Trends and Opportunities of Bivalve Shells' Waste Valorization in a Prospect of Circular Blue Bioeconomy

Bivalves aquaculture is already considered a very sustainable for of food production and might become an essential pillar on which to develop future global food security. However, with the increase in production, a correspondingly great amount of waste will be produced all around the earth, principally in the form of shells, which can represent up to 90% of the fresh mollusk weight. Nowadays, shell waste has no notable use and is commonly regarded as waste, often dumped in landfill, or thrown back into the sea, causing a significant level of environmental concern, and resulting in a loss of natural and valuable resources. Bivalve shells are mainly formed by CaCO3, giving them the potential to become a promising secondary raw material for several applications, from a circular economy perspective. This paper aims to review the scientific literature of the last twenty years and the principal recent trends in shell waste reuse, describing applications that are already in place on a large scale in agriculture and livestock, soils, water and wastewater remediation, biodiesel production and biofilters, as well as niche applications that now simply have the potential to be scaled up.