Compared with conventional soil additives, biochar has found successful application as an organic soil amendment to improve crop productivity coupled with climate change mitigation via carbon sequestration. In parallel, huge amounts of sludge are generated from water treatment facilities, requiring appropriate management strategies to avoid environmental deterioration. Utilizing wastage sludge for producing cost-effective biochar for crop cultivation would be an interesting research point to achieve a profitability scenario and meet sustainable development goals. Hence, this study investigated the synthesis of biochar from wastewater sludge, followed by its application for lettuce plant growth. The thermal treatment of sludge at 450 °C peak temperature, 20 °C/min heating rate, and 0.250 L/min nitrogen flowrate obtained a biochar yield of 0.48 kg/kg dry sludge with a fixed carbon content of 34.2%wt. Biochar was added to the soil at three dosages of 2.5%, 5.0%, and 10.0% (w/w) using pot experiments under greenhouse conditions (temperature 26±2 °C, relative humidity 75-88%, and light:dark period 14:10 h) for 35 days. Results showed that applying a biochar dosage of 5.0% significantly (p <0.05) increased plant height, leaf number, shoot fresh weight, shoot dry weight, and leaf area by 16.8±0.8%, 17.7±1.1%, 58.9±3.2%, 46.3±2.8%, and 29.4±2.2%, respectively, as compared with the control (0% biochar). Moreover, the biochar amendment dosage of 5.0% produced the optimum marketable lettuce yield of 8.96 t/ha and could fix 8.24 t/ha of CO₂. Biochar application demonstrated an economic feasibility scenario by using a low-cost preparation process, selling the produced lettuce, and CO₂ fixation, giving an annual income of 495 €/ha. Under this condition, a payback period of 3.2 years was expected to refund the initial investment (1600 €/ha). Hence, the study outcomes would contribute to eco-friendly crop management, soil conservation, and combat climate change, providing a reliable strategy for achieving the targets of SDG 2 “Zero hunger”, SDG 13 “Climate action”, and SDG 15 “Life on land”.