The sustainable management of shellfish by-products is critical for reducing environmental burdens and creating circular economies. This study investigates the valorization of shrimp shells through pyrolysis and K₂CO₃ activation to produce high-surface-area biochar (1,639 m²/g) for treating hydrothermal liquefaction aqueous phase (HTL-AP). The HTL-AP, characterized by high organic load (COD: 107,000 mg/L; TOC: 103,000 mg/L), was treated via coagulation-flocculation (PAC/PAM) followed by adsorption onto shrimp shell-derived biochar. The combined process achieved significant reductions in pollutants: COD (95.5%), phenols (99.9%), and total nitrogen (89.7%). The spent biochar was further evaluated for soil amendment potential, demonstrating compliance with international biochar standards (IBI): pH 7.8, cation exchange capacity (6.00 cmol/kg), organic matter content (47.7%), and seed germination rate (84%). Heavy metals and PAHs were below regulatory thresholds (PAHs: 0.32 mg/kg), confirming its feasibility for agricultural use. This work aligns with SDG 6 (clean water), SDG 12 (responsible consumption), and SDG 15 (life on land) by offering a dual-benefit solution: (1) efficient HTL-AP treatment using waste-derived biochar, and (2) circular reuse of spent adsorbent to enhance soil quality. The results support EU Horizon and global zero-waste strategies, providing industries with a scalable approach to tackle wastewater and solid waste.