Increasing and expanding the use of general service water is one of the measures for effective utilization of water resources. General purpose water is rainwater and reclaimed water. It was in 1978 that general service water attracted wide attention in Japan. Fukuoka Prefecture had a severe drought in 1978, and the nation and local governments prompted the introduction of general service water. In Fukuoka Prefecture, it is regulated by an ordinance that a large building with more than a certain area of floor space should use general service water for flush toilets. The Tokyo metropolitan government also asks developers of buildings bigger than a certain level to use general service water for flush toilets and install a system for rainwater infiltration.
The current sewage system is generally designed to cope with a rainfall of 50 mm per hour, but unexpected torrential downpours occur frequently these days because of global warming and the heat-island phenomenon, causing reverse flow in the urban district. Tokyo Sky Tree in Tokyo scheduled to be open next spring has an underground rainwater reservoir with a capacity of 2,635 cubic meters. In addition to this underground reservoir, the district where Tokyo Sky Tree is located has more than 300 reservoirs to prevent a flood caused by torrential downpours.
The government estimates that if all single-family houses in the Kanto area have a rainwater reservoir with a capacity of 0.5 cubit meters, rainwater of 270 million cubic meters will be retained. This volume is about 7% of the total water supply amount in this area. Should half of this volume be used for daily life water, the volume of water restriction will be mitigated by about 5%. Using rainwater and reclaimed water for water spray, car washes, artificial cooling, and fire fighting contributes to building a society strong against drought and flood. At the same time, it will reduce the load of sewage treatment facilities and contribute to the improvement of water quality. (To be continued)