## 4 Discussion
### Possible Application and Management  
The treatment of a water body using the approach proposed in this study requires the structures (laminates/modules) to be safely exposed in the water column and removed after the finished treatment. We generally imagine that these structures can be installed in water by suspending them on buoys, as schematically shown in Figure 1 in Section 1, or onto a floating platform. After the end of the operation, the modules can be cleaned of the used reactive material, refilled, and applied in the same or another location. In the case of laminates, the reuse of the carrier (laminate backing) will not be possible, which presents a challenge in terms of waste management [41].  
It is also worth taking a closer look at the quantitative aspects of the application of both solutions in the aquatic environment. When expressing the experimental design for 1 m\({}^{3}\) of lake water, we gain 0.41 m\({}^{2}\) of the total ray surface in five modules and 0.83 m\({}^{2}\) of laminates (11 strips with the size of 15 \(\times\) 50 cm). The corresponding calcite mass values are 1.65 and 0.020 kg, respectively. To compare this with conventional methods for P immobilization in lakes, we assessed the dosages needed for treating 1 ha of a lake area. And so, assuming the lake depth of 5 m, we obtained the mass of calcite of 1 t in the case of laminates and 82 t in the case of modules. The material dosage in the case of laminates was thus comparable to that applied when using the common P binging agent, Phosphock, in a single addition, which was 1.4-6.7 t ha\({}^{-1}\)[78]. The material consumption in the case of modules was substantially larger. Still, it was comparable to the amount of calcite needed when restoring the lake by sediment treatment. This required the formation of an active barrier on the sediment surface to prevent internal loading. The typical thickness of a calcite barrier is ca. 1 cm [46; 47; 79], corresponding to about 100 t per ha of the lake's bottom, which was close to 82 t ha\({}^{-1}\) in the case of our modules. If we need to scale the dimensions of the modules and laminates from our mesocosm experiment to field conditions, the estimated total surface area would be 40,000 m\({}^{2}\) in the case of laminates and 20,000 m\({}^{2}\) in the case of modules when treating the total volume of 1 ha of a lake with a depth of 5 m. Such dimensions are basically unrealistic. It is more reasonable to assume that one module should be installed, for example, for a treatment area of ca. 10 m\({}^{2}\) of a lake surface with an adjusted mass of the P binding agent. Then we obtain 1000 modules per ha of lake, which is still a very high number, especially when considering the application and removal of these structures. Limiting the number of modules would require their loading with a higher mass for the P binding agent, which would be possible by enlarging the number of trays and/or adjusting their depth. In this light, the method proposed can be difficult to implement in large-scale applications. It may be suitable for rather small water bodies instead, or specific areas, such as the vicinities of recreational zones or inflows, polluted bays, or deep lake regions (where the OP accumulates above the sediment due to internal loading).  
Although technical issues are an important challenge here, we believe that the numerous advantages of the proposed approach compensate for these difficulties. The most important benefits from implementing solutions for non-invasive P removal are the possibility of adjusting the depth and duration of the treatment and the removal of the P-loaded agent from the aquatic environment, enhancing the P export and reducing the risks of side effects of the treatment. We believe non-invasive P removal is a good direction for the development of P immobilization and removal methods to be used in aquatic environments, for minimizing potential adverse impacts and enabling the P binding agent and P reuse.