The Effectiveness and Durability of the Sequential Application of Iron and Aluminum Coagulants as a Tool for Phosphorus Inactivation in Different Types of Lakes

The efficiencies of the restoration of three lakes of varied water dynamics—the polymictic Mielenko Lake (7.8 ha; 1.9 m), the meromictic Klasztorne Małe Lake (13.7 ha; 20.0 m), and the holomictic Klasztorne Duże Lake (57.5 ha; 8.5 m)—with the use of the phosphorus inactivation method and with the sequential application of iron and aluminum compounds have been compared. The full dose of the Fe coagulant used for the restoration of these lakes varied between 9 and 93 tons (littoral zone). The full dose of the Al coagulant applied in the studied lakes ranged between 9 and 148 tons (profundal zone). After the application of the reagents, greater efficiency in phosphate removal from the water column was observed in the stratified Klasztorne Małe and Klasztorne Duże lakes. Notably, in both lakes, inhibition of the internal loading process was observed. Research carried out several years after the completion of lake restoration showed that the obtained effects persist quite well in Klasztorne Małe Lake and Klasztorne Duże Lake, and in the case of the shallow-water Mielenko Lake, there was a significant increase in the concentration of phosphorus compounds in the bottom parts of the water. This phenomenon may be related to the shallow depth of the lake, high coupling between water and bottom sediments, and the large surface area of the active bottom. The results obtained for the Klasztorne Lakes show that the use of two types of compounds makes it possible to reduce the cost of restoration, and moreover, the dosing of iron salts in the coastal areas of the lakes ensures a higher level of ecological safety.