<p>The seafood processing industry generates high amounts of wastewater treatment sludge (WTS), posing environmental and economic challenges due to high disposal costs. Composting WTS with appropriate bulking agents, such as water hyacinth, offers a sustainable solution for waste management. However, challenges such as high moisture, nitrogen loss, and low composting efficiency limit the effectiveness of traditional composting methods. This study investigates the impact of biochar supplementation on the composting process of WTS and water hyacinth. A composting experiment was conducted using nine boxes (1m³ each) with varying biochar supplementation levels (0%, 3%, and 5%), monitored over two months for temperature, moisture, pH, and nutrient dynamics. The results showed that biochar addition, particularly at 5%, accelerated compost maturation by increasing composting temperatures (&gt; 66&#xa0;°C), reducing moisture content, and enhancing nitrification. While total nitrogen decreased due to volatilization, biochar helped retain nitrogen more effectively, with an increase in nitrate (NO₃⁻-N) and a reduction in ammonium (NH₄⁺-N) and nitrite (NO₂⁻-N). Total phosphorus (TP) and P₂O₅ levels also increased, although the direct influence of biochar on phosphorus availability was less conclusive. The findings demonstrate that biochar supplementation improves composting efficiency and nutrient retention, offering a promising solution for managing water hyacinth and sludge waste while producing high-quality organic fertilizer for sustainable agriculture.</p>