Ecological adapted stormwater treatment in Kalmar, Sweden
Solution to which problem
With every rain, large amounts of pollutants flow directly into surface waterways or are channeled into storm sewers, which eventually discharge to the Baltic Sea. For this reason, stormwater needs to be managed in a sustainable way adapting to ecological treatment solutions and thereby preventing unnecessary discharge of harmful substances. The techniques used for dealing with stormwater runoff in urban areas have had a focus on diversion and flood prevention.
Technical conditions
Hagbygärdediket was the wetland system taken into account in Moment project. The main focus in this area was to improve the treatment of stormwater runoff within the City of Kalmar.
Hagbygärdediket is a wetland system consisting of three major basins connected in series. The existing stream is diverted by two 600 mm pipes, and an open ditch to the first sedimentation basin. This basin has an area of about 5 500 m2 and an average depth of 1.4 m. The lower part of the stream Hagbygärdediket is located at such low levels that pumping is required in order to lift the water to the wetland system. For this purpose, a pair of propeller pumps is located upstream the first sedimentation basin.
An oil separation device is located at the outlet of the first sedimentation basin. From the oil separation device, water is diverted further to a vegetation surface of about 1 700 m2, and subsequently leads to an additional sedimentation basin with an area of 4 200 m2. This sedimentation basin has an overflow that diverts into an open ditch. The ditch has a width of 2 m and the bed consists of ground limestone. The limestone filter strip is designed to reduce phosphorus which binds to the particles in stormwater. A culvert diverts the water to a narrower ditch, before it reaches the common outlet stream of Törnebybäcken and Hagbygärdediket.
Implementation
The main objective of this project has been to improve the treatment of stormwater within a heavy polluted part of Kalmar Municipality. Hagbygärdediket was the largest contributor regarding stormwater pollution discharge within the city.
The recipients for stormwater discharges consist of a group of water bodies within Kalmarsund, which is part of the Baltic Sea. At that time, there was a risk that the water bodies will not meet the EU Water Framework Directive (WFD) environmental objective of at least good status by the year 2015. The main problems were eutrophication for the ecological status, and concentrations of heavy metals for the chemical status. In order to reach good ecological status by the year 2021, and good chemical status by 2015, environmental quality objectives have been prescribed for the water bodies.
In catchment Hagbygärdediket, the land use is quite mixed, with a dominance of industrial and commercial areas. There is also a major national road running through the catchment with a daily traffic of about 15000 vehicles per day.
Result
The wetland system is receiving and purifying about 800 000 m3 of stormwater per year. The latest results from the monitoring program, taken in June 2013, show that the expected results are being met. The total amount of phosphorus retained by the wetland system is above 50% and the total amounts of nitrogen retained add up to 35%. This mean that the wetland system retains 110 kg phosphorus (P) and 470 kg nitrogen (N) per year. Once the wetland system is fully functioning the amounts of nutrients retained are expected to increase further.
The initial results from the wetland system indicate that it works in a satisfactory manner regarding reduced amounts of nutrients. One could further expect that the wetland system should function even better once the flora and microorganisms are fully developed.