Touching up water treatment

THE SWEDISH city of Malmo has carefully landscaped an area of about 10,000 ha, which functions as a natural stormwater treatment plant.

In Malmo, on the southern Swedish coast, several projects involving the natural treatment of stormwater are being implemented. One of them is the Toftanas Project in which stormwater from a newly developed, 200-ha industrial area is disposed in an artificially created green area, measuring 10,000 ha.

The entire area has been carefully designed as an ecosystem and it includes a pond with a volume of 58,000 cu m. Because the surrounding region is flat, the level of the whole wetland area is set 3 m below normal ground level so that gravitational water flow occurs throughout the wetland. During dry periods and when it rains wildly, stormwater from several outfalls flow through a meandering stream and exits to the downstream river (Fig 1). When it rains heavily, the whole wetland area gets flooded and three circular raised spaces within the area become islands.

Planting in the area, consisting of grass, bushes and trees, was especially chosen for their capacity to treat water. There are, in all, about 300 plant species in the pond. The three islands and the edges of the pond area are covered by Salix which has anaerobic properties in its rootzone that enables soil particles to absorb heavy metals. The aerobic zone of the flooded surface and the stream functions as an effective treatment plant. Though this system required a great deal of landscaping, the entire cost was only about four million Swedish kroners (approx. Rs 2 crore), which is less than what a conventional stormwater conduit system would have cost.