Lagunen is a natural area where treated waste water from the city of Slagelse runs through, before entering Tude Stream and finally The Great Belt at Næsby beach.
Lagunen is a well known picnic area used by the citizens of Slagelses. Many birds and large bushes and trees offering shelter, makes it a perfect recreational area.
Lagunen is shaped as a number eight, with paths suited for running, biking and hiking.
Due to the increased waste water flow through the sewage plant, it was decided 2015, the capacity of treated water in the lagoon should be increased too. A penlock had to be installed in order to control flow, sedimentation and settlement of sludge.