# Is It Possible to Restore a Heavily Polluted, Shallow, Urban Lake?
## 4 Discussion
### Conception of Restoration Treatments  
In the past, Karczemne Lake was overloaded by pollution from the catchment, which was a result of the long-term input of raw wastewater, including domestic, sanitary, storm and industrial waste. Under these conditions, it is necessary to use the optimal method of lake restoration after protection techniques are implemented in the catchment. The optimal restoration method for Karczemne Lake will be the removal of deposits due to their unusually high concentration of phosphorus. Additionally, for the stabilization of environmental conditions in the reservoir, the phosphorus inactivation method and biomanipulation will be applied. For the purpose of restoration treatments in Karczemne Lake, there are separate extraction fields (10) in which the outputs of the liquid and solid fractions will be successively carried out for removal. The field currently undergoing the deposit removal process will be separated from the rest of the lake by a plastic curtain to prevent the movement of the deposit. The removal of the deposits will be carried out by a method with the use of an innovative device, the only type of equipment in Europe (suction dredge with wormwheel), precisely designed for lake dredging. Extracted output will be pumped to a pipeline on the northern shore of the lake and later by hydrotransport to a sewage treatment plant. This technical solution guarantees prevention of the uncontrolled runoff of leachate back into the lake basin. The pipeline will be made of plastic (Figure 10). The proposed method does not require the construction of a completely new deposit treatment system consisting of presses, centrifuges, polymer-dosing stations, water-conditioning equipment and reaction pools (Figure 10). In the sewage treatment plant, the sediment will be dehydrated in centrifuges and then subjected to hygiization. The part of the sediment that is contaminated with heavy metals will be transported for utilization. Unpolluted sediments extracted from the lake and processed at the sewage treatment plant can also be valuable material for improving the soil structure, as they may contain premineralized organic compounds and easily digestible fertilizers, including macro- and microelements such as nitrogen, 1.5% N; phosphorus 3, 5% P2O5; calcium, 4.22% CaO; magnesium, 1.6% MgO and iron, 1.8% Fe2O  
Figure 10: The scheme of planned restoration. (average contents of elements in dry mass of the Karczemne Lake sediments). Another form of deposit utilization can be the reclamation of degraded areas, for example, landfills [52; 53; 54]. In relation to the current level of external loads, the quantities of phosphorus and nitrogen that will be extracted from the lake during reclamation (loads coming with surface inflow, outlets of rainwater collectors, recreation and atmospheric precipitation) correspond to the amount of pollution that reaches the lake in 1300 years for phosphorus and 300 years for nitrogen. This confirms the validity of the assumptions behind the reclamation methods adopted, for which the priority was to remove the bottom sediments outside the ecosystem. The removal of the bottom sediments from Karczemne Lake will be carried out within two years, from spring to the moment of the lake ice cover. In the extraction field in which the sediment will be removed, the first phase of phosphorus inactivation to chemically precipitate pollutants distributed in the water column as a result of the dredging will be carried out. After the entire lake basin has been dredged, the next four stages of phosphorus inactivation (spring, autumn, spring and autumn) will be carried out. Based on the experience and research results obtained by the Department of Water Protection Engineering in the field of lake reclamation by phosphorus inactivation [2; 7; 55; 56], two types of coagulants will be dosed: PAX 18 and PIX 111.