# Analysis of the Water Quality Improvement in Urban Stream Using MIKE 21 FM
## 1 Introduction  
The annual precipitation in South Korea is concentrated in the summer season. The river flow is extremely low from December to February, resulting in water quality deterioration, water deficit, and water distribution imbalance across different regions [1, 2]. Moreover, water quality deterioration and other problems exist because wastewater, sewage, and domestic sewage are discharged to urban streams without treatment. Furthermore, the base flow is insufficient in urban streams due to the lowered underground water level and reduced streamflow due to the development and supply of agricultural water. There are 369 urban streams in South Korea, and 36.3% (134 streams) are expected to undergo stream depletion [3]. Therefore, there is an increasing need for proper management and practical usage of limited water resources. The immediate solution is securing sufficient flow. Regarding the base flow of the domestic urban stream, there was some research that compared the effect of eluted underground water, sewage treatment-reclaimed wastewater, and reservoir discharge water [4]. As in the previous study, it is necessary to discuss the supply of various inflow water in order to secure the flow rate of the river. In particular, when water does not flow in a river during a drought, it has a great impact on the ecosystem, so it is most important to establish a plan for securing water for maintenance. Various studies utilized the sewage treatment-reclaimed wastewater as base flow within the urban stream [5, 6, 7]. There have been some published reports on the use of sand filter, ozone treatment, flocculation, precipitation, active-carbon adsorption, and membrane filtration process [8, 9, 10]. In particular, the ozoneoxidation process is widely used for purposes other than disinfection in water treatment. It has a bleaching and deodorization effect, oxidizes organic and inorganic matters, and improves biodegradation [11; 12; 13; 14]. Internationally, research on methodology and water quality analysis are conducted in relation to the application of reclaimed water to urban rivers [15; 16; 17]. It is necessary to quantitatively evaluate before and after the supply of reclaimed water using a numerical model, and this study can be used as analysis data on the effect of urban reclaimed water. For water quality improvement and maintenance in urban rivers, the exact effect of reclaimed water application should be identified. Additionally, it needs to be verified through the evaluation of observation data and numerical analysis models in parallel. In this study, water quality improvement was quantitatively analyzed when the reclaimed water was supplied to the actual stream as base flow. Gulpocheon, where sewage treatment discharged water was processed with ozone treatment, was selected as a target stream for this study. The domestic stream's water quality standards (Ministry of Environment) were established to protect the aquatic ecosystem from water pollution and preserve the water quality suitable for water use. In this study, numerical simulations were conducted to analyze the water quality factors (e.g., dissolved oxygen (DO), biochemical oxygen demand (BOD), total-nitrogen (TN), and total-phosphorus (TP)) that influence stream water quality standards for Gulpocheon. The treated sewage water was utilized as the base flow. In particular, the water quality improvement within Gulpocheon before and after supplying the reclaimed water was quantitatively analyzed using the two-dimensional numerical analysis model, MIKE 21 FM (DHI, [[https://www.mikepoweredbydhi.com](https://www.mikepoweredbydhi.com)]([https://www.mikepoweredbydhi.com](https://www.mikepoweredbydhi.com)), accessed on 20 August 2021). Reclaimed water is wastewater that has been treated and transformed into a product that is clean, clear, and odorless [18]. The installed ozone-treatment facility at Gulpocheon improves the water quality using chemical process by ozone and its facility uses the sewage water from near city.