## 1 Introduction  
Huge amounts of eggshell waste (ESW) have been produced worldwide annually, but the majority of it unfortunately ends up in landfills, presenting a significant ecological burden, as well as a potential biohazard [1; 2; 3; 4]. This is quite surprising since numerous scientific reports and patents [3; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16] clearly show the multiple usage possibilities of ESW. Regardless of the fact that ESW may be used to produce low-value added products, such as fertilizers or food for pets and animals, or to produce single value-added products, there is still a multitude of possibilities for ESW utilization, especially those oriented toward the production of high value-added products approaching the \"zero-waste\" model. ESW originating from fresh hen egg breaking in households, restaurants, as well as egg breaking plants, could be roughly divided into three structurally different parts each containing unique set of high value molecules of potential use (Figure 1). Adherent egg white layer comprising the inner part of ESW, present in the amount of about 3% of ESW on dry weight basis, is a rich source of valuable egg white proteins, such as ovalbumin, ovotransferrin and lysozyme [1, 4, 5, 17]. This sticky egg white layer is adhered to the eggshell membranes (ESM) that comprise about 3% of ESW on dry weight basis, and presents a rich source of collagen fibers, hyaluronic acid and other valuable glycosaminoglycans [1, 18, 19, 20, 21]. Finally, the outer part of ESW is a calcified matrix mainly composed of calcium carbonate comprising between 92 and 95% of ESW on dry weight basis, and some highly valuable antimicrobial eggshell proteins present in low amounts [1, 4, 16, 22, 23]. As we have just stated, each part of ESW could be used for the production of high value-added products, instead of being used partially, or in the worst-case scenario discarded on landfills. In this respect, our research group has been oriented towards the possibility of the complete utilization of eggshell waste, using the most common industrial processes, such as mixing, filtration, evaporation and drying, but with the aim of producing high-value-added products. Our most recent publication [1] has proven the possibility of ESW utilization for the production of egg white protein solution, calcium chloride dihydrate as a food-grade additive, and eggshell membranes, using ESW transformation with 5% (\(w/v\)) hydrochloric acid. In continuation of our efforts oriented toward the complete utilization of ESW, in our present research, we have examined the possibility of chemical transformation of ESW with acetic and \(o\)-phosphoric acid to the corresponding calcium salts and eggshell membranes. Although the use of \(o\)-phosphoric and acetic acid for the production of corresponding Ca-salts from eggshell waste, such as calcium hydroxyapatite [2, 8, 10, 13, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39], calcium triphosphate [30, 31, 32, 34, 38, 30, 32, 35, 36, 37, 38, 39, 40, 41, 42], calcium diphosphate [42, 43] and much less studied calcium monophosphate [42] or calcium acetate [44, 45], have been reported, the novelty of our research lies in the use of eggshell halves and pieces instead of milled ones, the chemical transformation of eggshell waste calcium carbonate with diluted acids instead of those with  
Figure 1: Structural parts of eggshell waste and their most valuable components. Contribution of each structurally different part of eggshell waste in total eggshell waste amount is approximated on the basis of reports by [PERSON] et al. [1], while the most valuable components of each part of eggshell waste comes from references [1, 2, 4, 5, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26]. higher percentages, the use of room temperature during the ESW chemical transformation into salt solutions and ESMs instead of higher ones, as well as the production of calcium salts as the major product and corresponding eggshell membranes as valuable by-products.