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.