# Structure of Microbial Communities When Complementary
## 3 Results and Discussion
### Anaerobic Digestion of PE and OMW Mixture
On the contrary, 20P + 80O presented a lower quantity of acids than the former but it has a lower proportion of PE and, in addition, the highest concentration of phenolic compounds, which may have contributed to the mixture acidification and justify the acidic pH (5.7) obtained in the digestate. Regarding VFA, PE digestion was able to reduce most acids (85%, Table 7), including the isobutyric and butyric acid (2.2 and 1.6 g/L as acetic, respectively, data not shown), confirming the presence of an active and balanced microbial population (bacteria and archaea) capable of converting these acids into CH\({}_{4}\). While in 70P + 30O mixture, more than half of the initial VFA amount (55%) was converted; in 50P + 50O, it does not happen and a VFA concentration increase was observed at the end of the experiment. Indeed, an accumulation of VFA with a larger chain than acetic acid (propionic, isobutyric, and butyric, at concentrations of 3.1, 0.93 and 0.53 g/L, respectively, data not shown) was registered in the digested 50P + 50O mixture. This unexpected result reveals that although the process was not able to convert the acids that have accumulated, it has not been completely inhibited. Looking at the behavior of 50P + 50O in Figure 1, it is observed that these units do not present any stabilized production line (plateau) at the assay end, as seen in other cases, but a notable growth trend in the biogas production in the final stage. In line with the previous comments, this performance can be regarded as the result of some substrates degradation, after a slow adaptation period of microbial population, where the VFA may be included. Given this composition of the substrates, following Table 3, it is clear that acetic acid is the main component of VFAs in OMW, while PE has higher concentrations in longer chain acids than acetic acid: 39% versus 61% (OMW) and 25% versus 75% (PE) in acetic versus propionic, isobutyric and butyric, respectively. It is expected that as the amount of OMW in the mixture rises, the proportion of acids favorable to the development of the process, mainly composed by acetate, will increase and the conditions for its conversion will improve. In fact, the acetic acid content corresponds to increasing percentages of total acids (expressed as acetic acid) from 24% to 65% as the volumetric contribution of OMW in the mixtures increased. Concerning the digestates, while the conversion of VFA in acetate increased during the process of PE and 70P + 30O (from 24% and 58% in influent to 83% and 68% in effluent, respectively), it came down in the two other cases, confirming their inhibiting conditions. The most disadvantageous situation was found in 50P + 50O, with values of 59% in influent and 31% in effluent, compared to 20P + 80O mixtures (65% in influent and 57% in effluent). Similar values were obtained for cumulative methane yield with anaerobic digestion of the mixture 70P + 30O and the sample PE, in which values of 489 and 434 L CH\({}_{4}\)/kg SV\({}_{\text{in}}\) were achieved, respectively (Table 8). In terms of energy, these substrates presented interesting values (between 4.87 and 4.32 KWh/kg SV\({}_{\text{in}}\)), which are of the same order