistent with the stream water quality environmental standards. Furthermore, based on the field investigation results after the facility's completion, it was identified that the transparency of the Gulpo stream water was enhanced, and its odor was significantly removed. The TP and TN concentrations in the treated sewage water were higher than those in the Gulpo stream water. Their concentration was partially increased compared with their concentration before the business. While the TP and TN concentrations tended to increase rapidly before providing the base flow, their water quality concentrations tended to be stable after supplying the base flow, suggesting it is favorable for the water quality management. Figure 6 shows the results of time series concentration comparison according to location by main water quality factors.