## 3 Result and Discussion
### Removal Mechanisms of Radial-Type Multifiber Media Filtration Process  
Figure 3 shows the removal efficiency of pollutants (SS, TOC, and BOD\({}_{5}\)) for the RMF process under different weather conditions. The process removed 89.6% SS, 45.3% TOC, and 58.4% BOD\({}_{5}\) in dry weather and 87.4% SS, 42.8% TOC, and 48.5% BOD\({}_{5}\) in rainy weather.  
The main mechanisms for removing SS and TOC in this filtration process are the sieving effect (size exclusion) and hydrophobic adsorption. Furthermore, the mechanism  
Figure 3: Removal of contaminants in combined sewer overflows (**a**) SS (Suspended Solids), (**b**) TOC (Total Organic Carbon), (**c**) BODs (Biochemical Oxygen Demand) in dry weather condition, (**d**) SS in rainy weather condition, (**e**) TOC in rainy weather condition, and (**f**) BODs in rainy weather condition. Operating condition: Initial velocity = 20.0 m\({}^{3}\)/h, temperature = 23 \({}^{\circ}\)C, and raining intensity = 5 mm/h.  
Figure 2: Characteristics of the fiber media filter as determined via (**a**) FE-SEM, (**b**) Zeta potential analysis. Blue line represents a isoelectro point of the multifiber media filter. Isoelectro point means zeta potential equals zero when the object meets at specific pH (pH 5.09), and (**c**) water contact angle. The red line represents a zeta potential of whole multifiber media filter at different pH range.  
operating in the roof fiber media filter may be electrostatic repulsion between the contaminants and the negatively charged surface of the RMF. Regarding the observed charge differences, the fiber media had a negative charge (\(-\)28.84 mV) at pH 7. This may lead to a reduction in the electrostatic repulsion between the fiber media and negatively charged contaminants (NOM; organics), resulting in acceptable removal efficiency [25; 26; 27]. In addition, the roof-type fiber media filter is characterized by a high hydrophobicity value (Figure 2c), due to hydrophobic-hydrophobic interactions between the filter and hydrophobic contaminants. Hydrophobic pollutants can therefore be easily adsorbed into hydrophobic material filters [28; 29; 30; 31]. Hence, large particles, such as suspended solids and large organic matter, were then filtered into the fiber media filter, leading to an increase in the removal efficiency.  
Previous studies have shown that the SS, TOC, and BOD\({}_{5}\) concentrations are significantly higher in rainy weather than in dry weather [32]. The SS and TOC concentrations of pollutants increased by more than 25% in rainy weather compared to dry weather, whereas the BOD\({}_{5}\) concentration increased by approximately 36.5%. Precisely, the SS, TOC, and BOD\({}_{5}\) concentrations in dry (rainy) weather were 122.5 (177.6), 66.8 (87.5), and 174.6 (273.3) mg/L, respectively. These results can be explained in terms of the properties of CSOs in urban areas. For most urban areas, a combined sewer is usually used to transport sewage into wastewater with a very slow flow in dry weather. The sewage accumulated under a sewer pipe forms a high concentration of pollutants. During a rainfall period, accumulated sewage may be overflowed from the combined sewer pipe with a high concentration of pollutants and mixed with municipal wastewater. These CSOs increase the concentration of pollutants.  
The RMF process exhibited stable SS removal efficiency in both weather types. Although the removal efficiencies of BOD\({}_{5}\) differed and TOC rejection showed stability in both weather types, the concentration of organic matter in CSOs can contaminate an ecosystem when untreated water discharges into rivers. Hence, it is necessary to develop the pretreatment or posttreatment process of RMF.