Abstract: BACKGROUND: Spent sulfite liquor is a by-product obtained in the process of manufacturing dissolving pulp by the acid sulfite method. A full physico-chemical characterization of this waste has been developed in order to determine if it is susceptible to valorization by means of biorefinery processes. RESULTS: The main components present in this spent liquor are sugars from hemicelluloses (25.9%), mainly xylose (18.2%) because hardwood is used as a raw material, and lignosulfonates (50.6%). Taking into account the results obtained, a literature search has been done in order to establish the biorefinery options for this lignocellulosic waste, giving lignosulfonates, xylitol, ethanol, polyhydroxybutyrateorpolybutylene-succinate,hydrogenandfurfural as thebest options.However, other compounds present in the liquor such as furans, acids and inorganic pollutants can complicate the valorization options. CONCLUSIONS: Spent sulfite liquor can be a good feedstock for biomass based processes. This paper will be useful for future work regarding the potential biorefinery opportunities of the sulfitemill.

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