## 3 Results and Discussion
### Anaerobic Digestion of the BWW and PE Mixture
The attained data indicate that the hybrid anaerobic reactor reacted positively to the change in operating conditions, showing capacity to accommodate this increase in organic load. On the other side, the reactor design was able to withstand high loads of organic-rich substrate and produce high volumes of biogas/methane, taking advantage of both suspended growth and attached-growth anaerobic microbial biomass as pointed out by [PERSON] et al. [57]. Gas production is a result of the organic matter conversion, and the partial removal of influent contents is consequently achieved. According to the parameters in Table 3, the decrease in substrate concentration, shown in each process stage, indicates that the mixture digestion took place properly under the tested experimental conditions, as expected. However, regarding the data obtained, COD removal decreased from 52 to 12% with the reduction in the HRT from 6.7 d to 3.0 d, contradicting the increase in biogas production (Figure 2). A possible explanation for this would be the presence of additional material in the effluent, such as suspended biomass that reached the top of the hybrid, and was counted as COD content, when the sample was taken for analysis. Regarding solids, the greatest removal amounts of 54% in TS, and 68% in VS, were obtained precisely at HRT 3 d, suggesting that microbial populations were able to convert the majority of the organic matter into biogas. Values of this order of magnitude (55-65% TS and 59-68% VS) were also reported by [PERSON] and [PERSON] [58] when digesting dairy manure in an anaerobic hybrid reactor, equipped with a floating support for biomass immobilization, yet these values were obtained at a higher HRT of 15 d. In another experiment, [PERSON] et al. [57] achieved COD removal of 57% using a hybrid UASB reactor to test the swine manure treatment. The neutral pH (7.0, Table 3), measured in the feed mixture resulted from PE characteristics that had counteract the acid conditions of BWW. Slightly higher pH values (7.9-8.1) were measured in the digested flow, indicating that there is a suitable occurrence between the components that provide buffering capacity to the medium and those that act as acidity promoters, such as nitrogen compounds and VFA, respectively, providing conditions for degradation of the acids contained in the substrate and those that are formed during the organic matter digestion, without further accumulation in the reactor. Regarding nitrogen compounds, despite the similarity between the input and output of TKN values during the initial stage (HRT 6.7 d), the decrease in concentration (21%) achieved after the regimen at HRT 3 d suggests protein degradation by microbial populations, within the hybrid anaerobic reactor, under this last operational condition (Table 3). The ammonium nitrogen data did not provide much additional information due to the small difference registered between the substrate and digestate concentrations. Considering that the proportion of ionized and non-ionized forms (the toxic one) in the medium is pH-dependent and that values in the basic pH range (7.9-8.1) were registered in this work, some toxicity from ammonia could be considered. Nevertheless, from the available data, the initial amount of 800 mg NH\({}_{4}\)+-N/L is low enough to have any inhibitory effect on the process [11]. As expected, ammonium nitrogen concentration increased in the digestate during the first stage, although not in a very marked way. In the next phase (HRT 3 d), a low removal of ammonium was registered, indicating that both the ammonium contained in the substrate and the ammonium obtained from the conversion of compounds were used in the process. In fact, the greatest VFA removal (95%, Table 3) was registered in the HRT 3 d operational condition, signifying the presence of an adequate buffer capacity in the medium. Global VFA removal resulted from partial removals of its constituents, as shown in Table 3. Acetic acid is the main component of inputs, comprising 62% of total VFA.