Co-pyrolysis of municipal sewage sludge and polystyrene waste: DoE optimization of operating conditions for oil production

Co-pyrolysis of sewage sludge (SS) with polystyrene (PS) was investigated to improve oil yield and quality while limiting the fraction of plastic in the feedstock. A design of experiments (DoE) approach was applied to evaluate the effects of temperature (350–650 °C) and PS content (20–70 wt%) on liquid yield and oil higher heating value (HHV). Quadratic models showed high statistical significance and good predictive capability, with low relative errors between experimental and predicted values. Temperature and PS content strongly influenced process performance, leading to maximum liquid yields of 80.4 wt% at 650 °C and 70 wt% PS, and oil HHV values up to 41.0 MJ kg⁻ 1 at 500 °C and 80 wt% PS, although very high temperatures promoted gas formation. Process optimization based on liquid yield identified two operating scenarios at 590 °C with 20 wt% PS and 610 °C with 41 wt% PS. Experimental results confirmed the reliability of the model predictions and showed higher liquid yields than those expected from the weighted contribution of the two materials pyrolyzed separately, indicating synergistic interactions during co-pyrolysis (54 vs 51 wt% and 68 vs 62 wt% for Scenario 1 and 2, respectively). The optimized oils exhibited improved properties compared with SS-derived oil, including higher carbon content, lower heteroatom concentration, and HHV values approaching 40 MJ kg⁻ 1 . Simulated distillation showed that more than 60 wt% of the oils fell within the naphtha boiling range, while GC–MS analysis revealed aromatic hydrocarbons as the dominant components. These results demonstrate that statistically optimized SS–PS co-pyrolysis effectively enhances both fuel yield and quality.