Impact of effluents from Lyby wastewater treatment plant on the nitrogen content of Lake Ringsjön and Rönne Å River, Sweden
Lake Ringsjön is situated in the south of Sweden, in the county of Skåne. It consists of three shallow lakes in series: Sätoftasjön, Östra Ringsjön and Västra Ringsjön, with the outlet in the river Rönne å. The lake has been eutrophic since the 1960s. One large point-source to the lake is the municipal wastewater treatment plant (WWTP) Lyby where most of the wastewater from Hörby is treated. In this paper, the impact of this pointsource on the water quality of the lake and the outlet, Rönne å, has been modeled with regard to the nitrogen content. A model of the three basins and six of the biggest effluents was built to try to see how the system would reply to changes in the inlets and if a decrease in the nitrogen concentration at Lyby WWTP would affect the concentration in Rönne å. The model indicated that the retention time was the most important factor for the outlet concentration of nitrogen in the lake, i.e. the closer to Rönne å a point source is, the more impact it has on the outlet. The results from the model were used when Hörby municipality got permit from the environmental court to continue to discharge wastewater to the lake without nitrogen treatment.