2026 Volume 49 Pages 37-44
The Suikawa River flows from suburbs through the urban area of Ichinoseki City in Iwate Prefecture, Japan. The inflow of wastewater from paper mills that operated in the region caused significant pollution until 2000, with the biochemical oxygen demand (BOD) 75% value in the Suikawa River exceeding 50 mg L-1. In 2002, wastewater treatment facilities were installed in a paper mill; subsequently, the BOD 75% value in the Suikawa River decreased to approximately 20 mg L-1. Sewage treatment has been implemented to reduce the pollution load inflow into the Suikawa River. In 2018, the BOD 75% value decreased to < 5 mg L-1 because of the closing of a paper mill in the region. At present, the low penetration rate of septic tanks remains a concern. The BOD average values in suburbs have increased slightly; however these levels do not affect the water body ecosystem. On the other hand, in the urban area, BOD values increased in response to certain events: during rainfall, an increase in electrical conductivity (EC) value was observed; however, the fluctuations of both parameters were temporary. It was suggested that water quality improvement through sewage treatment in the Suikawa River basin has made it possible to take measures for ecological recovery of the water environment.
