Case Studies

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The majority of night soil (human excrements) is source separated from other sewage water and treated at night soil treatment plants in Japan. Efforts have been made to achieve material recovery from this organic mass, together with other organic wastes such as kitchen wastes and manure, by expanding the functions at night soil treatment plants. These facilities with expanded function are called "sludge retreatment centers", which are promoted by the Japanese Ministry of Health and Welfare. Potential environmental impacts of sludge retreatment using two presently available systems in Japan are analyzed for comparison.

Systems compared are: 1) Mebius system, a high-speed fermentation process with methane gas recovery and compost production and 2) a comparable system with a basic composting process. The functional unit for this study is concurrent treatment of 40 t/d of kitchen waste, 40 m3/d of night soil, and 60 m3/d of private sewage treatment tank sludge. Impact assessment on Global Warming (IPCC 1996, 20 yrs.), Acidification Potential (De Leeuv – AP), Eutrophication Potential (De Leeuv – EP) and Resource Index (Fava/SETAC & Heijungs) all indicated that sludge retreatment with Mebius system provides a better environmental performance. The main reasons are: 1) production of power using recovered methane and 2) reduction of sludge volume by digestion, which leads to reduction of fuel required for sludge drying. The collection and treatment of night soil and kitchen wastes involves many economic and social factors. Therefore, more studies with different functional units on these systems should be made to obtain a more complete picture that can be used for decision-making processes. The results of this study can be used as a starting point.