Waste water treatment. A demonstration of the use of plants in waste water treatment. The plants are grown without soil, on the sewage. Bacteria digest ammonia in the sewage and turn it into nitrates for use by the plants. The plants also absorb phosphorous from the sewage. Aerobic bacteria live in the water and on the roots of the plants. These bacteria convert the organic matter in the sewage to carbon dioxide and other stable chemicals. Photographed at the Earth centre, Doncaster, UK.