THE EFFECT OF SANITARY SEWER OVERFLOW DURING RAINY EVENTS ON THE BEHAVIOR OF DISSOLVED ORGANIC MATTERS AND SANITARY MICROORGANISMS IN WATER OF THE RIVER FLOWING OUT OF LAKE BIWA

Public concerns have recently emerged due to sanitary microorganism in sanitary sewer overflows (SSOs) from sewerage systems in rainy events. The purpose of this study was to investigate the influence of SSO on chromophoric dissolved organic matter (CDOM) and sanitary microorganisms in a river, Seta River receiving a sewage treatment plant (STP) discharge at the outlet of Lake Biwa. To this end, we conducted field survey during four rainy events whose total rainfalls ranged 10 to 177 mm, and measured CDOM constituents in the STP discharge and the river water. EEM-PARAFAC analysis of CDOM revealed that there are three components of CDOM in the STP discharge and the river water. During the three rainy events, SSOs occurred in the STP. As the results, the CDOM component indicating protein origin in the river water (Comp.2) and summation of the three CDOM components (Comp.f) were confirmed to increase due to untreated sewage discharge in total rainfall events of 28 mm and 68 mm. However, in 177 mm rainfall event when SSOs had occurred before sampling campaign started, Comp.2 did not increase, while Comp. f had gradually increased due to untreated sewage discharge. In addition, in 64 mm and 177 mm rainfall events, increase in sanitary microorganisms in the river water was identified probably due to untreated sewage discharge.