## 3 Results and Discussion
### Anaerobic Digestion of the BWW and PE Mixture
#### 3.2.2 Molecular Characterization of Microbial Communities in the Hybrid Anaerobic Reactor
Other phyla were not considered in this discussion due to their low presence (0.01–1.2%) in the community: Armatimonadetes; Atribacteria; BRC1; Candidatus Berkelbacteria; Chlamydia; Chlorobi; Chloroflexi; Cyanobacteria; Deinococcus-Thermus; Fibrobactores; Hydrogenedentes; Lentisphaerae; Microgonomics; Nitrospine; Parcubacteria; Planctomycetes; SR1 (Absconditobacteria); Saccharibacteria; TM6 (Dependentiae); Verrucomicrobia; WS6; and unidentified bacteria. 3 d OUT, and most of the members of Firmicutes were assigned to _Clostridiales_ (35% and 37%, respectively, for HRT 6.7 d IN and HRT 3 d OUT). At the end of the HRT 3 d phase (Figure 6b), microbial populations of the order _Flavobacterialies_ became predominant (18%), and consisted mainly of members of the family _Flavobacteriaaceae_, genus _Moleibacter_ (17.8%), replacing the _Bacteroidales_ populations. _Moleibacter_ has been described as a strictly aerobic bacterium and without an important role in the breakdown of complex or other carbohydrates [68,69], which can explain its sudden appearance at this stage. Apparently, these changes in bacterial populations are meaningless and without much influence on the performance of the process. During this phase, _Basillales_ also became part of the composition structure of microbial communities (7.8%). _Bacillus_, whose order consisted mainly of members of the family _Planococcaceae_, and genus _Sporosarcina_ (7.4%), which are known as good carbohydrate-utilizing species [60], and as _Clostridiales_, also _Basillales_ can decompose protein, cellulose, fat, and carbohydrates to produce VFAs [65]. As can be observed in Figure 5, Proteobacteria, known degraders of polysaccharides and more complex substrates [65], predominate at phase HRT 3 d (19%). All five classes within the Proteobacteria were detected during the experiment, Beta- and Gammaproteobacteria dominated at HRT 3 d representing, respectively, 5% and 13% of the total bacterial sequences (Figure 6b). Betaproteobacteria is involved in the first steps of degradation during the anaerobic digestion process, and they are the main consumers of propionate, butyrate, and acetate [70]. Most of the members of Betaproteobacteria belong to the family _Alcaligenaceae_ that increased from 1% (6.7-d HRT, Figure 6a) to 4.8% (3-d HRT, Figure 6b) of total sequences. At HRT 3 d (Figure 6b), Gammaproteobacteria represented 28,784 sequences, with the genus _Pseudomonas_ representing 81% of those sequences and 11% of bacterial sequences. Pseudomonasaceae is a group with important environmental functions, such as bioremediation of toxic residues, as well as xenobiotic compound degradation. The presence of _Pseudomonas_ in the microbiota also has been related with phenol degradation and organic matter removal, during the effluents biotereatment processes [32]. During the anaerobic digestion of the mixture BWW + PE (60:40, \(v/v\)), slight changes in the structure of the bacterial populations were observed, mainly at HRT 3 d, in which the pronounced increase in _Bacillus_, _Pseudomonas_ and _Alcaligenaceae_ certainly may have contributed to the stabilization of the process at a continuous feeding regime with OLR of 10 g COD/L-d, achieving 95% VFA removal rates, and around 2 L biogas/L-d.  
Methanogenesis has three major pathways, each converting a different substrate into methane gas [71]: methylotrophic (decarboxylation of methyl alcohols/methyl amines/methyl sulfides); hydrogenotrophic (reduction of H\({}_{2}\)/CO\({}_{2}\)); and acetoclastic methanogenesis (decarboxylation of acetate). The pathway which leads to the methane production occurring in an anaerobic reactor only depends on the methanogenic populations and the availability of the suitable substrates for the anaerobic digestion process. Regarding the Archaea domain, Figure 7 shows considerable changes that occurred during anaerobic digestion in the hybrid anaerobic reactor.