摘要：A large quantity of discharge from sewage treatment plants flows into urban rivers. To evaluate the part this discharge plays in the organic pollution of river water, the distribution of urobilin was investigated. The amount of urobilin increased remarkably after rainfall, suggesting that the rivers are contaminated not only with discharge, but also with untreated sewage. Some rivers which run through both rural and urban areas were more polluted in their upper reaches than in their lower reaches, indicating that sewage was not treated properly in the upper reaches and flowed into the rest of the river. The contribution rate of untreated sewage, calculated from the amount of COD/urobilin of river water, was from 50 to 100% in the Kanda River in rainy weather. These investigations revealed that the main source of organic pollution of the river was sewage overflow.
