Energy Balance and Cost-benefit analysis of Biogas Production from Sewage Sludge by Thermal Pretreatment

This study was conducted to evaluate the applicability and economic feasibility of thermal pretreatment technology for improving energy recovery efficiency in the anaerobic digestion (AD) of sewage sludge. With the recent ban on marine sludge disposal and the global shift toward a carbon-neutral society, the development of advanced treatment technologies for the resource recovery and energy conversion from sewage sludge has become increasingly important. In particular, public wastewater treatment plants in Seoul face challenges in implementing co-digestion due to issues such as odor complaints, which limits biogas production in AD processes.</br>To address this issue, thermal pretreatment experiments were conducted using sewage sludge from public wastewater treatment plants in Seoul. The experiments, based on Response Surface Methodology (RSM), evaluated various temperature (60–180°C) and time (60–180 min) conditions to determine their effect on methane yield and methane production rate. As a result, the optimal thermal pretreatment condition was identified as 130°C for 176 minutes. Under this condition, methane yield and methane production rate increased by 35.5% and 95.8%, respectively, compared to untreated feed sludge (control).</br>Additionally, the economic feasibility of the thermal pretreatment process was assessed through cash flow analysis and a 20-year net present value (NPV) evaluation under the optimal pretreatment condition, confirming that the initial investment cost could be recovered. These findings indicate that the pretreatment process is also viable from an economic perspective.<br>This study proposes a practical direction for the development of pretreatment technologies that ensure stable sewage sludge treatment and maximize biogas recovery efficiency. The findings are expected to serve as a foundational reference for designing future carbon-neutral sewage sludge treatment systems.