RESIDENTS swim and even fish for salmon -- which is safe to eat -- in Lake Malaren, between the Parliament building and the prime minister's office in Stockholm, giving Sweden's capital a unique environment, compared with most other large cities.

Stockholm has 15 municipal beaches and plans to establish more, but its water resources are under constant threat.

In the Environmental Plan adopted by the city council in 1989, the water environment was a priority area. Actions included renovation of the waste water network and construction of a storm sewer overflow basin, as well as measures and campaigns to reduce the load on the pipes and treatment plants. Responsibility for these actions was put in the hands of Stockholm Water -- a municipally-owned company responsible for all water and sewage systems in the city.

Stockholm Water has concentrated on trying to reduce pollution of the waste water passed on by the industry and on minimising the input into sewers of hazardous pollutants such as lead, cadmium and mercury and on implementing measures at source. For example, the graphics industry and photo-processing laboratories are no longer allowed to discharge used developer or photo-fixing solution into the sewer system, but must separate and treat it. This has forced larger companies to acquire their own treatment plants.

As mercury is discharged into the waste water mostly by dentists, they have been required since the mid-1980s to have "amalgam separators" in their clinics. In 1992, Stockholm Water encouraged households to return their mercury thermometers to their local chemist, rewarding complying households with a personalised letter and a bonus offer. Almost 200,000 thermometers were returned, taking 400 kg of mercury out of circulation. They have been replaced by thermometers operated by batteries, which have their own environmental problems, but Sweden already has a well-developed system of waste-battery collection.

A major discharge problem concerns household detergents and so a "Wash Wisely" campaign was launched, aimed at households and service agencies. Information sheets listing suitable cleaning agents for various purposes were distributed.

The campaign identified as least suitable the products of a major supplier of detergents to day nurseries in Stockholm. As a result, the firm was black-listed and its products disappeared from the market, though it once commanded a 70 per cent share of the market.

Cars contribute to stormwater pollution through exhaust emissions and cause wear and tear on asphalt roads, especially in winter, when studded tyres are used. The only way of reducing these pollutants is to reduce traffic. Stockholm Water has yet to make progress in this area, though it has started impact studies relating to dioxins from traffic emissions and on other metals and compounds that are known to be injurious to health. Solutions being considered are a ban on studded tires in central Stockholm, separate conduction of stormwater from streets most travelled and reducing use of leaded petrol. The lead content in sludge from sewage water treatment plants has dropped by half since unleaded petrol was introduced less than a decade ago.

The city has also signed an agreement with local farmers to reduce the use of fertilisers and chemicals so as to protect Stockholm's reserve water supply -- Bornsjo Lake.