Waste to Watts: Unlocking Substantial Energy Offset in Wastewater Treatment Plants Through Sludge-to-Biogas Conversion Under Resource Security Pressure

Biogas production from sewage sludge is increasingly recognized as an important pathway for improving the energy performance of wastewater treatment plants (WWTPs). In this study, sewage sludge was confirmed as a stable substrate for anaerobic digestion, with 2023 identified as the most stable year (stability index 0.09) and 2020 as a period of instability (0.17) due to insufficient buffering capacity. Statistical analyses showed that higher biogas production improves energy balance, while increased electricity consumption reduces efficiency. Multivariate and time-series analyses confirmed clear relationships between sludge characteristics and biogas production, including temporal dependencies. Regression models such as ordinary least squares (OLS) and a generalized linear model with a Gamma distribution and log link (GLM Gamma) demonstrated that biogas production can be reliably explained and predicted based on sewage load, sludge composition, and process stability indicators (VFA/TIC). From a practical perspective, maintaining stable process conditions and adequate buffering capacity, together with proper sludge management, is essential for maximizing biogas yield and improving the energy efficiency of the plant. Our results showed that stable digestion conditions (VFA/TIC = 0.09) were directly associated with improved biogas production and better energy performance. Overall, biogas already covers a substantial share of WWTP energy demand and can contribute to full energy coverage. The results clearly showed that WWTPs can reliably produce energy from sludge and operate with relatively stable performance over time. This means they should no longer be treated only as waste management facilities, but as part of the energy system. In practice, this supports policies that promote biogas recovery and cogeneration as standard solutions, especially where energy security and efficiency are priorities.