Part of the drinking water demands of the city of Amsterdam and the surrounding district are met by abstraction from nearby Lake Loenderveen that is fed by two different kinds of surface water. The lake itself is subject to rainfall, evaporation and seepage. It acts partly as a stockpiling reservoir (about 2 weeks) and partly as a self-purifying medium (retention time about 100 days).

The design and management of the lake are such that a high degree of mixing of the inflowing water is obtained and an almost constant water quality, which greatly facilitates subsequent treatment to obtain high quality drinking water.

To combat eutrophication the nutrient-containing water that enters the lake is dephosphated. After a number of years working with an experimental and simplified method in the lake itself, a highly efficient new coagulation and settling plant was constructed. This has been in operation since 1984. The improvements in the quality of the lake water are discussed in this paper. In addition to a considerable reduction in the phosphate content (and that of other nutrients), improvements were registered in a number of other water quality parameters (DOC, heavy metals, colour c.a.).

A similar coagulation and settling system is also used to treat one kind of surface water that is fed into the adjacent Loosdrecht Lakes during dry summer spells.

These lakes are of great environmental importance; they are also unique features of the landscape. Pretreatment of the water that flows into the lakes has a substantial effect on the water quality. A monitoring system has been set up to check the resulting improvement.

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