RESTORATION CONCEPT FOR LAKE TEGEL, A MAJOR DRINKING AND BATHING WATER-RESOURCE IN A DENSELY POPULATED AREA

Although Berlin is rich in lakes and rivers, discharge rates are low. The water bodies are intensively used in many ways-for gaining drinking water from bank-filtered water, for recreational purposes, as recipients for treated sewage and stormwater, and as waterways. The example of heavily eutrophicated Lake Tegel shows the development of protection and restoration measures for securing surface water quality, especially with respect to drinking water and recreation. Numerous measures were undertaken to improve the water quality of Lake Tegel, such as replacement of oversaturated wastewater irrigation fields by sewage treatment plants, partial diversion of a heavily loaded inlet, treatment of stormwater runoff, further sewerage, aeration, sludge dredging in the harbor area, protection of the remaining reed belt, and construction of a phosphorus elimination plant directly at the major inlet of the lake. By a four-step process, this plant is capable of reducing total phosphorus (P) input concentrations from 5 mg/L P or more down to approximately 0.02 mg/L P and, therefore, proved to be the most important measure. By providing enough water with a low phosphorus content for exchanging the lake volume three to four times a year, this elimination plant caused a decline of phosphorus concentration in the lake from roughly 0.8 mg/L P in 1984 down to the present values of between 0.06 and 0.13 mg/L P (annual means). The algae are beginning to respond to this decrease by a reduction of maxima and an increase in water transparency. The Berlin experience shows that treatment of the inflow was the correct option in a situation where diffuse sources cannot yet be sufficiently controlled.