This study focuses on the valorization of waste oyster shells by grinding and calcining them into recyclable, porous oyster shell powder, which was developed into two functional products: Lightweight Expanded Clay Aggregate (LECA) and a rolled filter cartridge, both designed for wastewater treatment applications. Experimental investigations revealed that the calcined outer layer of the oyster shell, primarily composed of calcium oxide, exhibited a stronger heavy metal adsorption capacity than the inner nacreous layer, with maximum efficiency observed at pH 10. The adsorption performance of the rolled filter cartridge followed the order: calcined oyster shell powder ≈ powdered activated carbon > commercial CTO activated carbon filter, attributed to the increased surface area after calcination. Permeability tests showed that filters made with calcined oyster shell powder achieved higher flow rates than the commercial CTO activated carbon filter. Microscopic observations confirmed that calcination at 800°C generated a porous surface morphology that significantly enhanced adsorption. Additionally, the self-fabricated rolled filter cartridges can be regenerated and reused through acid washing, offering a cost-effective and sustainable alternative to conventional filtration materials. By recycling marine waste into functional adsorbents and filter media, this approach provides an effective solution for removing heavy metals from wastewater while promoting circular resource utilization. Overall, the development of calcined oyster shell–based filter materials aligns with the United Nations Sustainable Development Goals (SDGs), particularly SDG 6 (Clean Water and Sanitation) and SDG 14 (Life Below Water), by advancing access to clean water and protecting marine ecosystems.