Techno‐Economic and Life Cycle Assessment of Anaerobic/Aerobic Integrated Biosystem for Threefold “Biogas, Biochar, and Treated Effluent” Benefits

Although sequential anaerobic/aerobic processes have recently emerged as viable domestic wastewater treatments, their performance under higher organic loading rates (OLRs) requires further investigation to meet economic and sustainability benchmarks. This study evaluates an integrated up-flow anaerobic sludge blanket (UASB) reactor followed by a downflow hanging sponge (DHS) system, comparing its techno-economic performance to a standalone UASB unit. Chemical oxygen demand (COD) mass balance analysis revealed that 32%-54% of influent COD was converted to methane (CH₄) and 17%-22% to sludge, yielding 205-255 L CH₄ per kg of COD removed. The subsequent DHS unit provided robust polishing, achieving total removal efficiencies of 62%-94% for COD, 75%-95% for biochemical oxygen demand (BOD), 81%-94% for total suspended solids (TSS), 28-72% for total Kjeldahl nitrogen (TKN), and 63-100% for NH₄ ⁺-N at an OLR of 0.84-5.43 kg COD/(m³·d). Furthermore, sludge pyrolysis produced a nutrient-rich, calcite-composed biochar (yield: 0.54 g/g dry sludge) suitable for soil amendment. Economic analysis, incorporating biogas and biochar sales, carbon credits, and pollutant shadow pricing, confirmed the system's feasibility. As such, the profitability criteria were estimated as a payback period of 5.9 years, an internal rate of return (IRR) of 11.0%, and a net present value (NPV) of 3485 US$. Given the superior life cycle assessment (LCA) results regarding carbon footprint and ecosystem health, this UASB/DHS/pyrolysis strategy warrants further research into biochar's role in enhancing biogas and digestate quality throughout the project lifetime.