## 3 Results and Discussion
### Winery Wastewater Fermentation  
After studying the acidogenic fermentation of the main saccharides contained in the winery wastewater, the acidogenic fermentation of the synthetic winery wastewater was carried out. In these experiments the total monosaccharide concentration was the same as in the previous tests, 9 g L\({}^{-1}\). However, due to the presence of both saccharides--glucose and fructose--in the winery effluent the concentration of each of them was 4.5 g L\({}^{-1}\). The main results obtained during the acidogenic fermentation of the synthetic winery wastewater are shown in Figure 4. In this figure the substrate consumption, glucose and fructose content and biomass growth, as well as the hydrogen, carbon dioxide production and SCFA generation are presented. In Figure 4 it can be seen that the fructose was fully consumed before the full consumption of the glucose, which needed two hours more to be completely consumed. The different moments at which the substrates were consumed were reflected in the biomass growth, which showed a slow reduction in the biomass growth once the fructose was fully consumed after 9.5 h of fermentation. The average glucose and fructose consumption rates were representative of the process taking place in the bioreactor, with values of 0.47 g L\({}^{-1}\) h\({}^{-1}\) for the fructose and 0.37 g L\({}^{-1}\) h\({}^{-1}\) for the glucose. The full consumption of the fructose was also observed in the hydrogen and the carbon dioxide production, which presented different trends throughout the experiment. During the first hour of the experiment, the gas production increased according to an exponential trend; however, after this moment the trend slowed down to a linear trend. This linear trend drastically changed after approximately 9.5 h from the beginning of the experiment, presenting a drastic decrease, which can be explained by the full consumption of the fructose in the bulk solution. Finally, after 12 h the hydrogen and carbon dioxide generation finished, because of the full consumption of the organic pollutants contained in the synthetic winery wastewater. Parallel to the hydrogen, carbon dioxide and biomass production, several SCFAs were produced. The main SCFA produced was acetate, followed by butyrate and lactate. The amounts of propionic and formic acid produced by the acidogenic fermentation of the winery effluents were negligible. The product distribution obtained was close to that previously described in the literature when dealing with similar substrates [32; 33]. The biomass yield obtained when fermenting the winery wastewater was 0.10 g TSS g\({}^{-1}\) substrate fermented. This value was identical to that obtained when fermenting fructose and was very similar to the value obtained with glucose as the substrate. Because of this, the biomass yield obtained when  
Figure 4: **(a)** Substrate consumption, biomass growth, expressed as VSS and TSS. **(b)** Hydrogen and SCFA production during fermentation of the synthetic winery effluent. ferruning the synthetic winery wastewater could be explained by the proportional contributions of the fructose and glucose, without significant differences. The bio-hydrogen production obtained when fermenting the winery effluent, 350 mL h\({}^{-1}\), was also an average of the values obtained when fermenting the glucose and the fructose in isolation--370 mL STP g\({}^{-1}\) glucose was fermented and 334 mL STP g\({}^{-1}\) fructose was fermented, respectively. However, the fermentation products generated when fermenting the synthetic winery wastewater were different to those obtained when fermenting the glucose and the fructose in isolation, with the acetic acid generation being about a 30% higher than those observed when fermenting the glucose and the fructose separately. This behavior could be explained by the effect of co-ferrmentation on SCFA production, as described in the literature [34].