Development of a novel sewage treatment system for 21^<st> century to break out from the conventional activated sludge process

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Conventional sewage treatment systems such as activated sludge process are considered to be not suitable in developing counties due to high cost and much energy requirement. An “appropriate technology" of sewage treatment must be developed for developing countries. UASB is one of attractive technologies in developing countries because it guarantees low costs in construction and operation, production of energy as methane, and less excess sludge. However UASB cannot meet discharge criteria, so that some following reactor is needed to polish up UASB effluent without losing UASB advantages. As the appropriate system, we have proposed DHS (Down-flow Hanging Sponge) reactor containing polyurethane sponges as substratum to retain biomass. The concept is similar to that of tricking filter, except that the packing material is sponge, resulting in a significant increase in entrapped biomass and thus longer solid retention time (SRT). Without submerging sponges in DHS, oxygen is supplied from the air to dissolve into wastewater flowing down on the surface and inside. Therefore, there is no need for external aeration or any other energy inputs. Moreover, excess sludge production from DHS is negligible small as longer SRT provides considerable time for autolysis of sludge in the reactor. From these reasons mentioned, the combined system of UASB and DHS reactors would be also applicable for developed countries. In this study we investigated performances of a demonstration-scale DHS reactor with a treatment capacity of1,000m^3・d^<-1> installed in India and a lab-scale DHS reactor set-up in Nagaoka municipal sewage treatment plant site, which were receiving effluents of UASB reactors treating actual wastewater. In addition, new technologies of nutrient removal of nitrogen and phosphate using DHS reactor have been innovated. The results demonstrated that the combined system of UASB and DHS reactors has an ability to become dominant even in developed countries.