## 3 Results and Discussion  
### Eggshell Waste Transformation using 10% Acetic and 15% o-Phosphoric Acid  
The present study investigated the possibility of chemical transformation of ESW using acetic and o-phosphoric acid into value-added products, including corresponding calcium salts and eggshell membranes completely devoid of remnants of ESW calcium carbonate. In this respect, it was necessary to design a transformation process that fulfils both demands. Therefore, the starting point in designing the ESW transformation process using acetic and o-phosphoric acid was the report by [PERSON] and [PERSON] [46], in which the authors used four times greater stoichiometric amount of acids than necessary to completely dissolve the ESW calcium carbonate. Regardless of the proposed molar ratio of ESW calcium carbonate and molar ratio of acids used by [PERSON] and [PERSON] [46] for the complete dissolution of ESW calcium carbonate, we used 7.5, 10 and 12.5% (\(w/v\)) solution of acetic acid and 10, 12.5. and 15% (\(w/v\)) solution of o-phosphoric acid, and monitored the complete removal of ESW calcium carbonate from ESMs every 15 min for 4 h. The obtained results (Figure 3) show that the use of 10% and 12.5% (\(w/v\)) acetic acid was suitable for the complete dissolution of the ESW calcium carbonate in 3 h, while in the case of o-phosphoric acid, the complete dissolution of ESW calcium carbonate was achieved in 3 h only when using 15% (\(w/v\)) o-phosphoric acid. Therefore, the complete solubilization of ESW calcium carbonate using 10% (\(w/v\)) acetic and 15% (\(w/v\)) o-phosphoric acid in 3 h was used for the simultaneous production of calcium salt solutions and corresponding eggshell membranes. Figure 3: Time dependent solubilization of eggshell waste calcium carbonate using 7.5, 10.0 and 12.5% (\(w/v\)) acetic acid (**a**,**c**) and 10.0, 12.5 and 15.0% (\(w/v\)) o-phosphoric acid (**b**,**d**). Results are expressed as average values of three independent determinations each performed in triplicate. Following the aforementioned defined parameters for the simultaneous production of calcium salts and corresponding ESMs using acetic and _o_-phosphoric acid, the ESW transformation process was performed using the procedure shown in Figure 2, and results of the three independent acid transformation processes are shown in Table 1.  
The designed ESW transformation process (Figure 2) has been proven to be suitable for the simultaneous production of calcium salts and corresponding eggshell membranes (Table 1). The eggshell's calcium carbonate was completely dissolved with the use of 10% (\(w/v\)) acetic and 15% (\(w/v\)) _o_-phosphoric acid (Figure 3b,d) into the corresponding Ca-salts and ESMs completely devoid of the calcium carbonate remnants, at ESW conversion rate of 22.22 g/L h. The chemical transformation of 100 g of ESW containing approximately 85% of dry matter using 10% (\(w/v\)) acetic acid resulted in 111.41 \(\pm\) 2.13 g of calcium acetate monohydrate and 2.90 \(\pm\) 0.14 g of ESM, and a chemical yield in calcium acetate monohydrate of 79.16 \(\pm\) 1.18%. On the other hand, the transformation of 100 g of ESW using 15% (\(w/v\)) _o_-phosphoric acid yielded 77.06 \(\pm\) 6.21 g of calcium hydrogen phosphate and 2.94 \(\pm\) 0.08 g of ESM, with a chemical yield in calcium hydrogen phosphate of 71.36 \(\pm\) 5.58%. Chemical reactions between ESW calcium carbonate and 10% (\(w/v\)) acetic and 15% (\(w/v\)) _o_-phosphoric acid are shown by Equations (4) and (5):  
\[CaCO_{3}+\ 2 CH_{3}COOH\ \leftrightarrow\ \ Ca(CH_{3}COO)_{2}+\ {CO}_{2}+\ {H}_{2}O \tag{4}\]  
\[CaCO_{3}+\ {H}_{3}PO_{4}\ \leftrightarrow\ \ CaHPO_{4}+\ {CO}_{2}+\ {H}_{2}O \tag{5}\]  
The possibility of the production of calcium acetate using ESW have been reported by two patents [53, 54] and two scientific publications [44, 45]. The reported mass yield of calcium acetate using ESW was 70% [53], 88% [45], 95.3% [54], and 95.5% [44] which was quite lower than mass yield (111.41 \(\pm\) 2.13 g/100 g of ESW) achieved in this research.