Rationale and Implementation of a Strategy to Restore Urban Lakes in Berlin: Results after Ten Years of Phosphorus Removal
Abstract All lakes in the Berlin area have received excessive phosphorus loads during the last century. Restoration measures during the 1980s reduced the phosphorus input to three of these lakes considerably (from about 0.5-2 mg/L down to 0.005-0.02 mg/L total P). Although the chief measure—phosphate precipitation (flocculation, sedimentation and filtration)—reduced input to the greatest extent exercised so far worldwide, it became evident that such lakes may nevertheless need 4-6 years to show significant signs of recovery. Although the nutrient budget reacted rapidly, biocoenotic reactions differed between the lake types. The Berlin experience shows that urban lake areas cannot be restored by conventional sewage treatment and restoration techniques, but will need a great variety of additional techniques in order to show success. P- removal from the main inflow down to a few µg/L of total P is a minimum requirement. However, further well suited techniques have to be developed for the specific situation in each of the Berlin lakes in order to achieve nutrient removal to the extent which is in accordance with the present knowledge of eutrophication processes as well as with the special situation of each case. This refers to dosage of chemicals, removal of algae from treated surface water, filtration techniques, and hypolimnetic drainage. The costs of lake restoration in Berlin amount to less than 0.30 DM per m3 of treated water—a price which has demonstrated good results in the last few years. These results are of greatest importance for the further planning of water resources management, recreation, and for a safe drinking water supply for the growing urban area of reunified Berlin with its 4 million inhabitants.