## 1 Introduction  
Within the bio-refinery concept, glucose is probably one of the key starting raw materials because its abundance in lignocellulosic biomass, and the high chemical versatility of this carbohydrate to be transformed into a wide variety of bio-products through different chemical pathways (isomerization, dehydration, hydrogenation, oxidation, etc. and/or their combinations) [1, 2]. Nowadays, glucose is industrially produced by hydrolysis of amylaceous feedstock under acid conditions or by using specialized enzymes. Conventionally, starch-containing raw materials like potato, maize, wheat, and cassava are some of the crops frequently used for its production [3]. In this sense, environmental impacts associated to the production of starch, and the derived glucose, are affected by agricultural stages, which usually involve intensive consumption of natural resources (land occupation and transformation, use of fertilizers and pesticides, depletion of fossil fuels for machinery, etc.). In this context, an interesting alternative for improving the environmental profile of glucose is its production from residual biomass, an option which also avoids competition with the food market. The potential benefits of such an alternative has prompted several investigations focused on the conversion of a variety of residual biomass into glucose [4]. [PERSON] et al. (2007) [5] studied the transformation of the biodegradable fraction of municipal solid wastes into glucose, comparing different pre-hydrolysis and enzymatic hydrolysis treatments. Specific food residues have also been used by some authors to produce glucose through both chemical and enzymatic pathways [6; 7; 8; 9], evidencing the benefits of the latter because of the lower downstream requirements for recovering glucose. Other studies have also reported the treatment of various types of paper wastes by enzymatic processes [10] showing, among other remarkable conclusions, that non-recyclable paper can be used to obtain high-purity glucose syrups. Despite the significant examples above, lignocellulosic biomass residues are undoubtedly the most common type of waste tested for glucose production, mainly because of its renewable character and availability [4; 11]. Waste lignocellulosic biomass includes a huge number of different feedstock, such as agricultural wastes, woody wastes, residues from green areas, etc. However, the recalcitrance and complex composition of these feedstocks make the selective transformation of their constituting components (cellulose, hemicellulose, and lignin) into the corresponding monomers (e.g., glucose, xylose, aromatics) a challenge, and much more difficult than converting simpler and less recalcitrant residues. The recalcitrance of waste lignocellulosic biomass can be reduced through high-temperature and/or pressure conditioning steps (e.g., steam explosion technology) or advanced treatments using ionic-liquids [12; 13]. On the other hand, the complex composition of natural lignocellulosic biomass wastes requires an adequate separation technology [14], either applied to the starting biomass constituents (fractionation) or the complex mixtures obtained after biomass transformation (monomers, oligomers, aromatic compounds, and other extractable substances). Therefore, the conversion of lignocellulosic biomass wastes to get simple monosaccharides selectively and efficiently results in a complex process, whose environmental impacts and resources requirements should be quantified to evaluate its real sustainability as compared to conventional pathways for sugar monosaccharides production. LCA methodology has been previously applied to the environmental evaluation of the transformation of lignocellulosic biomass into valuable bio-products such as bioethanol [15], biofuels [16], bioenergy [17], or jet-fuel [18].