## 3 Results and Discussion
### Anaerobic Digestion of the BWW and PE Mixture
#### 3.2.1 Hybrid Anaerobic Reactor Profile  
The hybrid anaerobic reactor profile was created from the samples collected in ports placed at different column heights (Figure 1), at the end of the 3-d HRT phase. As shown in Figure 3a, the highest matter concentration represented by the COD, TS and VS parameters is found, as expected, at the bottom of the hybrid anaerobic reactor (P3, 14 cm, Figure 1), indicating the presence of a dense material known as a sludge blanket. Comparing samples at the P3 and at P2 (37 cm) sampling ports, a decrease in concentrations is observed which continues in the next section (P2 and P1) but in a more pronounced way. Considering that the P1 (48 cm) sampling port is in the packing bed section and the collected sample corresponds to biomass material from the support medium, the greatest decrease verified in the concentration, mainly in terms of COD, could be justified by the functional action of the packing bed in retaining the digesting material and promoting the substrate conversion. However, when comparing the values obtained in the influent and effluent, the difference concerning solids is greater than in the case of COD (54%, 68% versus 12% TS, VS and COD removal, respectively). As was already pointed out, this low removal capacity of COD does not agree with the stable behavior observed in a hybrid anaerobic reactor related to daily production of biogas/methane. The additional material in effluent could be associated to some suspended biomass, coming from the lower layers that passed through the packing bed medium, or even to be related to the aggregated biomass in it that was dragged in the gaseous flow and/or in the digestate exit. As shown in Figure 3b, the main consumption of VFA was carried out at the bottom of the hybrid anaerobic reactor, in the first column section, about 14 cm high (P3). Accordingly, identical information is given by the pH values. The main evolution of pH to more basic values happened in the same bottom section, which remain relatively stable throughout the column of the hybrid anaerobic reactor until the sample output (at 66 cm height). Figure 3c shows that acetic acid is the main constituent of VFAs at the influent and, together with the other components of VFAs, was converted at the bottom of the hybrid anaerobic reactor (P3). According to the composition of the reactor (lower and upper sections, Figure 1), the recorded data suggest that a stratification occurs inside the hybrid anaerobic reactor, and the majority of hydrolytic, acidogenic and acetogenic microbial populations are located at the lower layers, acting on the material in digestion and converting most of the compounds for methanogenesis and biogas production. A sludge bed formation in the bottom of the hybrid anaerobic reactor is suggested by the content increase obtained at the 14 cm (P3) sampling port section. The effect of the sludge bed biomass on the hybrid anaerobic reactor has already been observed in another study in which most of the removal of total phenols took place mainly at the reactor base [25]. In addition, the effectiveness of the packing bed placed on top of the hybrid anaerobic reactor used in this assay in promoting the retention of digested material was demonstrated by the fact that the greatest decline in parameters concentrations (COD and solids) were obtained in this section (48 cm column height).