Sky Water falling along a trunk of a tree though wheat or bamboo has been collected into an earthenware pot at Toshima Island in Japan and Jeju Island in Korea where have been suffered from severe water shortages. After stock of the sky water in a pot, bio-film has formed automatically inside its wall. It has been thought that it has made contribution to purify the stocked rainwater. Regarding this, DIN (Germany Industrial Standards) recommended to avoid washing out the surface of inside wall of the rain water reservoir and set calm inlet in the tank which can control of the speed of the water flow into the tank. It seems to take aim to protect bio-film of the rainwater tank.
Bio-film is the bio-complex of microorganism, such as bacteria, actinomyses, fungi and algae. They form slimes. They absorb contaminants（nutrients）in the water and purify them. As the result of this mechanism, stocked rainwater becomes cleaner. It is a key point to keep good water quality of the stocked rainwater to integrate sky water harvesting into fabric society. Institute on Sky Water Harvesting started to research on bio-film of rainwater tank in cooperation with TOHO Univ. Japan and Seoul Univ. Korea in 2009.