## 1 Introduction
An important limitation of this approach comprised losses of the P binding agent from the packaging due to the porous structure of the packaging material (which was necessary for ensuring the contact of the solution with the P sorbing medium) or a reduction in the OP uptake (compared to freely added agents) caused by less permeable packaging materials [35; 40]. To increase the exposure of the binding material (its area effectively interacting with the OP ions in the surrounding water), we developed another solution--the so-called laminates, which are strip-shaped, flat surfaces covered with a thin layer of the sorbing medium [39]. In a short-term experiment, the laminates showed good stability in the lake water and OP removal reduced by 21% (on average) compared to the lose P-reactive material [41]. However, laminates only allow for the application of a relatively small amount of the reactive agent if applied in a reasonable number [39]. Thus, we further developed another solution, with the so-called modules being tray-shaped structures filled with the P binding material and arranged in tiers. This approach makes it possible to apply larger doses of the agent per unit area of the carrier, still maintaining a relatively large surface of the agent exposed in water. The design of the modules employs the use of refillable trays and thus enables the exchange and recharge of the reactive material (which is not possible in the case of laminates). In this study, we performed a long-term experiment to compare the longevity of both developed solutions (laminates and modules) in terms of their sorption properties and effects on lake water chemistry. The study built upon short-term batch experiments on laminates and testing the prototype of modules in a long-term flow-through experiments, both performed in synthetic lake waters [39; 41]. As the P binding agent, we used mineral calcite in the form of a limestone rock. Calcite, consisting of calcium carbonate, is a natural component of aquatic ecosystems contributing to P cycling in the environment [42]. In lakes, it plays an important role in their self-purification, generally sequestering P from the water column during the process of co-precipitation and/or contributing to P retention in sediments [42; 43; 44; 45]. The source of calcite-based materials for different applications comprises carbonate rocks, such as limestones, or synthetic calcites [46; 47; 48; 49]. Bio-waste materials containing calcite (or its polymorph, aragonite), e.g., eggshell or oystershell, have received attention as well [50; 51; 52]. Calcite is generally considered a non-toxic and low-cost adsorbent [53; 54]. Depending on the physio-chemical conditions, the P uptake onto calcite can employ different mechanism: adsorption, co-precipitation, and/or phosphate precipitation with newly formed calcium salts [55; 56; 57; 58; 59], the latter process being generally more stable compared to reversible adsorption [56]. Calcite materials have been investigated as potential reactive agents with respect to phosphate, and other polluting substances, to be used in different settings, including lakes [46; 47], rivers [60], or wastewater [54; 61].