Water environment of the River Mirumae receiving the effluent from a sewage treatment plant was investigated based on the data on water quality, attached algal biomass and benthic animals. Although residual chlorine remained in the effluent, the amount of attached algal flora was increased in winter. The inflow of the effluent decreased the diversity of benthic animals and it changed the structure of benthic animals in the River Mirumae. It was concluded that the secondary treatment can not conserve the sound water environment in river receiving the effluent from sewage treatment plants.
