This strategy features in the framework of Agenda 21 as formulated at UNCED (Rio de Janeiro, 1992), now coordinated by the United Nations Commission on Sustainable Development and implemented through national and local authorities.
The entire sewer network of the city has been improved. Vast underground storage chambers have been built in key areas to prevent overflow after heavy rains. The four waste water treatment plants have also been upgraded with greater emphasis placed on biological treatment and chemical phosphorus reduction. Stockholm is now removing 95% of all phosphorus and organic material; Sweden, in concert with the other countries around the Baltic, is working to reduce nitrogen discharges by 50%. The heat in the treated waste water is recovered by the municipal energy company and used for district heating. In this way, Stockholm Water Company delivers nine times as much energy in heat as is consumed in the course of the whole treatment process. The sewage sludge is digested, and the resulting bio-gas provides the major treatment plant with 40-45% of its need for electricity. In 1995, over 60% of the digested sludge was used as fertilizer on farmland. Since this can only be safely done if environmentally hazardous substances are kept at low level, this involves the cooperation of the water consumers.
Fundamental to the integrated concept of the urban water cycle is the idea that the individual user of water is not only a consumer of clean water but a producer of waste water. Therefore stringent rules have been imposed on industrial customers for which substances may be disposed of in the waste water. Allowable limits may be based on toxicity, degradability, bio-accumulativity and corrosiveness. Mercury, cadmium and chlorinated organic compounds are prohibited. These and other special measures, including collection of fever thermometers containing mercury and cooperation with the Swedish Dental Association in order to reduce the discharge of amalgam by dentists -- has resulted in drastic decreases in the metal content of the sludge. In addition, a series of educational campaigns promote the use of environmentally safe washing and cleaning products, and teach the safe disposal of hazardous wastes by households.