On the shores of Pien-Saimaa, seven stormwater wetlands have been constructed in the urban area. The constructed wetlands act as natural filters. They collect solid material as well as nutrients and other impurities from run-off water, before it reaches Pien-Saimaa. In the summer, a well-functioning wetland may also be an important remover of nitrogen.
The wetlands capture solids, nutrients and other pollutants transmitted by stormwater runoff.
The wetland has three pool areas and a part resembling a stream. The pool areas slow down the flow of water, allowing the materials that burden Lake Saimaa to settle in the wetland pools. The pool areas have been constructed at different heights on the slope. Gravel structures have been used in the pool areas so that they are able to withstand water caused erosion. As water flow rates vary considerably, preparations included measures for handling overflows. The water is guided to the wetland via a stormwater network.
The network of wetlands also increases biodiversity and provides a habitat for many birds and insects. The wetlands enhance the scenery and amenity of the area as well.
Wetlands have traditionally been used for agricultural water pollution control. In Lappeenranta, the purpose of the wetlands is to stop nutrients and solid materials brought by stormwater, instead of nutrients from agriculture.