Humic substances discharged from sewage treatment plants

Lake Biwa, located in Shiga Prefecture, is the largest lake in Japan. It is an important water source for sonic 14 million people living in Shiga and neighboring prefectures. The sewage treatment plants established by the Shiga Prefectural Government and those established by several cities have worked to keep the lake water clean. The biochemical oxygen demand (BOD) value of the lake water has been decreasing during the last 2 decades, showing high efficiency of the sewage treatment facilities in biodegradation of labile organic materials. The chemical oxygen demand (COD) level, in contrast, shows a gradual increase over time, suggesting that the concentration of refractory organic matter has increased (SHIGA PREFECTURAL GOVERNMENT 2002). Humic substances (HS) dissolved in water are refractory organic compounds making up about 50% or more of the dissolved organic carbon (DOC) in freshwater ecosystems (THURMAN 1985). The distribution and chromatographic properties of HS in Lake Biwa and rivers were reported by YAMADA et al. (2000), NAGAO et al. (2003) and SUGIYAMA et al. (2005); however.. HS in the effluent from sewage treatment plants have been little characterized. We analyzed the treated water and lake and river water samples for HS by the high-performance-size exclusion chromatograph (HPSEC) and examined the distribution of effluent HS in the lake and its photochemical reactivity.