Comparative environmental assessment of integrated anaerobic digestion and pyrolysis systems for sewage sludge treatment: impact assessment using ReCiPe and USEtox methods

Growing concerns over pollutants in sewage sludge have increased interest in integrating anaerobic digestion (AD) and pyrolysis (Py) to enhance energy recovery, reduce toxic compounds, and minimize environmental impacts. This study evaluates six treatment scenarios: standalone AD or Py, and four integrated AD-Py configurations, with or without aqueous pyrolysis liquid (APL) recycling. Environmental impacts were assessed using ReCiPe 2016 and USEtox® methods. Scenario S4:AD-Py-AD, which recycles APL to AD, achieved the highest climate change mitigation (-300 kg CO₂-eq./ton hydrolyzed sludge (HS)) but also showed elevated freshwater ecotoxicity (increase by 52-fold compared to AD) due to persistent organic pollutants emissions in effluent. Scenario S3:AD-Py, without APL recycling, demonstrated the best overall environmental performance, with low climate impact (-293 CO₂-eq./ton HS) and ecotoxicity (same as AD). In contrast, S6:ADCo-Py-AD, which includes a high-rate AD reactor and APL recycling, had the poorest performance due to high ecotoxicity (980-fold increase compared to AD). These findings highlight that Py of digestate can be a beneficial pathway for sewage sludge treatment, showing the best environmental benefits compared to stand-alone processes.