# Structure of Microbial Communities When Complementary
## 3 Results and Discussion
### Anaerobic Digestion of PE and OMW Mixture
At the beginning of the experiment, the TP values increase with increasing OMW concentration in the mixture, 0.9 up to 2.7 g/L (Table 6), but the antioxidant activity remains constant, and the DPPH inhibition slightly decreases. This observation suggests the maintenance of compounds with antioxidant activity in all mixtures (about 1 mmol TEAC), and that the quantity of phenolic compounds with DPPH inhibitory capacity is much higher in the PE and 70P + 30O samples (81% and 79%, respectively). After anaerobic digestion, despite the low removal of TP in all samples, the values of DPPH inhibition are in accordance with the antioxidant activity, showing that the anaerobic treatment is a process able to remove or convert phenolic compounds, but it does not eliminate the antiradical power of the digested flows. This aspect was already reported by [PERSON] et al. [27]. The highest concentrations of nitrogen compounds, both total and ammonium, in the substrate, present in PE units, decreased as the volume of OMW in the mixture was increased (Table 7). In terms of anaerobic performance, it was again found that the 70P + 30O unit showed the highest increment of ammonia nitrogen after anaerobic digestion, indicating the existence of a balanced microbial population capable of degrading the organic matter. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Mixture**} & \multirow{2}{*}{**TP\({}_{\text{in}}\) (g/L)**} & \multicolumn{2}{c}{**TPr\({}_{\text{r}}\) (\%)**} & \multicolumn{2}{c}{**Antioxidant Activity**} & \multicolumn{2}{c}{**DPPH Inhibition**} \\  & & **TP\({}_{\text{r}}\)** & \multicolumn{2}{c}{**(mmol TEAC)**} & \multicolumn{2}{c}{**(\%)**} \\ \cline{5-7}  & & **(\%)** & **Initial** & **Final** & **Initial** & **Final** \\ \hline PE & 0.9 \(\pm\) 0.0 & 3 \(\pm\) 0.0 & 1.1 \(\pm\) 0.0 & 0.6 \(\pm\) 0.1 & 81 \(\pm\) 1 & 30 \(\pm\) 8 \\
70P + 30O & 1.7 \(\pm\) 0.0 & 35 \(\pm\) 0.0 & 1.1 \(\pm\) 0.0 & 0.4 \(\pm\) 0.1 & 79 \(\pm\) 0 & 10 \(\pm\) 7 \\
50P + 50O & 2.2 \(\pm\) 0.0 & 30 \(\pm\) 0.0 & 1.0 \(\pm\) 0.0 & 1.3 \(\pm\) 0.0 & 74 \(\pm\) 1 & 69 \(\pm\) 2 \\
20P + 80O & 2.7 \(\pm\) 0.0 & 7 \(\pm\) 0.0 & 0.9 \(\pm\) 0.0 & 1.3 \(\pm\) 0.0 & 68 \(\pm\) 0 & 69 \(\pm\) 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Anaerobic digestion of PE + OMW: Antioxidant activity and total phenolic (TP) content. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Mixture**} & \multicolumn{2}{c}{**COD**} & \multicolumn{2}{c}{**Total and Volatile Solids**} \\ \cline{2-6}  & **CODin** (g/L) & **CODr (\%)** & **TSin (g/L)** & **TSin (\%)** & **VSin (g/L)** & **VSr (\%)** \\ \hline PE & 93 \(\pm\) 5 & 63 \(\pm\) 6 & 47 \(\pm\) 1 & 17 \(\pm\) 2 & 32 \(\pm\) 1 & 29 \(\pm\) 2 \\
70P + 30O & 81 \(\pm\) 3 & 75 \(\pm\) 0 & 40 \(\pm\) 0 & 21 \(\pm\) 2 & 28 \(\pm\) 0 & 36 \(\pm\) 4 \\
50P + 50O & 77 \(\pm\) 3 & 48 \(\pm\) 1 & 38 \(\pm\) 0 & 16 \(\pm\) 3 & 28 \(\pm\) 0 & 22 \(\pm\) 3 \\
20P + 80O & 73 \(\pm\) 1 & 29 \(\pm\) 2 & 33 \(\pm\) 1 & 21 \(\pm\) 1 & 25 \(\pm\) 1 & 25 \(\pm\) 0 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Anaerobic digestion of PE + OMW: Performance and biogas/methane production. All assay mixtures presented almost neutral pH that varied from 6.2 (20P + 80O) to 7.3 (PE) before anaerobic digestion (Table 7). At the end of the process, except for 20P + 80O, the increase in pH suggests the medium conditions for the effective removal of acidic compounds. PE acted as a buffer solution during the experiment due to the presence of high nitrogen content, counteracting the acidic nature of OMW (pH = 5.1, Table 3). The presence of PE in 70P + 30O helps to maintain the medium pH in values appropriate to the development and maintenance of the anaerobic process despite the high VFA concentrations in the substrate (3.9 g/L).