and created concerns over the emission of hydrogen chloride and other hazardous substances from incinerators, an issue that attracted attention as an emerging environmental problem. This was the starting point for Japan’s efforts to ensure that waste incineration facilities had adequate environmental conservation capability, including measures against dioxins. To address the problem of night soil disposal, an advanced system for sanitary night soil disposal was established for traditional Japanese vault toilets, while sewage systems and johkasoh were made widely available in order to meet the needs of flush toilets. In particular, as a result of technological advances, new johkasou were developed which were small enough to be installed in homes yet still had the same capability for high-quality treatment as public sewage systems. These tanks are now widely used.

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## I ntroduction of sorted waste collection (Hiroshima City)

In the postwar high economic growth period, Hiroshima City experienced a sudden increase in waste generation, as did many other cities throughout Japan. In Hiroshima City, kitchen garbage, categorized as "Hiroshima garbage", was once used for fertilizing farmland in islands and other rural areas of the city. However, demand fell as chemical fertilizers came into general use. The existing waste disposal system was abolished in 1960. With no other place to go, most of the waste in the city was disposed of in landfills in the Showa 30s to 40s (1955-1974), but the city government increasingly found it more and more difficult to secure enough landfill sites. At Hesaka Junior High School, which was built on a former landfill site, there was even an incident in which methane erupted from the school yard.

In response, the Hiroshima City Government declared a state of waste disposal emergency in 1975 and urged the public to consider the waste disposal issue as their own problem. In an effort to reduce waste generation, in the following year (1976) the city introduced the sorted collection of waste for the first time in Japan. In this collection system, everyone was required to separate their own garbage into five categories: (i) combustible waste, (ii) non-combustible waste, (iii) recyclable waste, (iv) large-sized waste, and (v) hazardous waste. Although people were confused at the beginning, the new collection system gradually became established and has since come to be widely known throughout Japan as “the Hiroshima system”. This has now become a landmark in the history of waste disposal and a pioneering example demonstrating that even a big city like Hiroshima can operate a sorted waste collection system by gaining the understanding and support of its citizens.

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## The semi-aerobic landfill structure (the Fukuoka method)

One of the landfill systems now used for final disposal sites is the semi-aerobic landfill structure (the Fukuoka method), which was first proposed in the second half of the Showa 40s (1970-1974) by Fukuoka University and the Fukuoka City Government. Because of its ease of construction and maintenance and its ability to quickly stabilize waste and improve the quality of the leachate, this landfill structure was adopted by the former Ministry of Welfare as the national standard and has been used by many municipal governments when constructing their landfill sites.

This structure uses large-diameter leachate collection and drainage pipes installed at the bottom of the landfill site in order to drain out the leachate and allow outside air to naturally flow into the inner landfill through these pipes via convection induced by the heat of waste decomposition. This design increases the decomposition capacity of the aerobic bacteria present and thereby improves the quality of the leachate. The ease of construction and maintenance is another advantage of this method, and