## 4 Discussion
### Effect on Phosphorus
That was why the modules were able to ensure OP uptake for a longer time compared to laminates. Thus, as revealed by the mesocosm study, the laminates can be regarded as a constituting short-term measure (to be applied for short periods, such as days). This generally verified the results of our previous short-term experiment using laminates [41]. If a prolonged effect is necessary, repeated applications of laminates for short periods should be considered rather than longer exposure in the aquatic environment. To achieve a longer lasting effect (weeks, months) without the need to exchange carriers, modules represent a more advisable solution. As a consequence of technical design, the modules and laminates also showed contrasting OP removal features when comparing unit area or unit mass of the calcite applied using the two solutions (Table 1). In terms of OP uptake per surface area of the carrier, modules were clearly more efficient as they fixed about six times more OP compared to laminates (Table 1). This was because they contained 168 times more calcite per m\({}^{2}\) of the carrier, as previously mentioned. What was more, the performance of laminates was also affected by the calcite fixation by the glue layer reducing the OP removal onto laminates by up to 33% compared to loose (powdered) calcite [41], making part of the calcite layer not accessible for the water. At the same time, when comparing laminates and modules based on the OP mass removed per unit mass of calcite applied, the modules were able to remove about 0.2 mg PO4\({}^{3-}\) g\({}^{-1}\) while the performance of the laminates was almost 30 times higher (6.0 mg PO4\({}^{3-}\) g\({}^{-1}\)) (Table 1). This was the result of a much larger solid-to-solution ratio in the case of the treatment with modules. At the same time, it showed that a large part of the calcite material applied in modules did not contribute to the OP uptake and was still available, at least in theory, for the adsorbing ions. This was in accordance with the observation that at the end of the experiment, the OP concentration was still declining in the mesocosms with modules (Figure 5a), so the OP removal could be expected to last for a longer time compared to the experiment duration. This was highly likely as continuous OP uptake lasting as long as 7 months could be achieved when testing the modules prototype in a flow-through laboratory experiment [39]. The differences in OP uptake between laminates and modules impact their potential in developing a P recovery strategy following use in lake treatment. Taking into account the large solid-to-solution ratio when using modules, and the resulting low OP mass removed per unit mass of the calcite material (0.2 vs. 6.0 mg PO4\({}^{3-}\) g\({}^{-1}\) in the case of laminates, Table 1), modules appear to be a less favorable solution in terms of phosphorus recovery (less phosphorus per mass of calcite to be possibly recycled). Nevertheless, in technical terms, the recovery of calcite and phosphorus from a module is technically simpler. This is because it only requires the removal of the mesh and calcite from trays. Meanwhile, in the case of laminates, the P binding material needs to be separated from the backing, which will not be possible selectively, i.e., without the glue layer. The inevitable side effect will be the production of waste as the laminate's backing material (food foil) after such a separation will likely not be reusable. At the same time, modules present a more sustainable solution compared to laminates as their construction enables the reuse and refilling of the frame and trays with the new portion of the P binding agent after the saturation of modules with the OP will be achieved. The above discussion shows clear advantages of modules in terms of their OP removal potential, simplicity of application and reuse, and possible P recycling. However, to gain a substantial elimination of the OP available in the lake water in this study, the modules needed at least a few weeks (Figure 5a).