## 4 Discussion
### Effect on Phosphorus
One of them is using synthetic calcite showing typically higher surface areas and OP capacity compared to ground limestones, at, however, a higher material cost [46; 48; 64]. The mechanical processing of limestones is a further option as it can boost the sorption properties of powders by modifying their granularity and surface area [61; 65]. The material used in our design was mechanically activated by grinding, which increased its affinity for the OP by at least seven times (see Section 2.1). A short grinding time of 0.5 h was applied as it was practically convenient. Prolonging the grinding time or increasing the force of grinding (e.g., by modifying the grinding speed) can further boost the sorption potential [66]. Using calcite with another natural P binding agent is also possible. [PERSON] et al. [67] enhanced the P adsorption of calcite by up to two times by combining it with dolomite. A common method for improving the CaCO\({}_{3}\) reactivity is its calcination to produce more reactive CaO. Using this approach, [PERSON] et al. [54] enhanced the OP uptake of eggshell waste by 80%. However, the higher reactivity of CaO means its rapid dissolution and a resulting pH increase, reaching a very high level of around 12 [54]. Although calcination offers an excellent possibility to gain efficient P removal, the strongly alkaline properties of CaO exclude this as an option in our target field of applications (lakes and other aquatic ecosystems). At the same time, it has to be noted that in real world applications, the efficiency and kinetics of OP sorption onto modules (and laminates) will strongly depend on the actual in-lake conditions and may vary from those observed under our experimental design. Surfaces submerged in water (such as the laminates and modules tested in this study) should be expected to undergo a rapid colonization by periphyton, even within few days, as reported for example by [PERSON] [68] and [PERSON] et al. [69], who applied artificial structures for enhancing self-purification in water bodies. Thus, laminatesor modules can be easily overgrown by aquatic organisms, including algae (assuming sufficient light access). If primary producers develop onto the modules/laminates, an additional biologically mediated OP removal can decrease the OP availability and, by this, support the performance of laminates/modules in limiting phytoplankton growth. Such a process has been reported by [PERSON] et al. [69] for artificial substrates applied in lakes. Assuming the generally higher underwater area of modules, compared to laminates, this effect is to be expected to be more important in the case of modules. What is more, biological processes taking place in the ecosystem during module/laminate exposure can also affect the behavior of calcite, e.g., by promoting [70] or inhibiting mineral dissolution [71] with possible consequences for material reactivity and OP sorption. We discussed these potential interactions in [PERSON] et al. [41]. Furthermore, lake water properties during treatment will also influence the performance of non-invasive P removal in lakes when using calcite as a P binding agent. These are mainly temperature, pH, saturation with respect to calcite, and OP concentration, as they affect the occurrence and efficiency of OP uptake by adsorption, co-precipitation, and precipitation (see the review provided in [PERSON] [62]). As these parameters change in lakes with season and depth, the performance of calcite-based solutions for P inactivation and removal will depend on the timing and depth of application [41,62].