Sumida is a district in the centre of Tokyo. Many large residential areas with single-family homes are located here. The Numada's home was built in 2005 in a gap between other buildings of less than 50 m² floor space, designed by Shinji Munakata.
Rainwater harvesting makes a lot of sense in Sumida for 2 reasons: The district administration pays EUR 400 per m³ of storage volume. Meaning that the building owner is given approx. 45% of the overall costs of his rainwater system in the form of subsidies. The second reason is the annual precipitation of 1,500 l/m².
With a filter efficiency of more than 90% from the WISY filter collector, 36 m³ of drinking water can be replaced with rainwater every year. It is mostly used for flushing the toilet. The pipe feeding the flushing cistern is designed to accommodate a rainwater pipe and a drinking water pipe. Via a simple lever, the feed pipe can be switched whenever the cistern is empty. Where the car is parked on the ground floor level, a tap is installed that is connected to the rainwater system for cleaning the car. For safety reasons, it is integrated with a removable top valve. All pipes and tapping points are marked with "Rainwater" to avoid confusion.