In Japan, night soil, or black water, was formerly used as fertilizer for agriculture. However, during the rapid economic growth that took place after 1960, chemical fertilizers began to be adopted for agriculture and night soil was almost abandoned. How to treat the excess of night soil then became a big challenge to tackle. From that time a variety of night soil treatment technologies have been developed and the Japanese government has actively promoted the construction of night soil treatment facilities all over the country. As measures for domestic wastewater treatment, sewerage systems were spread in high-density urban areas, while a decentralized system called johkasou was diffused in rural areas with low population density. Johkasou is a unique system that was devised in Japan and mainly treats domestic wastewater on-site. Night soil treatment systems and decentralized wastewater treatment systems used in Japan are introduced in this paper, in the hope that the experience acquired in Japan will contribute to improve the sanitation conditions in other countries.

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