Wetlands treat water in Soderhamn, Sweden
Solution to which problem
Constructed wetlands are engineered systems that have been designed and constructed to utilize the natural processes involving wetland vegetation, soils, and the associated microbial assemblages to assist in treating wastewaters. One of the main goal of this implementation in Söderhamn was to reduce the nutrient loads to the recipient. The discharges from the sewage treatment plant had previously gone straight into the Söderhamn River, the municipality wanted to change this practice.
Technical conditions
Granskär WTTP has a capacity of 22 500 inhabitants (real value 14 500 PE) and the average daily inflow rate is 6800 m3/d. The last step of purification is made by a constructed wetland situated in Söderhamn area. The wetland acts as an additional purification step for the wastewater treated in Granskär wastewater treatment plant.
Granskär’s wetland has an area of 9 hectares of which five are filled with water. The water depth vary from 50 to 200 cm and the water flow is around 150-600 m3/h. In total there are 8 ponds.
The size of the wetlands is based on the volume, total volume is around 65 000 m3. The volume-based methods use a hydraulic retention time to assess the pollutant removal, in this case it is 11 days, although, field studies have indicated that it is shorter. In order to increase the retention time actions will be performed.
Implementation
The municipality of Söderhamn created a wetland alongside a sewage treatment plant. The wetland was created by excavating an overgrown sea bay with a rich bird life and abundant vegetation. The removal material was deposited in the area, partly as embankments between the ponds that were created. The municipality dug new channels from the sewage treatment plant to the wetland.
Phosphorus and nitrogen is taken up by plant and algae, the process is therefore the most intensive during spring and summer. Nutrients can be transported to other ecosystems to animals that eat the plants, phosphorus is also absorbed to minerals which settle to the sediments. The process requires adequate oxygenation otherwise eutrophication can occur. For that reason the last ponds are shallow, in order to secure good oxygenation of the water.
Nitrogen is reduced within the wetland and released to the atmosphere by the denitrification. The first ponds within the wetland are deep in order to create optimal condition for denitrification (access to organic carbon and low oxygen levels). Creating sustainable and ecocycle-adapted water and wastewater systems is important both for the infrastructure of society and for environmental efforts.
Created wetlands have important water-managing functions by acting as treatment plants, reducing flooding and contributing towards reduced eutrophication. Wetlands also account for a significant share of our biodiversity.
Result
The wetland now acts as an extra treatment stage for nutrients and metals and reduces the nutrient load on the inlet of Söderhamnsfjärden. Thanks to this good practice a lot of positive environmental and economic impacts occurred. The inputs to water have decreased in 2016 by 460 kg/year for phosphorus and by 5.7 tons/year for nitrogen. BOD load decreased by 1.6 tons/year. The wetland is now a significantly more varied environment, benefiting biodiversity. The wetland treatment makes it possible to optimize the chemical purification step at the sewage treatment plant, thereby making it possible to use less chemicals, saving both money and the environment.
At the same time, the residents of Söderhamn gained a new recreational area that has also become a resting and breeding site for many birds.
With the aid of the wetland, the objective of moving the discharge point was achieved and emissions were substantially reduced, at a lower cost than the alternatives.
The wetland is the most northerly in the world for the treatment of sewage.