## 3 Results and Discussion
### Anaerobic Digestion of the BWW and PE Mixture  
During the first phase of 19 days of this trial, conducted under the HRT of 6.7 d and OLR of 4.5 g COD/L-d, biogas production ranged between 0.4 and 1.2 L/L-d, mainly composed of methane in concentrations of 63-78% (Figure 2). A biogas production rate of 0.996 L/L-d and 0.928 L/L-d, containing 65% methane, was registered after the digestion of pre-treated manure (140 \({}^{\circ}\)C, 40 min) and without pre-treatment, respectively [12]. The values for biogas and methane content are of the same order of magnitude than those obtained in this work (1.1-1.2 L/L-d), however they were performed under identical OLR (2.5 g VS/L-d versus 2.2 g VS/ L-d) but advantageous operating conditions. They resulted from a higher digestion process temperature and longer HRT than the conditions in this work (55 \({}^{\circ}\)C, 18 d HRT versus 37 \({}^{\circ}\)C, 6.7 d HRT) [12]. Thermophilic conditions of temperature, such as 55 \({}^{\circ}\)C, promote higher biogas productions than the mesophilic ones (37 \({}^{\circ}\)C) as shown by [PERSON] [53]. On the other hand, the increase in HRT allows for a better conversion of organic matter and, consequently, greater volumes of biogas will be expected as concluded by [PERSON] et al. [54]. Bearing these considerations in mind, the data obtained in the present work are comparatively of relevant interest because they indicate that it is possible to obtain a similar production in biogas (energy) and treat a higher volume of effluent in the same period and, at lower process temperature, conditions that will certainly provide an energy balance more attractive for the operating unit. Considering the co-digestion of Pennisetum hybrid and pig manure (50:50) under a mesophilic range of temperature (35 \(\pm\) 1 \({}^{\circ}\)C) [55], the results obtained are comparable to those of this work, although with a lower methane content: biogas production of 1.16 and 2.07 L/L-d with 77.8 and 79.5% CH\({}_{4}\) (OLR 2.22 and 4.96 g VS/L-d) versus 0.87 and 1.92 L/L-d with 56.7 and 60.3% CH\({}_{4}\) (OLR 2.0 and 5.0 g VS/ L-d [55]) were registered, respectively. The variations in biogas volume under HRT 6.7 d (Figure 2) were certainly stressed by the failure to feed the reactor on Sundays. Despite these operating conditions, it was found that the unit always reacted positively, providing successive increments in biogas volume during every week of testing. Thus, the work continued according to the experimental plan outlined, but with an additional objective: to test the unit ability to withstand or not withstand more aggressive operating conditions by decreasing HRT and increasing the loading rate. Figure 2: Anaerobic digestion of BWW and PE mixture under fed-batch conditions in the hybrid anaerobic reactor: biogas production and methane composition. HRT = hydraulic retention time. In the next phase (HRT 3.0 d), there was a clear intensification in the biogas volume (1.1-2.3 L/L-d), suggesting that both hydraulic and microbial reactor conditions were able to support the OLR rise to 10.0 g COD/L-d (4.06 g SV/L-d). The good process response was confirmed by the improvement in the biogas quality, on which a slight increase in concentrations from around 76 to 80% was registered (Figure 2). Another relevant aspect is the stability in biogas production indicated by the constant values measured in the last week of operation (Figure 2). In fact, the methane level of about 80% in the biogas obtained at the stable period was confirmed by measurements following this experimental phase (data not shown). [PERSON] et al. [56] reported comparable methane content (77.9%) yet operating under an HRT of 17.86 d (OLR of 1.16 g COD/L-d, 0.45 g SV/L-d) to digest swine effluent. At this point, roughly comparing these two operational periods, it seems that by doubling the organic load (4.5 to 10 g COD/L-d, Table 1), approximately twice the volume of biogas was obtained (1.2 up to 2.3 L biogas/L-d).