This study is to improve the efficiency of BTL (Build Transfer Lease) project operation by comparing the infiltration rate based on the data of 5 years of infiltration of the separate sewer system and combined sewer system. In the survey site, the separate sewer system area consists of eight flowmeters in seven treatment basins, and the combined sewer system area consists of eight flowmeters in five treatment basins. The infillration rate was analyzed by night-time domestic flow evaluation method, and the average infiltration rates of the separate sewer system and combined sewer system were 13% and 16%, respectively. Combined sewer system was about 1.3 times higher than the separate sewer system. The average BOD of separate sewer system was 233 mg/L, which was about 2.4 times higher than the combined sewer system was 107 mg/L. In the comparison of the average pipe diameter-length infiltration of separate sewer system and combined sewer system, the separate sewer system and the combined sewer system were about 0.150 m3/d/mm/km and about 0.109 m3/d/mm/km, respectively. The floating population in mixed residential and commercial areas has been identified as the cause. Therefore, we propose a method to calculate the infiltration rate in consideration of the margin ratio in the area where the night active population is concentrated.

The purpose of this study was to analyze the effects of sewerage facilities through I/I analysis by rainfall by selecting areas where storm overflow diverging chamber is remained due to the non-maintenance drainage equipment when the sewerage system was reconstructed as a separate sewer system. Research has shown that wet weather flow(WWF) increased from 106.2% to 154.8% compared to dry weather flow(DWF) in intercepting sewers, and that the WWF increased from 122.4% to 257.6% in comparison to DWF in storm overflow diverging chamber. As a result, owing to storm overflow diverging chamber of partially separate sewer system with untreated tributary of sewage treatment plant, rainfall-derived infiltration/inflow(RDII) has been analyzed 2.7 times higher than the areas without storm overflow diverging chamber. Meanwhile, infiltration quantity of this study area was relatively higher than that of other study areas. Therefore, it is necessary to reduce infiltration quantity through sewer pipe maintenance nearby river. Drainage equipment maintenance should be performed not to operate storm overflow diverging chamber in order to handle the appropriate sewage treatment plant capacity for rainfall because it is also expected that RDII due to rain will occur after maintenance. In conclusion, it is necessary to recognize aRDII(allowance of rainfall-derived infiltration/inflow) and to be reflected it on sewage treatment plant capacity because aRDII can occur even after maintenance to the complete separate sewer system.

The purpose of this sudy is to get characteristics of water quality in separated sewer overflows. In dry weather the water quality of sewage in the outfall of the separated sewer was an average BOD of 2.7 mg/L, CODmn of 4.9 mg/L, TN of 3.8 mg/L, TP of 1.9 mg/L, and SS of 5.3 mg/L, Because the quality except for TP was less than the water quality standard of wastewater treatment plant (WTP), a prevention facility was needed to prevent the sewage inflow to interceptor. When the separated sewer overflows (SSO) were overflowed, the maximum pollutant load was be found to be about from 2.7 to 5.4 times as a BOD base, about from 3.2 to 4.6 times as a SS base higher than average load. There was a big difference according to rainfall intensity and rainfall continuous time. In case the quality of SSO was less than water quality standard or water quality in dry season, SSO was required to discharge to the stream not to inflow to WTP by operating the floodgate in the outfall with water quality data using sensors, such as turbidity sensor, etc. Because removal efficiency by an intercepter load was calculated as low as about 10~20% as a BOD base, removal efficiency could be increased by installing a separated wall within the sewer.

This study is to evaluate control effects of separation wall by surveying water quality and sewer overflows during dry and wet periods in combined sewer and separated sewer systems. Ravine water from the combined Seokgyo outfall with the separation wall was separated about four times larger than sewage flow during dry periods. The water quality of the combined Seokgyo outfall with separation wall during dry periods is flow weighed average T-N 22mg/L, T-P 2.4mg/L, the combined Chenseokgyo outfall without the separation wall is average T-N 21mg/L, T-P 2.6mg/L, and the separated Pyoungsong center outfall is average T-N 12mg/L, T-P 1.0mg/L. The T-N, T-P concentration in separated outfall make form about 44%, 42% of the combined outfall, and this means the separated outfall (i.e. storm sewer) is polluted by inflow of sewage. The overflow load of the separated outfall is ten times higher than the combined outfall and its overflow load per rainfall is three times than combined outfall during the wet periods. Therefore, the control plan of overflow load is required in storm sewer. The control effects of the overflow load increased 100% by setting the separation wall in the combined sewer, and showed 44% increase without the separation wall in separated sewer, but forecasted over than 82% increase of effects with the separation wall.