# Structure of Microbial Communities When Complementary
## 3 Results and Discussion
### Anaerobic Digestion of PE and OMW Mixture  
Cumulative biogas production was registered in all tested mixtures without any \"lag\" phase (Figure 1) and around 0.12 L was obtained in all units, 13 days upon start-up. From then onwards, it was observed that the units containing 70P + 30O and PE reached the highest volumes in cumulative biogas (about 0.78 L) and with the best gas quality of the entire experiment (70-71% CH\({}_{4}\)). Comparatively, the better behavior showed by 70P + 30O mixture than by PE (Figure 1) suggests that PE is an unbalance substrate and may contain inhibitory components for the anaerobic process. Effectively, the slow evolution in biogas production that was observed at the beginning (around the first 10 days), indicates that some biomass adaptation process occurred. Subsequently, the sharp drop in biogas after day 34 still suggests a limitation in the load degradation capacity. Units with 50P + 50O presents a typically diauxic curve, with two clearly distinct growth phases. First, a stable period without increasing biogas production was maintained for a period of about 20 days, reaching a volume of 0.22 L, approximately. Then, more biogas was produced until the end of the assay and values of around 0.33 L were obtained. This performance can be understood as the result of the sequential metabolization of substrates, simultaneously with the adaptation of microorganisms to degrade the polyphenols present in OMW. In consequence, substrates were more available for biogas producers, as methanogens. This inhibition phenomenon is clear evident in the digestion of 20P + 80O mixture, where biogas volumes and methane proportion remained very low throughout the experiment (0.12 L cumulative biogas with 6% CH\({}_{4}\)). The highest percentages of PE (\(\geq\)70%) in the mixture provided greater organic matter removals (63-75% COD, 17-21% TS, 29-36% VS, Table 5). On the contrary, the proportion increase of OMW in the mixture has a negative effect on the performance of the anaerobic treatment, showing how strong the inhibitory capacity of this effluent is on the microbial process only standing a small proportion of about 30% in the mixture with PE. On the other hand, it is interesting to note that the inhibitory effect caused by OMW seems to have been compensated through the balance found in the complementarity with acidity and nutrients, therefore the mixture 70P + 30O provided better results in the removal of solids and COD  
Figure 1: Anaerobic digestion of PE and OMW mixture: biogas production. STP—Standard temperature and pressure. (Table 5) than PE without mixtures. The mitigating inhibitory effects were also reported when complementary substrates were anaerobically digested [46]. Concerning polyphenols, an increase in concentration was observed as the OMW volume rose in the substrate, as was expected to occur (Table 6). The PE and 20P + 80O mixtures showed the lowest removal values, and despite the small initial phenolic concentrations of PE, the microorganisms did not display the ability of removing them. On the other side, the highest concentration of phenolic compounds in 20P + 80O mixture prevented the removal process due to high inhibitory effect of OMW. Anaerobic process of 70P + 30O and 50P + 50O mixtures provided the best results in removing the phenolic compounds (35% and 30%, respectively), indicating that their microbial consortia were able to degrade this kind of molecules, however, comparing the biogas volumes production, the 50P + 50O units needed about 27 days to start to accumulate gas and, probably, to degrade part of the initial phenolic compounds present in a higher concentration than in the 70P + 30O digesters (2.2 versus 1.7 g/L). Some phenolic compounds have antioxidant activity due to their chemical and functional characteristics, not exactly because of the amount they are found in the sample.