Despite the fact that the described system is a fully artificial, man-constructed wetland, its shape, appearance and many features perfectly mimic those of natural wetland habitat. As it can be seen in Fig. 1, these reservoirs are shaped into serially connected canals that make up a 9-hectare system. They have various shapes and the depths of their bottoms vary from 20 cm, through 70 cm, up to 1.5 m, changing repeatedly along the entire length of the canal. As a result, zones with the different stormwater flow rates are created, which allow effective removal of pollutants in suspended form by sedimentation. Besides, such the diverse wetland ecosystem contributes to the removal of dissolved compounds, in particular nitrogen and phosphorus, as well as heavy metals and other pollutants. The retention time of stormwater in an entire system of such the meandering canals is approx. 5-10 days. In such a long time, thanks to the activity of microorganisms that inhabit the ecosystem, the nitrogen in all forms is converted into harmless gaseous form, phosphorus and other pollutants can be also removed efficiently. Besides that, the effluent from CWs is recirculated to their inflow, thus in a rainless period (with no fresh stormwater inflow), the retention time is extended, improving the treatment efficiency and retention.
The average annual stormwater flow through the described system is about 600,000 m3, with the maximum observed annual sum reaching even 1,000,000 m3.The average nutrient removal efficiency of Vattenparken is 55% for total phosphorus, 70% for ammonia and 40% for total nitrogen. Heavy metals are also removed relatively efficiently by the CWs at Enköping, thus cadmium is removed with an average efficiency of 51%, zinc 56%, chromium 53% and lead as high as 83%. Generally the performance of this system should be assessed positively, especially taking into an account the fact the concentrations of these pollutants are relatively low in stormwater water flowing through these CWs. Most importantly, the system effectively protects the ecosystem of Lake Malaren against the eutrophication and the toxic effects of heavy metals washed off roads and other urban paved surfaces of the city of Enköping. The scale of the positive impact of these CWs on nature is evidenced by the fact that annually circa 3-5 tons of nitrogen and nearly 1 tons of phosphorus are removed from stormwater, which otherwise would go directly to Lake Malaren, and subsequently to the Baltic Sea.
Stormwater treatment and retention, the protection against eutrophication and contamination are important, but not the only functions which Vatenparken provides. It is also a natural habitat for many species, improving local biodiversity and the attractiveness of this municipal area (Fig. 2). It is also a popular destiny among inahabitants of Enköping and the guests, as an ideal place for walks, bicycle trips, sports activities, family picnics and a good opportunity to relax in nature. The wealth of species of birds and other representatives of fauna and flora causes that numerous educational activities for children and adolescents are held here.
Project Size: 9 ha (water table); 15 ha (the whole complex)
Wetland Type (System/Cells):
Free-water system; recircilation of treated stormwater; the plot for snow and ice removed from the city of Enköping
Company/Organization: Enköping Kommun (Municipality of Enköping)