## 3 Results and Discussion  
### BMP Tests  
A series of biomethanogenic potential tests (BMP) was performed, following the procedures and parameters proposed in [PERSON] et al. (2016) [26]. Keeping an inoculum to substrate ratio of 2 based on volatile solids, both substrates were tested, as well as a number of mixtures of the substrates. Due to the heterogeneous nature of DGSBs, a significant sample of the matter (1000 g, making sure by visual inspection that the different components were present, in the same apparent proportion as shown in the bulk sample) was ground and blended until its particles were small and uniform enough to the naked eye. BSGs were also ground coarsely. Milling of the samples was performed taking into account the need of not overheating the samples, with the consequent loss of volatile compounds and water and alcohol contents in the original substrates. The different mixes of substrates used in this series of tests were performed keeping in mind that both substrates (but mainly DGSBs) have a complex chemical composition, and so interactions and reactions could be expected throughout the biological process, not only because of the original composition but also because of the different nature of by-products created through the process in the decomposition of the original ones. Thus, the series was planned from samples 100% DGSBs (which we named M1) to 100% BSCs (which we named M6), with intermediate steps changing the mixture proportions in 20% (by original weight) each. This way, the tests performed with a mixture of 80% DGSBs and 20% BSCs were the M2 samples, the ones with 60% DGSBs and 40% BSCs were the M2, and so on. The results of the different series of BMP tests can be seen in Figure 1.  
Two things become apparent from the evolution of the accumulated biogas yield. Firstly, the time needed for the total exhaustion of the biological process is very high. This might be the result of the usage of substrates with a complex nature, a high content in cellulose and lignin as well as other substances, and with a complex degradation through which intermediate compounds (which can pose difficulties for the process) are created. There is a correlation (\(p<0.05\)) between the speed of the process and the final biogas yield with the content of DGSBs. In effect, the higher the proportion of DGSBs in the sample, the lower the slope of the yield curve on the first stages of the process (on the first two weeks) and the lesser the final biogas production (it should be reminded that the total content in VS in each sample was the same, regardless of the proportions of the co-substrates used). It was expected a high degree of variability in samples with a high DGSBs content due to the heterogeneous nature of the substrate and the small size of the sample, which implies differences in the presence and proportion of some components in each sample. Secondly, in relation to the shape of the curves throughout their evolution with time, there is a first phase in which biogas is created normally, with a daily yield that reduces with time as the available substances for anaerobic digestion are reduced in the batch  
Figure 1: Total specific methane production in \(\rm{I_{N}}\) CH\({}_{4}\)kgVS\({}^{-1}\) from the BMP tests (SD for each sample: M1 (100% DGSBs), 12; M2 (80% DGSBs, 20% BSCs), 15; M3 (60% DGSBs, 40% BSCs), 19; M4 (40% DGSBs, 60% BSCs), 23; M5 (20% DGSBs, 80% BSCs), 15; M6 (100% BSCs), 22.  
process. However, in all cases and after a lag in the process appears, there is a second relative increase in biogas production, appearing as an inflection point. This second relative increase appears in all cases regardless of the composition of the sample and proportions in the mixture of DGSBs and BSGs. However, it is to be noted that, the higher the content of DGSBs, the later this inflection point appears throughout the experiment. It is also noteworthy that this phenomenon is more remarkablewith the higher content in DGSBs of the sample, at least apparently.