Thermochemical Co-Valorization of Sewage Sludge and Plastic Waste: Co-Processing and Sludge-Derived Catalysis

Plastic waste and sewage sludge (SS) are abundant, heterogeneous waste streams whose utilization and recycling are constrained by contamination, variable composition, and limited resource recovery. This review evaluates their integrated thermochemical conversion, focusing on co-incineration, co-pyrolysis, co-gasification, hydrothermal co-carbonization, hydrothermal co-liquefaction, and the use of SS-derived char (SSC) and sewage sludge ash (SSA) as catalytic materials. Co-processing benefits arise from complementary feedstock properties: plastics provide high carbon, hydrogen, volatile matter (VM), and heating value, whereas SS contributes oxygenated intermediates and mineral phases containing Si, Al, Fe, Ca, Mg, and P. Among the reviewed routes, co-pyrolysis shows the strongest evidence of non-additive interactions, including hydrogen transfer, altered decomposition kinetics, and changes in liquid composition. Co-gasification can enhance H2 formation and tar conversion, while hydrothermal routes are especially attractive for wet SS but remain comparatively underexplored. The review further distinguishes SSA, dominated by mineral acid–base and redox functionality, from SSC, which combines these mineral effects with adsorption and surface reactions on a porous carbon matrix. Key barriers include catalyst deactivation, chlorine and sulfur poisoning, heavy-metal migration, feedstock variability, and limited pilot-scale validation. Future development requires mechanistic controls, catalyst stability testing, techno-economic analysis, life-cycle assessment, and integrated nutrient recovery to establish practical circular waste-management systems.