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A “green roof” that cleans up waste bath water for flushing toilets has been developed by London’s Metropolitan Water Company (GB 2375761). A pool of waste water around 12 centimetres deep, which forms part of the roof, is covered by a rainproof membrane. Flowers grow through the membrane with their roots in the water. A solar-powered pump circulates the water around the roots, where micro-organisms digest any organic matter. The cleaned-up water is then diverted to the toilet cistern. The roof water stays at around 5 °C, keeping the house warm in winter and cool in the summer.