Only 1 per cent of the world's water resources is freshwater available for human use. This limited resource has to provide a thirsty world with all its needs for agriculture, industry and human consumption. As urban growth undergoes a staggering increase, many regions of the world are already experiencing severe water stress. Aside from the unnecessary death and suffering that results from lack of safe water, economic development is seriously hampered, food production becomes expensive and many production and service industries can grind to a halt. Thirsty cities impart a severe environmental impact on water resources, both by over exploitation of fragile freshwater reserves and unacceptable disposal of wastes and toxic substances.
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 Stockholm Water Company has combined environmental and economic thinking into a single integrated concept of the water cycle and applied it to its far-reaching responsibilities – securing and safeguarding raw water sources and ensuring the quality of the receiving waters in a process in which virtually all by-products are used. To protect the quality of the main source of water, Lake Mälaren, a number of measures involving the cooperation of many parties have been instituted over a twenty year period. Now all waste water is discharged to the Baltic side of the city and there is strict monitoring of the use of fertilizers and herbicides in the surrounding area. To protect the secondary water supply, Lake Bornsjön, its entire drainage area has been acquired by the Company.
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.