Engineering synergies between thermochemical and biochemical processes for municipal solid waste management and sustainable fuels supply.

Municipal solid waste (MSW) valorization is constrained by the mismatch between heterogeneous feedstock properties and the operating requirements of individual conversion technologies. This review synthesizes integrated thermochemical and biochemical pathways and treats MSW valorization as a system design problem organized around carbon cascading, in which each carbon fraction is routed to the process able to convert it and performance is governed by interactions between processes rather than by individual process yields. Four hybrid routes are examined: gasification-syngas fermentation, pyrolysis-anaerobic digestion, hydrothermal processing-anaerobic digestion, and reverse configurations in which digestion precedes thermochemical conversion. Across these routes, intermediate quality rather than process yield governs feasibility. Tar (1-150 g/Nm3), ammonia (up to 14,000 ppmv), and sulfur species must be reduced by two to three orders of magnitude before downstream bioconversion, relocating the critical engineering problem from the reactor to the interface. Environmental and economic evidence indicates that integration benefits are real but conditional. Coupling anaerobic digestion with pyrolysis approximately doubled the climate mitigation of standalone digestion, yet the advantage is not uniform across impact categories, and one configuration achieved the greatest climate benefit while increasing freshwater ecotoxicity 52-fold. Reported production costs for integrated routes ($0.31-1.95/L ethanol) exceed those of standalone biochemical conversion, though integrated systems convert carbon that cheaper routes cannot access, and scale and product value influence viability more than process configuration. Integration is therefore justified by feedstock coverage and interface management rather than by coupling processes as such, and full-chain demonstration on real MSW remains the principal research challenge.