Integrated assessment of carbon stability and environmental performance of sewage sludge-derived biochar from a small wastewater treatment plant

Municipal sewage sludge (MSS) from small wastewater treatment plants (WWTPs) is difficult to manage because decentralized facilities often lack economically viable treatment options. This study evaluated biochar produced by slow pyrolysis of MSS from a small WWTP (< 10,000 population equivalent) treating exclusively domestic wastewater. Pyrolysis at 400 and 600 °C progressively increased carbonization and structural stability, as indicated by a decrease in the H/C org ratio from 1.84 to 0.31, an increase in the thermostable fraction from 14% to 79%, and an increase in the recalcitrance index from 0.41 to 0.56. Carbon sequestration estimates were method-dependent: the R 50 -based carbon sequestration potential (CS R50 ) was higher for B400 (17.5%) than for B600 (15.3%), whereas the H/C org -based 100-year permanence approach predicted greater long-term carbon retention for B600 (0.12 vs. 0.10 kg C/kg dry MSS). These estimates should therefore be regarded as method-dependent predictions. Although total heavy metal concentrations increased during pyrolysis, their mobility decreased markedly, reducing the ecological risk index from 940 to 20. Among priority PAHs, only phenanthrene was detected in B600 at 0.011 mg/kg. Pyrolysis also strongly reduced the phytotoxicity of raw MSS. Overall, slow pyrolysis improved carbon stability and environmental performance. These findings indicate that pyrolysis can provide a locally applicable sludge valorization route for small decentralized WWTPs, combining waste reduction with carbon stabilization and reduced environmental risk.