Abstract
Under the concept that a sewage treatment plant should serve as a base for the circulation of materials and energy in a watershed, a case study of the Omono River basin in Akita was conducted. Through the improvement of the energy self-sufficiency ratio of Rinkai Sewage Treatment Plant (STP) with wide-area cooperation, the energy self-sufficiency ratio of 31% of the basic system increased to 35% as a result of night soil acceptance. After public sewerage integration, the energy self-sufficiency ratio remained only 28% which is less than that of the basic system, because of the increase in electric energy consumption. However, the watershed STP energy self-sufficiency ratio of the public sewerage integration improved to 28%, which is 9 points more than the 19% of the basic system. From the viewpoint of phosphorus recovery, the 73 kg of the basic system increased to 109 kg after combined treatment with night soil. Furthermore, it became maximum with a phosphorus recovery ratio 25%, which is equivalent to 123 kg, after public sewerage integration. On the basis of the estimates for a model watershed, I suggested that an evaluation of the whole watershed is important, rather than that of an independent sewage treatment plant.
