A STUDY ON DEVELOPMENT OF ENERGY SELF-SUFFICIENT SEWAGE TREATMENT SYSTEM

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The first, a questionnaire survey to the sewer institutions was conducted. Questionnaires were sent to 377 cities in Japan to seek for the energy consumption rate of the current sewer system. An answer was provided from 296 cities. As a result, it was appeared that a quantity of necessary electricity to treat unit sewage volume is 0.46 kWh and amount of 0.006 m^3 sludge come from unit sewage volume after treatment. Chemical heat capacity within unit sewage volume was approximately 1000 kcal. It was calculated that self-sufficient of electricity was more than 60% if unused energy was possible to apply a drying process of sludge.Next, energy recovery from sludge was estimated about next three methods. An index of estimating was energy consumption rate (ECR). This index means that a ratio of the amount of input energy to the amount of recovering energy. The first was sludge dehydration and drying processes with freezing and thawing pre-treatments. In this system ECR became 1.8 despite the effective use of a heat pump in drying process. The second was digestion. ECR became 0.93. Although using a cogeneration process was assumed, the value of ECR is near 1.0. The third was transformation to oily liquid from sludge. It was estimated that ECR was 0.55. This value means that the amount of energy recovering was approximately 2 times of input energy. Although various problems in actual plants, the transformation to oily liquid from sludge has some possibility of actualizing a energy self-sufficient sewage plant.