# Shell Waste Management and Utilization: Mitigating Organic Pollution and Enhancing Sustainability
## 7 Conclusions  
Although we have a history of recognizing bivalve and crustacean shells as valuable resources, shells produced nowadays are largely considered as waste, despite the fact that CaCO\({}_{3}\) is one of the most highly exploited resources globally. Shell waste is still orders of magnitude less than the yearly CaCO\({}_{3}\) demand [115]. Nevertheless, shell waste repurposing and valorization are highly important for the determination of strategies to make them a standard valuable resource for a number of applications. Some of these applications may demonstrate more sustainable modus than already established processes, some may be less sustainable in industrial implementation, but adjustments should be appraised for the sake of the environmental benefit. Furthermore, bivalve aquaculture alone has the potential to act as a carbon sink, sequestering several million metric tons of CO\({}_{2}\) per year globally [116]. Unfortunately, most of the aqua-cultured bivalves and crustaceans are traded fresh and their shells are disposed of as domestic waste. Much of that waste is \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Shell Type** & **Replacement Component** & **Dosage (\%)** & **Physical Properties/Max. \% of Decrease** \\  & & & **or Increase** \\ \hline Oyster & Fine aggregate & 20–40 & Water absorption/21.60 decrease \\  & Cement & 5–20 & Compressive strength/23.70 increase \\  & Coarse and fine aggregate & 25–100 & Splitting tensile strength/12.50 decrease \\  & Coarse aggregate & 25–75 & Compressive strength/38.00 decrease \\  & Fine aggregate & 20–60 & Compressive strength/10.00 decrease \\  & Coarse and fine aggregate & 10–50 & Compressive strength/32.00 decrease \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The effects on the product performance when using various types of shells as cement additives or concrete aggregates. The presence of waste shells generally reduces the strength of concrete (particularly as the replacement dosage increases) but it remains within the acceptable range. Adapted from [PERSON] et al. [7]. handled in furnaces where high temperatures of incineration release the previously trapped CO\({}_{2}\) from CaCO\({}_{3}\) back to the atmosphere [116]. Thus, in order to be subject to upgrading, waste shells should be collected in an organized manner and their quantities estimated. For any large-scale applications, a stable supply of shells needs to be ensured, particularly in order to design (at least a partial) substitution of the limestone with the shell-derived CaCO\({}_{3}\). Certainly, care should be taken that during the repurposing of waste shells, the environmental impact of the process should be reduced [117], particularly in terms of energy and water demand. To that end, the use of cleaner energy sources could ameliorate the environmental impact of the repurposing operations. However, any shell repurposing operation should be evaluated not only through the environmental footprint, but also in terms of economic value and impact. The reuse and valorization of waste shells must take into consideration the collection, cleaning, storage, transport, and utilization costs. In any case, to gain a full momentum, sustainable waste shell bioeconomy and repurposing needs an integrated support from the legislative standpoints, local governments, and related industry operators. Conceptualization, N.T.P. and V.L. ; methodology, N.T.P. ; visualization, I.S.-P. and R.C.-R.; writing--original draft preparation, N.T.P. and V.L. ; writing--review and editing, N.T.P. ; funding acquisition, R.C.-R. All authors have read and agreed to the published version of the manuscript. This research was supported by the Scientific Centre of Excellence for Marine Bioprospecting-BioProCro, a project co-financed by the Croatian Government and the European Union through the European Regional Development Fund--the Competitiveness and Cohesion Operational Programme (KK.01.1.1.01). Not applicable. The authors declare no conflict of interest.