Hwong-Wen Ma

Digestate, the solid by‑product of anaerobic digestion, can cause nutrient leaching, greenhouse‑gas release and heavy‑metal accumulation. Slow pyrolysis offers a promising route for upgrading digestate into biochar, yet its high energy demand and uncertain economics must be carefully weighed. This study evaluates the functional unit of 1 t solid digestate, the conversion of digestate to biochar at a pig farm in Pingtung, Taiwan, and compares the results with composting and incineration. Life‑Cycle Assessment and Techno‑Economic Assessment covered five scenarios: composting (S1); solid digestate pyrolysis at 500 °C (S2), 600 °C (S3) and 700 °C (S4); and incineration (S5). Because S2 required the least process energy, it yielded the lowest global-warming potential of 67.7 kg CO₂-eq t⁻¹. In contrast, S4 produced higher-value biochar and showed the strongest economic performance, with a positive Net Present Value of USD 371,480 and a payback period of 11 year under the scenario without carbon fees. Sensitivity tests (± 20% capital expenditure, operating expenditure and benefits) confirmed S4’s economic resilience, while S2–S3 were vulnerable to market shifts. A Technique for Order Preference by Similarity to an Ideal Solution multi-criterion ranking placed S4 first under the stakeholder-derived weighting scheme, mainly because of its stronger economic performance and product value. These results suggest that digestate-to-biochar conversion can improve digestate management, but the preferred pyrolysis temperature depends on the decision priority. In this case, 500 °C showed stronger environmental performance, whereas 700 °C provided greater economic potential when high-value biochar use and energy recovery were assumed.

Due to increasing market demand and limited resources for utilization and allocation, the global economy faces mounting pressure. Construction and demolition waste (C&DW) is therefore considered a potentially significant source of secondary materials for the future. To prolong the service time of these end‐of‐use products within the construction sector, innovative business models for effectively utilizing these urban mines are emerging, with direct reuse strategies being one practical solution. Among various types of C&DW, the inherent properties of steel make it particularly favorable for direct reuse. This study develops a hybrid input–output analysis and environmental extended input–output analysis model to assess the economic and environmental impact of applying a direct reuse strategy in the construction sector, with a focus on end‐of‐use structural steel products from end‐of‐service buildings. Under the scenario that end‐of‐use structural steel is directly reused, the economy may experience slight output value losses owing to reduced demand for raw material and remanufacturing sectors. However, it can result in significant carbon reduction impacts on energy and raw material sectors. Moreover, the carbon reduction achieved through the direct reuse strategy is around 25% of that achieved by transitioning from blast furnace‐basic oxygen furnace (BF‐BOF) steelmaking process to direct reduced iron‐electric arc furnace (DRI‐EAF) steelmaking process. These findings indicate that the innovative direct reuse business strategy could effectively serve as a catalyst for reducing carbon emissions across the entire economy.