## 4 Discussion  
The studied environmental impact categories (Figure 9) can be divided into two groups: indicators clearly more affected by the MS-bp system (HT, PMF, and TA, with a relative contribution of the WBR-bp lower than 30%), which will be classified as cluster A for further discussion, and indicators closer for both systems (CC, FD, and OD, with a relative contribution of the WBR-bp higher than 30%), which will be included in cluster B. Attending to the results obtained for the LCA study on MS-bp, the main drawback ascribed to this process came from the production of the starting raw material, starch, with 60-96% of the relative contribution in all the studied impact categories. Previous studies assigned an influence around 70% to the starch manufacture within the environmental footprint of glucose production from this polysaccharide [3], relative to carbon footprint and water depletion indicators. Indeed, starch manufacture accounts for most of the generated impacts even when considering the transformation of the glucose into other products. Accordingly, we have previously reported the large influence associated with starch production on the glucose chemical and enzymatic synthesis routes, and the prevalence of those impacts when considering the subsequent glucose hydrogenation to produce sorbitol [2]. Similarly, [PERSON] determined that within the production of sorbitol, at least 60% of the impacts came from the production of glucose in every category described [29]. In this sense, the agricultural phase is associated with severe environmental weaknesses due to the land transformation, large area occupation, elevated water consumption, use of pesticides, fertilizers, machinery, etc. Similarly, field emissions and agricultural production of comparable raw materials used in sugar production have been noted to withhold greater potential for acidification, eutrophication, or greenhouse emissions than the subsequent processing [19]. Since the agricultural phase appears to be responsible for most of the impacts in the MS-bp, WBR-bp has the potential to constitute a more sustainable alternative by substituting the agricultural issues by the use of wastes. Considering the results shown in Figure 9, the impacts caused by the application of pretreatment and conditioning processes seem to be lower than those produced by the cultivation and harvesting of the maize crops. Regarding the pretreatment of the biomass of the WBR-bp, it has been the focus of a large number of studies over the past years [30; 31]. One of the challenges for pretreatment technologies is to develop scalable and efficient processes with low energy requirements [32]. However, the reported alternatives to conduct the depolymerization of the complex structural components of wood, still show large environmental impacts associated. Accordingly, the pretreatment stage was the one with the highest contribution to the impact categories produced in WBR-bp. Relative contribution varied from 50% to 70% and was superior to 65% in all the categories considered in cluster A (see Figure 6). Specifically, cluster A indicators are mostly affected by screw feeders used for displacing the mixture from the deacetylation tank to the acidification reactor (see Section 3.2.1). This operation involves a very large consumption of electricity due to the difficulties for moving the highly viscous biomass slurry (despite previous deacetylation treatment, the substrate is still rich in insoluble and highly interconnected structural polymers, which confers it a high viscosity). On the other hand, purification sub-processes used in the pretreatment step, namely flash evaporation and ammonia neutralization, prevail as the most impactful sub-steps in those indicatorsincluded in cluster B (see Figure 7a). In this sense, the flash tank should be noted as the critical equipment of the purification section to cluster B indicators due to its operational heat requirements.